Coastal Hydrodynamics and Circulation Exchange Between the Arabian Gulf and the Sea of Oman

Abstract

The Arabian Gulf (Persian Gulf) and the Gulf of Oman are two basins, physically linked but comprising different hydrodynamic features. The shape of the basins, the weather, and the exchange through the Strait of Hormuz all affect circulation in the system. This review synthesizes current knowledge of hydrodynamic processes leading to water circulation in both basins, including wind forcing, density-driven exchange, tidal dynamics, and mesoscale changes. The Arabian Gulf has shallow depths, intense evaporation, and limited exchange, which promote hypersaline conditions, long residence times, and greater sensitivity to environmental stress. In comparison, the Gulf of Oman has a deeper, more exposed system, governed by monsoon-driven circulation, upwelling, and mesoscale processes. The present review highlights how various hydrodynamic regimes influence stratification, changes in temperature and salinity, and exchange processes, with direct impacts on nutrient transfer, oxygen delivery, and ecosystem dynamics. An important control point, the Strait of Hormuz, connects the Arabian Gulf and the Gulf of Oman. This review also stresses the importance of observational data and modeling capacity, as well as their limitations, for subsurface processes and connections across scales. It also provides an inclusive assessment of hydrodynamics and water circulation in the two basins. Moreover, it underscores the need for coordinated observational and modeling approaches to improve understanding and enhance management of these globally sensitive aquatic systems.

1. Introduction

Hydrodynamics govern circulation, stratification, mixing, sediment transport, nutrient redistribution, and pollutant dispersion in marine systems, thereby shaping their structure, function, and long-term evolution. These processes exert particularly strong control in semi-enclosed and arid marginal seas, where prolonged residence times and restricted exchanges amplify both natural variability and anthropogenic pressures [1,2]. Water exchange through straits, density-driven flows, and wind forcing are therefore key determinants of environmental stability, ecosystem productivity, and pollutant fate [3].

The Arabian Gulf (Persian Gulf), and the Gulf of Oman form a physically coupled system with contrasting hydrographic characteristics. The Arabian Gulf is a semi-enclosed basin with an average depth of 30–35 m and is characterized by intense evaporation, hypersalinity exceeding 40 psu and reaching 50–70 psu in embayments, and pronounced seasonal temperature variability [4]. By contrast, the Gulf of Oman is a much deeper basin, reaching depths of approximately 3000 m, and forms part of the Arabian Sea–Indian Ocean system, where circulation is influenced by monsoonal forcing, upwelling, and mesoscale variability [5]. The Strait of Hormuz functions as the principal control point between the two basins, maintaining density-driven inverse estuarine exchange in which comparatively fresher surface waters enter the Arabian Gulf, while dense, evaporation-enriched hypersaline waters exit at depth into the Gulf of Oman [6]. This exchange is modulated by tides, winds, and seasonal density gradients and is further complicated by double-diffusive processes and hydrostatically stable inversion layers [7]. The relatively long water renewal time in the Arabian Gulf, estimated at 3–5 years, increases its sensitivity to biogeochemical alteration and contaminant accumulation [8].

In the Arabian Gulf, circulation is primarily governed by Shamal winds and reverse-estuarine cyclonic flow, whereas the Gulf of Oman is characterized by monsoon-driven gyres and energetic mesoscale eddies. These contrasting regimes generate distinct patterns of water movement and mixing between the two basins [9,10,11]. Differences in wind forcing arise from exposure to different climatic regimes, with Shamal-dominated variability in the Arabian Gulf and monsoon-dominated variability in the Gulf of Oman. Together with regional orography and basin geometry, these wind systems contribute to the seasonal variability and complexity of wind-driven circulation, promoting the formation of gyres and eddies that substantially influence oceanographic and ecological processes across the Arabian Gulf–Gulf of Oman system [11].

Beyond their physical connectivity, the Arabian Gulf and the Gulf of Oman hold major strategic, ecological, and socioeconomic importance. The region contains substantial oil reserves and major shipping corridors, underscoring its central role in global energy security [12]. It also supports fisheries and desalination-dependent coastal communities, as well as high-value ecosystems, including coral reefs, mangroves, seagrass meadows, and productive upwelling zones [13]. Nevertheless, the coupled system is increasingly exposed to environmental pressures, particularly in the Arabian Gulf, where climate change has intensified marine heatwaves and elevated baseline temperature and salinity conditions [14].

The Gulf region accounts for a substantial proportion of global desalination capacity, generating large volumes of brine discharge that can alter salinity, temperature, and circulation patterns [15]. In addition, coastal expansion, dredging, oil and gas activities, and contaminant inputs further degrade habitats and interact with hydrodynamic processes that regulate nutrient cycling and sediment transport [16]. Because circulation in the Arabian Gulf and the Gulf of Oman is physically connected, environmental impacts are not spatially isolated but may be transported across the boundaries of the two basins [6].

Sediment transport in the Arabian Gulf and the Gulf of Oman is controlled by the interaction of tidal currents, wind-driven waves, density-driven circulation, and regional bathymetry. The relative importance of these processes varies spatially: in the northern Arabian Gulf, tidal forcing and aeolian dust inputs are dominant, whereas in the Gulf of Oman, bathymetric complexity and tide–wind interactions exert stronger control on sediment dynamics. Understanding these drivers is essential for predicting sediment distribution, supporting navigation management, and assessing environmental impacts in this distinctive marine system [17].

Sedimentation is an important coastal management issue in shallow marine and semi-enclosed environments because sediment transport is affected by river inputs, wave–wind forcing, tidal currents, and coastal structures [18]. In the Arabian Gulf, broad shallow shelves, weak flushing, dredging, reclamation, and riverine inputs can influence sediment retention, resuspension, and shoreline stability. Recent sedimentation studies have shown that bathymetric change, sediment grain size, wave forcing, and shoaling analysis are useful for assessing coastal structure vulnerability and maintenance needs [19].

Despite advances in process-based studies and hydrodynamic modeling, critical research gaps remain. Observational networks are spatially sparse and temporally discontinuous, particularly along the Omani coastline and in deep-water environments [20]. In addition, subsurface and transitional circulation processes, especially variations in deep outflow and their interactions with monsoon-driven dynamics, remain insufficiently resolved in the Arabian Gulf–Gulf of Oman system [21].

Furthermore, the roles of the Strait of Hormuz and the Musandam Peninsula in species dispersal, biogeochemical exchange, and the maintenance of regional circulation remain important but insufficiently resolved dimensions [22]. Although climate-change projections have only recently begun to be incorporated into hydrodynamic simulations, substantial uncertainties remain in predicting long-term stratification, hypoxia risk, and shifts in nutrient regimes [23,24].

This review provides a novel and integrated assessment of hydrodynamics and water circulation in the Arabian Gulf and the Gulf of Oman by treating the two basins as a single coupled system rather than as independent water bodies. It advances current understanding by linking physical circulation with biogeochemical processes, groundwater interactions, and anthropogenic pressures within a unified framework. In particular, it identifies the Strait of Hormuz as a critical control point regulating inter-basin exchange and broader system behavior, while emphasizing the roles of mesoscale variability, stratification, and ecological responses.

Accordingly, the objectives of this review are to (i) characterize the dominant hydrodynamic processes in the coupled Arabian Gulf–Gulf of Oman system, (ii) compare circulation and mixing regimes between the two basins, (iii) evaluate hydrodynamic controls on nutrient pathways and productivity, (iv) assess anthropogenic and climate-driven influences on circulation and ecosystem functioning, and (v) identify future directions for integrated observational and modeling frameworks.

By integrating physical, ecological, and hydrological perspectives, this review provides a comprehensive systems-based synthesis that can advance scientific understanding and support sustainable management of the Arabian Gulf–Gulf of Oman region.

2. Methods

2.1. Literature Selection Criteria

Studies were selected based on their relevance to hydrodynamics, water circulation, exchange processes, temperature–salinity variability, tidal dynamics, wind forcing, numerical modeling, ecological interactions, groundwater exchange, or sediment-related processes in the Arabian Gulf, Gulf of Oman, Strait of Hormuz, and adjacent Arabian Sea. Articles were included if they provided direct information on physical oceanographic processes, modeling approaches, observational datasets, or coupled hydroecological processes. Studies were excluded if they were unrelated to the study region, focused only on terrestrial systems, lacked relevance to marine or coastal circulation, or did not provide sufficient scientific detail for synthesis. In addition to Scopus, Web of Science, and Google Scholar, additional relevant sources were considered during the narrative literature synthesis when necessary to provide context on hydrodynamics, sediment processes, and regional environmental conditions. For temperature, evaporation, and salinity, the trends were not recalculated from raw observational datasets in this review. Instead, long-term patterns were synthesized from published studies, reanalysis products, and model-based assessments. Therefore, the reported trends represent literature-derived evidence rather than newly computed time-series analyses.

2.2. Literature Search Strategy—Bibliometric Analysis

A bibliometric analysis was conducted to assess research trends, geographic distribution, and thematic development in studies on hydrodynamics and water circulation in the Arabian Gulf and the Gulf of Oman. Bibliographic records indexed in Scopus for the period 1994–2025 were analyzed using VOSviewer (version 1.6.20) to examine keyword co-occurrence patterns, thematic clusters, and country-level research contributions.

Scopus was selected because it provides broad coverage of peer-reviewed scientific literature, citation metadata, author affiliations, keywords, abstracts, and export formats compatible with VOSviewer. Using a single database also reduced duplication and improved reproducibility of the bibliometric workflow. Web of Science and Google Scholar were not used for bibliometric mapping because differences in indexing structure, metadata format, and duplicate handling can affect reproducibility.

The literature search was conducted using regional and process-based terms to capture studies relevant to the Arabian Gulf, the Gulf of Oman, the Strait of Hormuz, and the adjacent Arabian Sea. The search strategy was formulated as follows:

TITLE-ABS-KEY (“Arabian Gulf” OR “Persian Gulf” OR “Gulf of Oman” OR “Sea of Oman” OR “Strait of Hormuz”) AND TITLE-ABS-KEY (hydrodynamic* OR circulation OR “water exchange” OR tide* OR salinity OR “wind-driven” OR “density-driven” OR upwelling OR stratification).

The search returned approximately 234 records for the Arabian Gulf and 165 records for the Gulf of Oman. Records were screened according to titles, abstracts, and keywords. Studies were retained if they examined hydrodynamic processes, circulation patterns, water exchange, tidal dynamics, temperature–salinity variability, numerical modeling, or ecohydrodynamic processes within the study region. Publications were excluded if they were outside the geographic scope, focused exclusively on terrestrial systems, were unrelated to marine circulation, or lacked sufficient relevance to the study objectives. Duplicate records were removed before analysis.

Keyword data were manually standardized before cluster generation by merging synonymous terms and removing irrelevant entries. The resulting thematic clusters were evaluated against highly cited and region-specific publications to verify that the identified themes accurately reflected the scientific literature (see Supplementary Data S1—Excel file).

In addition to the bibliometric assessment, information on sea surface temperature (SST), salinity, tidal range, and other hydrodynamic characteristics was compiled from peer-reviewed articles and technical reports. Where multiple values were reported, representative ranges were selected to support consistent comparison between the Arabian Gulf and Gulf of Oman and to highlight the principal hydrodynamic contrasts between the two basins [6,7,8,10,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40].

During the preparation of this review, ChatGPT (GPT-4o, OpenAI) was used to assist in the design and creation of Figures based on data reported in the collected literature, including wind vectors, current components, tidal elevations, and basin-scale depression averages. The authors reviewed, revised, and validated the generated content and take full responsibility for its accuracy.

2.3. Methodological Approaches in Hydrodynamic Studies of the Arabian Gulf and the Gulf of Oman

The Arabian Gulf lies between 24° N and 30° N latitude and 48° E and 57° E longitude [41], whereas the Gulf of Oman, located in the northwestern Arabian Sea, extends from approximately 21° N to 26° N latitude and 56° E to 63° E longitude [10]. Figure 1 illustrates the study area and major circulation pathways within the Arabian Gulf–Gulf of Oman system. Hydrodynamic investigations in these basins employ diverse methodological frameworks that reflect their contrasting physical characteristics. The Arabian Gulf is shallow, highly evaporative, and semi-enclosed, whereas the Gulf of Oman is deeper, dynamically connected to the Arabian Sea, and strongly influenced by monsoon-driven circulation. Consequently, although many techniques are shared, their applications and relative emphasis differ between the two regions.

2.4. Field-Based Observations

Field-based observations in the Arabian Gulf have primarily focused on coastal and nearshore environments, where shallow bathymetry and anthropogenic pressures are dominant. Common measurements include water levels from tide gauges, current velocities, temperature and salinity profiles, wind forcing, sediment characteristics, and water-quality parameters. For example, sediment and seawater samples were collected from 67 stations across Saudi Arabian waters [42]. In addition, tides, currents, temperature, and salinity were measured in the Ruwais coastal basin in the United Arab Emirates to support hydrodynamic model calibration. Sampling campaigns commonly target seasonal contrasts between winter and summer, particularly to resolve variability in Shamal-driven circulation [43]. Overall, most field studies rely on dense coastal station networks and shallow-water monitoring.

Field studies in the Gulf of Oman primarily examine exchange processes through the Strait of Hormuz, water-mass interactions, stratification, and frontal systems. For example, surface and subsurface current systems, including the high-salinity outflow from the Arabian Gulf, have been analyzed [39]. However, deep-water observational coverage in the Gulf of Oman remains limited because of steep bathymetry and operational constraints. Consequently, most investigations have relied on hydrographic cruises and exchange-flow monitoring.

2.5. Remote-Sensing Methods

Remote sensing has been applied in both the Arabian Gulf and the Gulf of Oman, although its use and emphasis differ between the two basins. Satellite-derived sea surface temperature (SST) products, including GHRSST Level 4 datasets, are widely used for regional monitoring. In the Arabian Gulf, remote sensing is primarily used to detect extreme summer warming and shallow-water thermal gradients. In the Gulf of Oman, by contrast, satellite observations are more commonly used to identify upwelling signatures and track monsoon-driven variability. Satellite-derived data are typically validated against buoy observations, with root-mean-square deviations generally ranging from 0.5 to 0.9 °C, particularly for seasonal detection and model validation [40].

2.6. Numerical Hydrodynamic-Modeling Frameworks

Numerical modeling is the dominant methodological approach in both regions, although model design reflects basin-specific physical and management priorities. In the Arabian Gulf, commonly used models include Delft3D-FLOW, COSMOS, and GULFHYDRO [8,41,42]. These frameworks are primarily three-dimensional, baroclinic, and hydrostatic and are used to simulate stratification, thermohaline circulation, and coastal exchange. Depth-averaged two-dimensional tidal models have also been applied in localized settings [43]. Earlier Gulf studies commonly relied on structured grids [43], whereas more recent applications increasingly incorporate unstructured meshes to better resolve complex coastlines and nearshore processes [11,44]. Typical forcing inputs include ERA5/ECMWF winds, multiple tidal constituents, Shatt Al-Arab River discharge, and seasonal Shamal winds. These applications commonly address pollutant dispersion, oil spill transport, industrial discharge assessment, climate change scenarios, and coastal infrastructure impacts [45,46]. Overall, numerical modeling in the Arabian Gulf is strongly oriented toward environmental impact assessment and coastal management applications.

In the Gulf of Oman, hydrodynamic modeling studies are dominated by high-resolution three-dimensional unstructured-grid frameworks, including the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), the Massachusetts Institute of Technology General Circulation Model (MITgcm), and finite-element or hybrid-element approaches. Unstructured meshes are particularly important for resolving the region’s complex bathymetry, narrow coastal zones, and exchange processes through the Strait of Hormuz [15]. These applications commonly address exchange-flow dynamics, regional circulation patterns, wave–structure interactions, and dispersion under complex boundary conditions. Overall, Gulf of Oman modeling emphasizes high-resolution, physics-based representations of exchange processes and regional hydrodynamics.

2.7. Analytical Approaches

Analytical approaches, including eigenfunction expansions, are rarely applied in the Arabian Gulf because of its complex geometry and irregular bathymetry. In the Gulf of Oman, however, such methods are useful for idealized formulations, numerical model benchmarking, and sensitivity analyses [47]. These approaches provide rapid solutions and transparent parameter relationships, but they require simplified geometries and assumptions. The analytical methods, key parameters, model types, and application areas used in hydrodynamic studies are summarized in Supplementary Table S1 [6,15,48,49,50,51].

2.8. Coupled and Application-Specific Models

In Arabian Gulf studies, coupled modeling frameworks commonly integrate hydrodynamics with water-quality modules (e.g., Delft3D-WAQ), microplastic tracking, near-field dilution models (e.g., CORMIX), and pipeline-flow models [48,52]. By contrast, Gulf of Oman studies have placed greater emphasis on physical–biological coupling and on wave–structure interaction modeling [53].

2.9. Operational Prediction Systems

Operational forecasting systems for the Arabian Gulf commonly use GULFHYDRO and Applied Environmental Management tools to simulate hydrodynamics and water circulation [42]. In the Gulf of Oman, comparable studies have applied the Gulf MetOcean Forecast System and cloud-based forecasting approaches to support regional prediction and decision-making [15].

The methodological landscape of hydrodynamic research in the two basins reflects both shared modeling foundations and region-specific priorities. Although three-dimensional numerical modeling provides the principal framework in both systems, Arabian Gulf studies place greater emphasis on coastal monitoring and environmental impact assessment, whereas Gulf of Oman studies more commonly rely on high-resolution unstructured-grid modeling and analytical benchmarking to resolve complex exchange dynamics.

Table 1. Methodological approaches applied in Arabian Gulf and Gulf of Oman studies.

Methodological Approaches	Arabian Gulf	Gulf of Oman	Methodological Benefit	References
Field-Based Observations	Coastal monitoring campaigns measuring tides, currents, temperature, salinity, and sediments	Hydrographic cruises and exchange-flow observations near the Strait of Hormuz	Delivers empirical validation and seasonal characterization	[8,41,42,51]
Remote Sensing (SST & Surface Monitoring)	Monitoring extreme summer warming and shallow-water gradients	Recognition of upwelling, frontal systems, and monsoon variability	Basin-scale surface validation and seasonal trend findings	[43]
3D Hydrodynamic Models (Structured Grid)	Delft3D-FLOW, COSMOS, GULFHYDRO for thermohaline circulation and coastal processes	MITgcm and regional-scale structured frameworks	Determines stratification, tides, and basin-scale circulation.	[15,41,44,46,53]
Unstructured Mesh/Multiscale Models	Increasing use of pollutant dispersion and the coastal infrastructure effect	Extensively applied to exchange dynamics and complex bathymetry	High spatial flexibility and fine-scale resolution	[52,54]
2D/Depth-Averaged Models	Applied for tidal-dominated studies in shallow sectors	Rarely used due to deeper bathymetry	Computational proficiency in shallow regions	[55]
Coupled Eco–Hydrodynamic Models	Water quality and contaminant fate modeling	Biophysical coupling in idealized or regional simulations	Connects circulation with ecological processes	[21,56]
Transport & Dispersion Modeling	Oil spills, radionuclides, and industrial discharge calculation	Regional tracer and exchange simulations	Environmental risk assessment	[47,52]
Analytical Approaches	Rare due to geometric complexity	Consumed for benchmarking and basic hydrodynamic circumstances	Fast computation and sensitivity insight	[53,54]
Operational Forecast Systems	GULFHYDRO and applied coastal management tools	Regional prediction and cloud-based systems	Real-time decision support	[15,42]

Generative AI Use: During the preparation of this study, ChatGPT (GPT-4o, OpenAI) was used to assist in the design and creation of Figures. The authors reviewed and validated the generated content and took full responsibility for its accuracy.

summarizes the principal methodological approaches applied across the Arabian Gulf–Gulf of Oman system.

3. Results and Discussion

3.1. Patterns of Bibliometric Analysis of the Arabian Gulf and the Gulf of Oman

The bibliometric analysis indicates a clear increase in research output on hydrodynamics and water circulation in the Arabian Gulf and the Gulf of Oman, with a marked acceleration after 2010. This trend reflects growing scientific attention to regional issues, including climate variability, coastal development, desalination, water quality, and environmental management. In the VOSviewer maps, larger nodes denote more frequently occurring keywords, stronger links indicate greater co-occurrence, and different colors represent thematic clusters.

For the Arabian Gulf (Figure 2), the main keyword clusters are centered on hydrodynamics, salinity, numerical modeling, seawater, evaporation, water quality, desalination, and coastal circulation. This pattern indicates that research on the Arabian Gulf has largely focused on hypersalinity, restricted exchange, modeling approaches, and anthropogenic pressures. By contrast, the Gulf of Oman network (Figure 3) shows stronger clustering around monsoons, sea surface temperature, ocean currents, water masses, the Strait of Hormuz, upwelling, seasonal variability, and biogeochemical processes, reflecting the stronger influence of open-ocean exchange, monsoon forcing, and water-mass connectivity in this basin.

Overall, the bibliometric results show that research on both basins has expanded, although thematic emphasis remains uneven. The Arabian Gulf literature is more strongly concentrated on salinity, pollution, desalination, and coastal management, whereas Gulf of Oman studies are more closely associated with monsoon-driven circulation, upwelling, and water-mass exchange. The analysis also indicates that integrated studies linking hydrodynamics, groundwater processes, and ecological responses remain limited. These findings highlight the need for more multidisciplinary research across the coupled Arabian Gulf–Gulf of Oman system.

The increase in publication volume reflects both the broader expansion of scientific output and a progressive shift toward applied coastal issues, including desalination impacts, pollution, modeling, and environmental management along developed Gulf coastlines. Nevertheless, the literature remains geographically uneven. Underrepresented areas include the Musandam–Hormuz transition zone, deep-water sectors of the Gulf of Oman, the Iranian side of the exchange pathway, and groundwater-influenced coastal margins. These gaps indicate that bibliometric growth has not yet translated into balanced spatial coverage across the coupled Arabian Gulf–Gulf of Oman system. Although publication output has increased, the bibliometric maps indicate that research coverage remains spatially and thematically uneven, with relatively few studies addressing deep-water circulation, the Musandam–Hormuz transition zone, groundwater-influenced coastal margins, and integrated ecohydrodynamic processes.

3.2. Physical Characteristics and Their Comparison of the Arabian Gulf and the Gulf of Oman

The physical characteristics of the Arabian Gulf and the Gulf of Oman are summarized in

Table 2. Physical characteristics of the Arabian Gulf and the Gulf of Oman.

Characteristic	Arabian Gulf	Gulf of Oman	References
Basin Type	Shallow, semi-enclosed basin	Deep, open marginal sea	[21,57]
Length	1000 km	950 km	[19,58,59]
Maximum Width	338–370 km	340 km	[58,59,60]
Average Depth	30–36 m	>1000 m (75% of basin)	[19,55]
Maximum Depth	100 m (Hormuz)	3200–3500 m	[5,21]
Surface Area	226,000–251,000 km2	about 94,000 km2	[59,61]
Evaporation Rate	1.84–2 m/year	Lower, ocean-influenced	[14,55]
Precipitation	0.07–0.1 m/year	Low but less dominant	[58,59]
River Discharge	35–133 km3/year	Limited direct input	[62]
Salinity (Typical)	38–44 psu	35–37 psu	[53,63]
Salinity (Extreme)	Up to 50–70 psu	Relatively stable	[64,65]
SST (Summer)	32–36 °C (peaks > 40 °C)	30–33 °C	[19,38]
SST (Winter)	<13 °C	22–23 °C	[46,66]
Water Exchange	Restricted (Hormuz-controlled)	Strong open-ocean exchange	[5,64]
Water Renewal Time	1.2–5 years	Shorter (well-flushed)	[55,63]
Circulation Pattern	Cyclonic, inverse estuarine	Gyres + upwelling circulation	[56,65]
Shelf Structure	Broad shallow shelves	Narrow shelf, steep slope	[67]
Topographic Controls	Limited	Ridges (e.g., Murray Ridge), eddies	[39,68]
Monsoon Influence	Indirect (via winds)	Strong (SW & NE monsoon)	[34,38]
Water-Mass Exchange	Surface inflow + deep outflow	Two-layer exchange system	[65,69]

Note: The values in Table 2 are compiled from multiple published studies and do not represent a single homogeneous observational period. The table is intended as a comparative synthesis of characteristic ranges rather than a direct time-synchronous comparison.

. The values were compiled from different studies and time periods and are used to compare characteristic physical ranges rather than exact contemporaneous conditions. The Arabian Gulf is distinguished by high evaporation rates (1.84–2.0 m year⁻¹) and very low precipitation (0.07–0.10 m year⁻¹), which promote hypersaline conditions and prolonged water-renewal times of approximately 1.2–5 years. By contrast, the Gulf of Oman is subject to lower ocean-influenced evaporation and limited precipitation, resulting in comparatively stable hydrographic conditions and shorter residence times associated with more efficient flushing. Bathymetric differences further reinforce this contrast: the Arabian Gulf is characterized by broad, shallow shelves, whereas the Gulf of Oman has a narrow shelf and a steep continental slope that enhances vertical mixing and exchange processes. Together, these differences demonstrate the combined influence of atmospheric forcing and basin morphology on regional hydrodynamics.

Hydrodynamic regimes play a central role in shaping coastal morphology and sediment budgets by regulating wave energy, tidal currents, longshore transport, and sediment resuspension [70,71]. In the Arabian Gulf, the broad, shallow shelf and restricted circulation promote local sediment retention and repeated resuspension, whereas the narrow shelf and steeper slope of the Gulf of Oman favor stronger cross-shore and along-shore sediment exchange. Recent coastal sediment studies underscore the value of integrating sediment properties, wave forcing, and shoreline modeling to evaluate coastal erosion and shoreline response under energetic conditions [72].

Although such studies are not specific to the Arabian Gulf–Gulf of Oman system, they provide a useful framework for linking hydrodynamic forcing with sediment transport and shoreline stability.

Riverine inputs exert a significant influence on sediment distribution and nearshore morphology, particularly in estuarine and deltaic environments [73]. In the Arabian Gulf, the Shatt Al-Arab system supplies localized freshwater and sediment inputs that modify salinity, turbidity, and depositional patterns in the northern Gulf [74]. Estuarine investigations indicate that river discharge can dominate sediment transport over tidal forcing in some river-mouth systems, underscoring the need to consider river flow, tidal currents, and sediment load collectively when assessing coastal sedimentation processes. This integrated perspective is particularly relevant for understanding sediment retention, resuspension, and pollutant binding in shallow gulf environments [75].

3.3. Strait of Hormuz and System Connectivity (Exchange Processes)

The Strait of Hormuz serves as the principal gateway linking the Arabian Gulf and the Gulf of Oman. It maintains a predominantly inverse estuarine circulation, in which comparatively fresher surface waters enter the Arabian Gulf, whereas dense, hypersaline waters exit at depth [6]. Although the strait is often described as a two-layer exchange system, observations indicate a more complex vertical structure, including subsurface intrusions and temperature-inverted layers [54]. Evidence for three-layer behavior and double-diffusive processes in the Strait of Hormuz remains limited and is mainly based on selected observational and theoretical studies. Therefore, these processes are interpreted here as localized or episodic features superimposed on the dominant two-layer exchange, rather than as a persistent basin-wide circulation mode. The dominant exchange remains the surface inflow of relatively fresher water and the deep outflow of dense hypersaline gulf water. The connectivity of the Strait of Hormuz with the two basins is illustrated in Figure 4.

Exchange through the Strait of Hormuz is further influenced by the complex topography of the Musandam Peninsula. This topographic setting forms a natural boundary between the two basins, where steep bathymetry and an irregular coastline enhance tidal flow, local mixing, and internal wave generation. Consequently, bathymetric control plays an important role in modulating water-mass exchange and stratification within the strait [11,67].

Hypersaline water formed in the Arabian Gulf (39–41 psu) exits through the Strait of Hormuz and forms a distinct subsurface salinity maximum in the Gulf of Oman at depths of approximately 150–300 m, thereby influencing the regional thermohaline structure [24,76]. Tidal interaction with steep bathymetry further generates internal waves that enhance tracer dispersion and vertical mixing [77].

The variability and strength of exchange are primarily controlled by evaporation-driven density gradients, with additional modulation by winds, tides, and seasonal forcing [78]. Estimates of deep outflow range from 0.1 to 0.28 Sverdrup (Sv), whereas Arabian Gulf residence times range from 1.2 to 5 years, indicating limited flushing and persistent vulnerability to pollutant accumulation [21,79].

It is essential to distinguish between model-derived exchange estimates and direct observational evidence. Most reported transport rates through the Strait of Hormuz are derived from numerical simulations or short-term field campaigns rather than sustained, full-depth measurements across the strait [80,81]. Existing in situ observations remain limited in spatial coverage, deployment duration, and vertical resolution, constraining the validation of modeled inflow–outflow rates at seasonal-to-interannual timescales [82]. Reducing these uncertainties requires sustained observational programs, including long-term moored ADCP transects, bottom-pressure recorders, repeated CTD sections, thermistor chains, autonomous gliders, and moored temperature–salinity–oxygen sensors across the Strait of Hormuz [82,83]. The core hydrodynamic exchange features and their implications for circulation, salinity gradients, and tracer transport are summarized in Supplementary Table S2 [7,38,39].

The transfer of dense Arabian Gulf water into the Gulf of Oman also influences regional biogeochemical processes and oxygen distribution, underscoring the central role of the Strait of Hormuz in cross-basin connectivity [10]. The main characteristics of this exchange are summarized in

Table 3. Main characteristics of exchange via the Strait of Hormuz between the Arabian Gulf and the Gulf of Oman.

Feature	Typical Description/Range Reported	References
Dominant exchange structure	Reverse-estuarine exchange with surface inflow and deep outflow; two-layer flow is broadly dominant	[6,84]
Evidence of added complexity	Temperature inversions and subsurface intrusions signifying intermittent three-layer behavior; double diffusion processes (salt fingering/layering)	[7]
Deep outflow properties	Dense gulf water; salinity frequently around 39–41 psu	[10,78]
Depth of gulf water signature in the Gulf of Oman	Subsurface spreading is often observed around 150–300 m, with salinity maxima near 250 m	[63,85]
Internal wave generation	Internal waves at density interfaces; possibly linked to tidal flow over shelf-edge/topography	[7]
Main drivers	Evaporation-driven density gradients as the main control; modulation by winds/tides	[6]
Seasonal variability	Exchange strength and water-mass intrusion vary seasonally; IOSW intrusion is stronger in early summer	[80]
Outflow transport magnitude (Sv)	Deep outflow is often estimated at 0.1–0.28 Sv; high speeds are 0.2 m/s	[81,82]
Annual volume transport (km3 yr−1)	Inflow 2696–7250 km3/yr; outflow 2375–6620 km3/yr (method-dependent)	[10,79]
Residence/flushing time	1.2–5 years, depending on method/period; some field-based estimates 350–500 days	[40,79]
Environmental implications	Restricted exchange and multi-year residence increase sensitivity to pollutant retention; export of dense water affects the Gulf of Oman subsurface conditions and possibly oxygen/biogeochemistry	[6,23]
Climate-scale modulation	Possible effect of climate modes (e.g., IOD/ENSO) on variability through winds/upper-ocean forcing	[83]

. Overall, the Strait of Hormuz functions as a critical control point that regulates water exchange and facilitates the transfer of biogeochemical properties, including salt and heat, between the Arabian Gulf and the Gulf of Oman.

3.4. Density-Driven Circulation

Thermohaline, or density-driven, processes are central to circulation in both the Arabian Gulf and the Gulf of Oman, although they operate under distinct physical controls. In the Arabian Gulf, intense evaporation and restricted exchange promote an inverse estuarine circulation, whereas in the Gulf of Oman, density structure is shaped by monsoon forcing, interactions with Indian Ocean waters, and inflow of dense Arabian Gulf water [6,86].

Hydrography in the Arabian Gulf is governed by a strongly negative water balance. Evaporation, estimated at approximately 1.4–5 m yr⁻¹, exceeds freshwater input and produces persistent hypersalinity, commonly greater than 39 psu and locally exceeding 70 psu [6,39,87]. This density contrast drives reverse-estuarine circulation, characterized by surface inflow from the Gulf of Oman and dense bottom outflow through the Strait of Hormuz [7,44].

The circulation structure is characterized by a two-layer exchange, with dense outflow typically occurring below approximately 40 m and velocities approaching 0.2 m s⁻¹ [6,88]. Residence times exceeding 1 year increase the vulnerability of the system to pollutant accumulation [89]. Although density forcing is dominant, circulation is further modulated by tides, Shamal winds, and seasonal variability, which influence exchange intensity and generate transient flow features [78,79]. Riverine input contributes locally to buoyancy and exchange processes, but its influence remains minor compared with evaporation-driven forcing [15].

The outflow of Arabian Gulf water, together with Indian Ocean circulation, influences the deep basin of the Gulf of Oman, where dense outflow forms a distinct subsurface layer at depths of approximately 100–300 m. This layer serves as an important pathway for salt and associated biogeochemical signals [5,10]. Circulation patterns in the Gulf of Oman are shaped not only by density-driven processes but also by interacting gyres, mesoscale variability, and wind forcing [59]. Fine-scale thermohaline mechanisms, including double-diffusive mixing such as salt fingering and isopycnal spreading of dense water, further modify the exchange process [7]. Seasonal variability is evident, with surface temperatures ranging from approximately 22.6 °C in winter to more than 31 °C in summer. Under climate change, projected thermocline intensification may reduce deep-water oxygen levels, linking changes in density structure to ecological stress [10].

The thermohaline framework is commonly quantified using potential temperature (θ) and potential density, which are derived from in situ temperature (T), salinity (S), and pressure (p) following TEOS-10 formulations [88,90]. The following equations characterize thermohaline structure and density.

i.

Potential Temperature

$$\theta =f(T,S,p)$$

(1)

ii.

Potential Density Anomaly

$${\sigma }_0=\rho (\theta ,S,p=0)-1000\left({\mathrm{kg} \mathrm{m}}^{-3}\right)$$

(2)

iii.

Deep Reference Density

$${\sigma }_{2000}=\rho (\theta ,S,p=2000)-1000$$

(3)

The seawater density is calculated by the thermodynamic equation of state:

$$\rho =\rho (S,T,p)$$

(4)

TEOS-10, which is based on the Gibbs function, provides a thermodynamically consistent framework, whereas EOS-80 is retained in some applications for comparison [23,90]. These parameters define the density gradients that control water-mass formation and thermohaline circulation pathways in the Arabian Gulf, particularly under strong evaporation-driven salinity contrasts [88].

3.5. Wind-Driven Circulation

Wind forcing governs surface circulation, vertical mixing, and short-term variability in both basins. In the Arabian Gulf, wind stress interacts with tides and density gradients to shape surface currents and dispersion patterns. In the Gulf of Oman, by contrast, wind forcing is strongly influenced by monsoon-driven circulation, generating greater seasonal variability and more complex dynamical responses [6,91].

Shamal winds are recurrent northwesterly winds that intensify over the Arabian Gulf during winter. They exert strong control on regional circulation, with wind speeds reaching approximately 16 m s⁻¹ and promoting rapid surface cooling of up to 10 °C. These events also enhance convective mixing, deepen the mixed layer to approximately 30 m, and temporarily weaken stratification [15,75].

These winds also intensify surface circulation and contribute to the development of the Iranian Coastal Current (ICC), southward transport, eddy formation, and coastal connectivity [44]. In addition, Shamal-driven currents interact with density-driven circulation to produce layered flow patterns, in which surface transport is primarily wind-dominated, whereas deeper flow remains regulated by thermohaline processes [6].

In the Gulf of Oman, circulation is strongly influenced by the seasonally reversing Indian monsoon system, which regulates basin-scale mixing and circulation [38]. During the southwest monsoon, wind speeds reach approximately 15–20 m s⁻¹, generating coastal upwelling along the Oman margin that brings nutrient-rich waters to the surface and enhances productivity. These upwelled waters can be advected hundreds of kilometers offshore, extending the influence of upwelling beyond the coastal zone [14,92]. During the northeast monsoon, upwelling weakens, and convective mixing becomes more important, particularly in the northern Arabian Sea [93]. Thus, the Gulf of Oman functions as a transitional system shaped by both regional winds and large-scale monsoon forcing, with these patterns further superimposed on mesoscale eddies that promote strong mixing and lateral transport [32].

Wind forcing modulates exchange through the Strait of Hormuz at the system scale, whereas the background inflow–outflow structure is primarily governed by density-driven circulation. Short-term variability is regulated by wind-driven changes in mixing intensity and surface transport pathways [11,44]. Thus, wind acts as a dynamic regulator of connectivity between the Arabian Gulf and the Gulf of Oman, influencing short-term exchange variability while operating alongside density-driven processes that control longer-term circulation. A comparative analysis of wind-driven processes is presented in

Table 4. Wind-driven circulation features in the Arabian Gulf and the Gulf of Oman system.

Region	Main Wind System	Important Wind-Driven Circulation Features	Seasonal Properties and Biophysical Consequences	References
Arabian Gulf	Shamal (NW winds)	Strong wind-stress control on surface currents; ICC development; wind-driven eddies; enhanced exchange variability near Hormuz	Strongest in winter; SST cooling up to around 10 °C; mixed-layer deepening up to around 30 m; stratification erosion and enhanced dispersion	[11,44,64,72]
Gulf of Oman	Monsoon influence + local winds	Monsoon-linked current variability; interaction with Gulf outflow; seasonal gyre/eddy activity and coastal flow changes	Strong seasonal restructuring; transition-season intensification; eddy-driven mixing influences nutrient transport and productivity	[38,94,95,96]

.

Wind-driven circulation is a major source of variability across the coupled two-basin system. Episodic Shamal winds in the Arabian Gulf intensify surface currents, enhance vertical mixing, and generate short-term variability in transport and stratification [44,64]. By contrast, the monsoon-dominated Gulf of Oman is characterized by seasonal wind reversals that drive upwelling, large-scale circulation, and productivity linked to the Arabian Sea [38,92,97]. Together with density-driven exchange through the Strait of Hormuz, these wind-driven processes regulate basin connectivity and the transfer of heat, salt, and suspended or dissolved materials [6,98].

3.6. Ecological Consequences of Wind-Driven Circulation

In the Arabian Gulf and the Gulf of Oman, wind-driven circulation plays a critical role in regulating ecosystem functioning by controlling vertical mixing, nutrient transport, and stratification dynamics [99,100].

In the Arabian Gulf, Shamal winds generate intense vertical mixing, erode stratification, and deepen the mixed layer [48]. These processes enhance nutrient availability in the euphotic zone and can temporarily increase primary productivity. Because of the basin’s shallow depth and relatively long residence time, wind-driven dispersion interacts with density-driven retention, producing marked seasonal variability in circulation and ecosystem responses [101]. Figure 5 illustrates the Shamal wind pattern over the Arabian Gulf.

Winter mixing improves bottom-water ventilation and reduces the risk of hypoxia; summer stratification may reduce vertical exchange and increase susceptibility to oxygen depletion, especially in semi-enclosed basins [11]. Through the Strait of Hormuz, the wind-modulated further enhances the nutrient transfer by controlling the inflow from the Omani Sea [48].

The Gulf of Oman’s ecosystem depends on monsoon-driven circulation. The nutrient-rich water introduced to the surface via the upwelling along the Omani coastline results in high productivity and phytoplankton bloom [97]. Through eddy transport and advection, these impacts extend offshore and cause spatial changes in biomass [102]. However, ecological productivity decreases during the northeast monsoon (i.e., weak upwelling), yet due to convective mixing, nutrient supply remains sustained [92]. The strong stratification and thermocline advance also affect oxygen supply, and reduced ventilation may raise hypoxia risk in warming settings [63].

Together, wind-driven and density-driven processes regulate the nutrient cycling, ecosystem variability, and ecological productivity in the two basins. The wind modulates retention and flushing in the Arabian Gulf, and, through upwelling, nutrient supply increases in the Omani Sea [5,6]. These exchanges regulate key ecological processes, including nutrient availability, chlorophyll dynamics, and oxygen dynamics. This underscores strong coupling among hydrodynamics and ecosystem functioning.

The wind-driven circulation is defined by momentum and continuity equations under hydrostatic and Boussinesq assumptions in 3D hydrodynamic models (e.g., SCHISM, HYCOM) [6,26], or reduced-gravity formulations for large-scale monsoon-driven systems [99]. The following are some important formulas:

i.

Wind Stress

$$\tau ={\rho }_{air} C_d U_{10}^2$$

(5)

where $\tau$ = wind stress (N m⁻²),${ \rho }_{air}$ = air density, $C_d$ = drag coefficient, and $U_{10}$ = wind speed at 10 m [103].

ii.

Wind Stress Curl (Gyre/Upwelling Forcing)

$$\nabla \times \tau ={\displaystyle \frac{\partial {\tau }_y}{\partial x}}-{\displaystyle \frac{\partial {\tau }_x}{\partial y}}$$

(6)

The controls the Ekman pumping, upwelling, and large-scale circulation forms [99,103].

iii.

Momentum Equations-Simplified Form

$${\displaystyle \frac{\partial \mathrm{u}}{\partial t}}+(\mathrm{u}·\nabla )\mathrm{u}=-{\displaystyle \frac{1}{\rho }}\nabla p+\nu {\nabla }^2\mathrm{u}+{\mathrm{F}}_{wind}$$

(7)

where $\mathrm{u}$ = velocity, $\nu$ = eddy viscosity, and ${\mathrm{F}}_{wind}$ = wind forcing term [48].

iv.

Parameterization-Regional Models

$$\nu \approx 5 {\mathrm{m}}^2{\mathrm{s}}^{-1},n\approx 0.03$$

The eddy viscosity and bottom friction coefficients are used in simplified 2D models [104].

v.

Wave–Wind Interaction

$$F_{bw}=C_b(U_{10}-U_0)T_p$$

(8)

This represents wave-induced mixing (Arabian Gulf settings) [105]. The wind fields (e.g., NOAA, satellite) are interpolated to model grids to simulate circulation, eddies, and coastal currents [106,107].

3.7. Tidal Changing Aspects and Mixing

Tides influence the system through two distinct mechanisms: tidal transport and tidal mixing [108]. Tidal transport refers to the horizontal movement of water masses by oscillatory currents, particularly through constricted areas such as the Strait of Hormuz [109]. Tidal mixing refers to turbulence generated by tidal shear and bottom friction, which enhances vertical exchange and can reduce stratification. In the Arabian Gulf, tidal mixing is most important in shallow and constricted areas, including the head of the gulf, near the Strait of Hormuz, island-fringed channels, and broad, shallow shelf regions [110]. In these areas, tidal energy can penetrate much of the water column and limit summer stratification. However, in deeper or more strongly heated parts of the basin, stratification can still persist despite tidal forcing [83].

In both basins, tides are the primary forcing that affects horizontal transport, vertical mixing, and stratification. The two basins differ in bathymetry and basin geometry, although both exhibit mixed tides, with distinct amplitudes and hydrodynamic impacts. In the Arabian Gulf, semidiurnal constituents including the principal lunar semidiurnal tide (M2), the principal solar semidiurnal tide (S2), and diurnal components such as the luni-solar diurnal tide (K1), and the principal lunar diurnal tide (O1) are dominant [32]. The basin’s geometry causes resonance and generates different amphidromic systems, including semidiurnal nodes at the basin’s ends and a diurnal node near Bahrain. The ranges of tides are usually modest with spatially concentrated influence, almost 1 m, with currents (up to around 0.5 m s⁻¹) mostly closer to the head of the Arabian Gulf and to the Strait of Hormuz [33]. In contrast, the Gulf of Oman experiences largely semidiurnal tides that co-oscillate with those of the Arabian Sea, with amplitudes exceeding 2.5 m, indicating strong open-ocean connectivity [10]. In the Arabian Sea, most tides are mixed and governed by the lunar elliptic semidiurnal tide (N2), K1, O1, M2, and S2, with spatial changes that rise northward [36]. The Gulf of Oman is largely influenced by the larger-scale tidal system and, through the Strait of Hormuz, facilitates the exchange of variability. Additional information on dominant tidal constituents, approximate tidal ranges, and their role in vertical mixing is provided in Supplementary Tables S3 and S4 [6,10,32,33,34,35,36,37,111,112].

Spatial Tidal Variability and Mixing Role

In the Arabian Gulf, tidal energy is strongly regulated by basin geometry, leading to spatial variability in diurnal and semidiurnal regimes [32]. The shallow depth enables the tidal forcing to effectively combine the entire water column, despite the smaller amplitudes. This reduces stratification and redistributes sediments and nutrients [6]. The Gulf of Oman has stronger tidal energy due to its open-ocean coupling and depth, where vertical mixing is driven by mesoscale processes, tides, and density gradients. Internal tides (particularly M2) play an important role in turbulent mixing and thermocline changes [34,35].

The contrast in tidal characteristics reveals clear regional differences in tidal energy and mixing processes. Figure 6 illustrates a comparison of tidal ranges across the Arabian Gulf, the Gulf of Oman, and the Arabian Sea. The Omani Sea had the highest tidal range, exceeding 2.5 m. This indicates stronger tidal forcing and greater vertical mixing, linked to mesoscale processes and internal waves. In comparison, the Arabian Gulf has reduced tidal ranges of about 1 m. However, the strong currents and shallow depths induce effective mixing and decreased stratification. The Arabian Sea exhibits moderate tidal variability, while internal tides play an important role in controlling thermocline structure and nutrient circulation.

Overall, tidal processes, with their influence amplified in the shallow waters of the Arabian Gulf, dynamically propagate into the deeper Gulf of Oman.

The relative contribution of different driving forces was distinguished through a seasonal synthesis of the reviewed literature. Density forcing, wind forcing, tides, exchange through the Strait of Hormuz, and mesoscale variability were ranked according to their reported dominance in different seasons and basins. This synthesis is presented in Table 5 and Figure 6.

Table 5 demonstrates the seasonal hierarchy of forcing mechanisms in the Arabian Gulf and the Gulf of Oman [110]. The Arabian Gulf is mainly controlled by evaporation-driven density gradients, with Shamal winds and tides becoming especially important during winter and in shallow or constricted areas [83]. In contrast, the Gulf of Oman is more strongly influenced by monsoon forcing, particularly during the southwest monsoon, when upwelling and mesoscale activity intensify [113]. This hierarchy provides the physical context for the mesoscale and sub-mesoscale processes discussed in the following section.

Table 5. Seasonal hierarchy of dominant hydrodynamic forcing mechanisms in the Arabian Gulf and the Gulf of Oman.

Season/Period	Arabian Gulf: Dominant Forcing	Gulf of Oman: Dominant Forcing	Main Hydrodynamic Response	References
Winter	Shamal winds, density gradients, tides	Northeast monsoon, cooling, convective mixing	Enhanced mixing, reduced stratification, stronger ventilation	[10,48,99,114]
Spring transition	Density gradients, tides, changing winds	Transition winds, eddies, gyre adjustment	Reorganization of circulation and exchange pathways	[26,115]
Summer	Evaporation, salinity gradients, stratification	Southwest monsoon, upwelling, mesoscale eddies	Strong gulf stratification; upwelling and offshore transport in Gulf of Oman	[15,38,100,103]
Autumn transition	Wind variability, tides, weakening stratification	Weakening monsoon, eddies, water-mass adjustment	Increased variability in mixing and circulation	[115,116]
Year-round background	Inverse estuarine exchange through Hormuz	Open-ocean exchange and Persian Gulf Water intrusion	Cross-basin heat, salt, and tracer transport	[5,6,7,89]

Table 5. Seasonal hierarchy of dominant hydrodynamic forcing mechanisms in the Arabian Gulf and the Gulf of Oman.

Season/Period	Arabian Gulf: Dominant Forcing	Gulf of Oman: Dominant Forcing	Main Hydrodynamic Response	References
Winter	Shamal winds, density gradients, tides	Northeast monsoon, cooling, convective mixing	Enhanced mixing, reduced stratification, stronger ventilation	[10,48,99,114]
Spring transition	Density gradients, tides, changing winds	Transition winds, eddies, gyre adjustment	Reorganization of circulation and exchange pathways	[26,115]
Summer	Evaporation, salinity gradients, stratification	Southwest monsoon, upwelling, mesoscale eddies	Strong gulf stratification; upwelling and offshore transport in Gulf of Oman	[15,38,100,103]
Autumn transition	Wind variability, tides, weakening stratification	Weakening monsoon, eddies, water-mass adjustment	Increased variability in mixing and circulation	[115,116]
Year-round background	Inverse estuarine exchange through Hormuz	Open-ocean exchange and Persian Gulf Water intrusion	Cross-basin heat, salt, and tracer transport	[5,6,7,89]

To clarify the relative dominance of hydrodynamic forcing mechanisms, Figure 7 reviews the seasonal hierarchy of physical drivers in the Arabian Gulf and the Gulf of Oman. The ranking is based on the synthesized literature reviewed in this study (based on Table 5), where lower rank values indicate stronger seasonal dominance. The Arabian Gulf is primarily controlled by evaporation-driven density gradients throughout the year, with Shamal winds and tidal mixing becoming important during winter and in shallow or constricted areas. In contrast, the Gulf of Oman is more strongly influenced by monsoon forcing, particularly during the southwest monsoon, when upwelling and mesoscale variability intensify.

The dominant tidal constituents comprise semidiurnal $M_2,{ S}_2$ and diurnal $K_1,O_1$, with $M_2$ typically being dominant. Amphidromic systems and basin resonance regulate spatial changes in amplitude and phase [32,51,117].

Representative conditions [33,118]:

$$\begin{gathered} \mathrm{Tidal} \mathrm{range}\approx 1 \mathrm{m} \left(\mathrm{Gulf}\right),>2.5 \mathrm{m} \left(\mathrm{Oman}\right) \\ {U}_t\approx 0.5{ \mathrm{m} \mathrm{s}}^{-1},>1{ \mathrm{m} \mathrm{s}}^{-1} \mathrm{near} \mathrm{Hormuz} \end{gathered}$$

Governing Equations

i.

Tidal circulation is modeled using continuity and momentum equations [43,108]:

$${\displaystyle \frac{\partial \eta }{\partial t}}+\nabla ·(H\mathrm{u})=0$$

(9)

$${\displaystyle \frac{\partial \mathrm{u}}{\partial t}}=-g\nabla \eta +\nu {\nabla }^2\mathrm{u}-{\displaystyle \frac{C_d\mid \mathrm{u}\mid \mathrm{u}}{H}}$$

(10)

where $\eta$ = surface elevation, $H$ = water depth, $\mathrm{u}$ = velocity, $g$ = gravity, and $\nu$ = eddy viscosity.

ii.

Tidal Mixing Parameterizations

a.

Mixing Parameter

$$\chi ={\displaystyle \frac{H}{U_t}}$$

(11)

Higher $\chi$ → stronger mixing in shallow regions [119].

b.

Tidal Mixing Efficiency

$$M={\displaystyle \frac{C_d{U}_T^2}{\omega N_0^{2}S}}$$

(12)

where $C_d$ = drag coefficient, $U_T$ = tidal velocity amplitude, $\omega$ = tidal frequency, and $N_0$ = buoyancy frequency [120].

c.

LSJ02 Parameterization [121]

$$N^2=-{\displaystyle \frac{g}{\rho }}{\displaystyle \frac{\partial \rho }{\partial z}}$$

(13)

$$k_0={10}^{-5}{ \mathrm{m}}^2{\mathrm{s}}^{-1}, \mathsf{\Gamma }=0.2,q=0.3$$

Overall, tidal forcing, shared with wind and density gradients, regulates mixing, stratification, and tracer transport across the Arabian Gulf and the Gulf of Oman system [33].

3.8. Mesoscale and Sub-Mesoscale Eddies and Frontal Systems

The transition periods between the southwest and northeast monsoons are particularly important for eddy evolution in the Gulf of Oman [122]. During these intervals, weakening or reversal of wind stress alters coastal currents, shear zones, and water-mass advection pathways. These transitional instabilities can promote eddy formation, while reduced wind forcing may allow existing eddies to persist and interact with Persian Gulf water intrusions. As a result, transition seasons may influence eddy genesis, longevity, and stability, as well as the redistribution of heat, salt, and nutrients [113,123].

One of the dominant features of the Arabian Gulf and the Gulf of Oman is the presence of mesoscale eddies. Mesoscale eddies typically range from 10 to 100 km in diameter and persist for days to months. Baroclinic instability of coastal currents produces these eddies. During the southwest monsoon, they become highly energetic, specifically along the boundary of the Gulf of Oman. Along the margin of the Omani Sea, eddy kinetic energy peaks and connects to coastal jets, frontal systems, and upwelling filaments [69,115,124,125,126].

The cyclonic eddies increase nutrient enrichment and upwelling, while the anticyclonic eddies enhance downwelling and stratification. This pattern causes spatial changes in biogeochemical and hydrographic conditions [127]. Moreover, lateral transport of water masses is facilitated by the eddies (i.e., the Arabian Gulf water affects the upper-ocean heat supply) [128]. Sub-mesoscale and frontal system processes are directly coupled to eddy dynamics, intensifying mixing at eddy boundaries and in the vertical direction [129]. Furthermore, monsoon-driven upwelling delivers nutrient-rich waters that are conveyed offshore via filaments and eddy advection along the coast of the Gulf of Oman [130]. Also, nearshore processes and internal waves further improve vertical mixing and cross-shelf exchange, affecting nutrient distribution and oxygen supply [131,132].

Overall, the above-mentioned processes highlight the variability in stratification and water-mass distribution across the region [129]. The mesoscale features of the region are summarized in

Table 6. The main mesoscale and sub-mesoscale processes that form the Gulf of Oman–Arabian Sea margin.

Process/ Characteristics	Chief Region(s)	Usual Seasonality	Core Physical Outcome	Important Ecological/Biogeochemical Impacts	References
Mesoscale eddies (cyclonic/anticyclonic)	Arabian Sea, Gulf of Oman	Year-round; enhanced during SWM	Upwelling/downwelling, relocation of heat/salt, modifies stratification and mixing	Patchy productivity; cyclonic eddies enhance upwelling, nutrient supply, and chlorophyll concentrations, whereas anticyclonic eddies promote downwelling and stratification and are generally associated with lower nutrient availability and chlorophyll concentrations.	[69,114,115,124,125,126,127]
Eddy kinetic energy hotspot	Omani coast/western margin	Peak during SWM	Intensified variability in SSH and currents near the coast	Supports offshore export of coastal waters via eddy-filament interactions	[124,125,126]
Fronts at eddy margins + frontal instability	Omani margin; eddy perimeters	Episodic; strongest when gradients sharpen	Strong vertical motion: fronts can spawn eddies and sub-mesoscale action	Improves vertical exchange, nutrient injections, and biological patchiness	[129]
Sub-mesoscale processes (0.1–50 km)	Gulf of Oman; frontal/eddy zones	Episodic; coupled to mesoscale activity	Anomalous vertical heat transport; enhanced vertical mixing	Supports clarifying short-term bloom variability and strong small-scale gradients	[11,33,40,100,115]
Coastal upwelling and Omani coastal current	Omani coast	Strongest in SWM; reversals/changes in NEM	Upwelling, frontal formation (incl. Ras Al Hadd); filament export offshore	Nutrient enrichment, strong chlorophyll variability, and blooms	[100,112,115,116]
Filaments/jets exporting upwelled water	From the Omani margin to offshore waters	SWM	Lateral export of cool, nutrient-rich water >100 s km offshore	Extends productivity influences far from the coastline	[7,105,112]
Internal wave/nearshore mixing linked to jets and eddies	Coastal Gulf of Oman	Year-round; modulated by eddies/fronts	Strengthens nearshore mixing and cross-shelf exchange	Effects oxygen/nutrient redistribution and subsurface variability	[35,109,111,113,114]
Water-mass transport (PGW, salinity maxima waters)	Gulf of Oman and the adjacent Arabian Sea	Strongly controlled by mesoscale activity	Lateral advection of salinity maxima alters heat content and fluxes	Changes biogeochemical gradients and habitat settings	[105,114]

. These multi-scale exchanges emphasize the role of mesoscale variability in linking the large-scale circulation to local mixing processes and the region’s hydrographic structure.

3.9. Patterns of Seasonal Circulation in the Arabian Gulf and the Gulf of Oman

The seasonal circulation in the northern Arabian Sea is driven by monsoon-driven wind reversals; these wind systems enter the Gulf of Oman and influence exchange processes toward the Arabian Gulf [133]. Furthermore, the Arabian Sea shows some of the strongest seasonal circulation variability worldwide. The Somali current is a classic example of a current that reverses direction semi-annually in response to monsoon winds. In the summer season in the southwest, the Findlater jet becomes stronger, causing strong upwelling and driving intense surface currents, while in the winter season, the Somali current flows southward, and the circulation becomes shallower and shifts westward north of the equator [133,134,135]. This large-scale monsoon system directly affects the Gulf of Oman and, secondarily, influences exchanges with the Arabian Gulf.

Circulation in the Gulf of Oman responds strongly to both the Indian Ocean monsoon forcing and the Arabian Gulf water. As previously mentioned, during the southwest monsoon (summer), upwelling becomes strong along the coasts of Oman and Somalia. The offshore Ekman transport brings cold, nutrient-rich waters to the surface. This increases the vertical transport and offshore advection by eddies. Moreover, the upwelled waters may extend 600–700 km offshore, significantly affecting hydrography and biological productivity [99,100]. Also, the surface mixed-stratification layer deepens in regions of strong wind forcing and upwelling, occasionally in the Arabian Sea, reaching depths of around 100 m [100,133]. Nonetheless, in the Gulf of Oman, localized conditions can generate a comparatively thin surface mixed layer in summer, particularly in areas subject to wind forcing and spatial heterogeneity [10].

During the northeast monsoon, especially in the northern region, northeasterly winds weaken upwelling, and surface cooling induces convective overturning [101]. However, productivity may increase in the northern regions due to greater vertical mixing. The mesoscale eddies and gyre assemblies also organize transport and mixing across the Gulf of Oman [115]. The winter sea temperature in the Gulf of Oman decreases to around 23.8 °C, and the mixed layer thickens compared to summer conditions [10]. The circulation patterns often shift toward anticyclonic systems, and mesoscale inconsistencies become more pronounced.

The Arabian Gulf is strongly influenced by seasonal wind changes (Shamal winds) and the region’s atmospheric forces, as mentioned earlier. In spring and summer, cyclonic overturning circulation becomes more common, and, due to high surface heating, stratification strengthens. Also, in the western–southern Gulf, sea temperatures reached 33–34 °C, while inflows from the Gulf of Oman moderate temperatures in the eastern–southern part [14,116]. Moreover, the higher baroclinic exchange through the Strait of Hormuz causes a coherent cyclonic structure through the basin. In the autumn and winter seasons, surface cooling reduces stratification, wind forcing becomes stronger, turbulent circulation becomes dominant, and mesoscale eddies increase. These conditions suppress the Iranian coastal jet. In the northern regions, the winter sea surface temperature can fall below 20 °C. The stratification breakdown enables lateral stirring and mixing [14,118].

Stratification and Interannual Variability

In both basins, stratification is high in spring and summer and low in autumn and winter. The interannual variability remains secondary to the main seasonal cycle. In the Gulf of Oman, the emerging warming period suggests that future intensification of stratification may modify oxygen circulation and ecosystem stability. As previously discussed, seasonal stratification trends are totally associated with the monsoon-driven forcing [136,137].

Climate models estimated that in the Gulf of Oman, further strengthening of summer thermoclines will likely decrease dissolved oxygen in subsurface waters [10]. The oxygen minimum zone (OMZ) in the Arabian Sea is maintained by vertical mixing processes and circulation reversals [136]. In the Arabian Gulf, in the late summer, the sea surface temperature (SST) reached a maximum of nearly 33 to 34 °C, and in the late winter, the SST minimum reached around 17 to 20 °C, while cyclonic circulation supports stratification in the spring–summer season and eddy in the winter season, as the Arabian Gulf basin is shallow, as explained earlier [116,118]. Overall, the Gulf of Oman shows more continuous stratification due to its depth and monsoon effects, whereas the shallow Arabian Gulf experiences rapid mixing.

Interannual variability occurs but is naturally smaller than seasonal variability. The SST in the Arabian Sea exhibits variability of almost 0.5 °C, while long-term warming trends are more pronounced [24,52]. In the Gulf of Oman, SST increased by 0.58 °C per decade between 1992 and 2009 [10]. The main factors of interannual variability are fluctuations in monsoon strength, ENSO events, and phases of the Indian Ocean Dipole [115,133]. These climatic influences are smaller than seasonal variability but are influenced by large-scale climate drivers. A comparative summary of seasonal circulation and stratification patterns in the Arabian Gulf, Gulf of Oman, and Arabian Sea is provided in Supplementary Table S5 [10,14,116,118,133,134,136].

3.10. Seasonal and Interannual Variability in Temperature, Salinity, and Hydrographic Structure

The interannual and seasonal variability in salinity and temperature showed distinct differences between the Arabian Gulf and the Gulf of Oman, in response to atmospheric forcing and oceanic exchanges. The SST and salinity comparisons shown in Figure 8 and Figure 9 are based on the synthesized values provided in Supplementary Table S6 [10,25,26,27,28,31,45,114,138,139,140].

3.10.1. Temperature Variability

The Arabian Gulf sustained an intense seasonal SST variability of around 5 to 8 °C, and almost 20 to 32 °C temperature ranges offshore, typically, while in the shallow coastal regions, in extreme conditions, the values vary from around 15 to 40 °C [25,45]. Regional atmospheric changes and exchanges through the Strait of Hormuz reflect the northwest–southeast temperature gradient. For example, Kuwait waters have a mean yearly seawater temperature of around 23.8 °C, reaching a maximum in July to August and attaining a minimum in January to February [45].

Moreover, the longer-term warming trends are apparent, with a rise of almost 0.08 °C yr⁻¹ in the northern region, accompanied by more frequent temperature extremes, resulting in increased thermal stress on oceanic systems [45]. Also, the temperature “tails” are becoming more noticeable, with the hot extremes becoming more frequent and increasing by about 95%. Likewise, the cold extremities show intensity across various assessments, with reductions of up to 5%, indicating a high risk of thermal stress in shallow ecosystems [27,45].

The Omani Sea also exhibits warming; however, the basin seasonal trend is moderated by depth, open-ocean exchange, and the regional monsoon cycle, as explained previously. The average annual SST is around 27.5 °C, with monthly mean temperatures ranging from 22.6 °C in February to 31.4 °C in July [10].

The Gulf of Oman seasonal SST amplitude decreases spatially from around 5 °C near the Strait of Hormuz to less than 1 °C toward the southeastern margin of the sea, indicating a shift from subtropical to more tropical conditions and the influence of deeper water interactions with the Arabian Sea/Indian Ocean [10].

As explained earlier, the Arabian Sea also influences these dynamics, with monsoon-driven upwelling decreasing summer SST and increasing spatial unpredictability. The seasonal SST variations can increase up to 6 °C, while the interannual variations are relatively lower, almost 0.5 °C [52]. In the summer, SST is colder basin-wide, typically around 3–5 °C, while on the western side, the strongest cooling occurs, likely due to Ekman-driven upwelling along the margins of the Gulf of Oman. In contrast, in the winter season, the sea surface temperature is typically warm, around 1 to 2 °C, with the highest temperatures mostly reported in the eastern Arabian Sea [52,53].

Based on the existing literature, the Arabian Gulf shows the highest temperature variability of 20–32 °C (min–max) (Figure 8), revealing shallow depth and strong evaporation, while the Gulf of Oman (22.6–31.4 °C (min–max) and Arabian Sea show moderated variability due to open-ocean exchange and monsoonal forcing.

In the recent decade, the Gulf of Oman has also shown substantial warming, with surface mixed-layer temperatures rising to around 1.5 °C (over the past 50 years), driven by weakened large-scale atmospheric forcing, including a weakening Siberian high and decreased zonal wind velocity. Moreover, in various climate scenarios, it is predicted that warming will increase further throughout the upper ocean up to 1000 m, with winter increases of +1.2, +1.6, and +2.0 °C and summer surface mixed-layer increases of +1.9, +2.5, and +3.4 °C under Representative Concentration Pathways (RCPs) 2.6, 4.5, and 8.5 [10].

This warming is expected to reinforce the thermocline, possibly reducing vertical ventilation and increasing the risk of subsurface oxygen depletion. Likewise, the same warming trends are detected in the Arabian Gulf, even though its shallow basin outcomes cause stronger temperature extremes and a faster reaction to atmospheric forcing [27,45].

3.10.2. Salinity Variability

In the Arabian Gulf, salinity is high because evaporation exceeds freshwater input. Normally, gulf-wide readings mean that the salinity is almost 40 psu, and in the southern coastal areas, it can reach up to 50 psu [45,46]. From October to March, salinity reached its maximum, and from July to September, minimum salinity was usually noted [45].

The main highlight in the Arabian Gulf is interannual variability, which can exceed seasonal variability in the basin’s mean salinity. The highest salinity reported yearly ranges from 39.5 to 40.5 psu, while the minimum value varies from 39.3 to 40.0 psu [46,55]. This shows that the year-by-year change in salt content is significant. As an interpretation, these interannual fluctuations are sustained by exchange across the Strait of Hormuz rather than solely by atmospheric flux. In addition, the freshwater effect remains locally vital near the Shatt Al-Arab [46]. In winter, salinity near its mouth can fall to nearly 35 psu or lower; however, recent studies have reported a strange maximum salinity over the last 30 years showing that seasonal freshening has been weaker and has remained constant despite reduced freshwater runoff to the gulf [113,115].

In contrast to the Arabian Gulf, the Gulf of Oman maintains a narrower salinity envelope, typically ranging from 35 to 37 psu, with comparatively moderate seasonal variability owing to open-ocean exchange and greater mixing pathways [15,47]. In the Arabian Sea, particularly in the upper northern zone (50 m), salinity showed considerable interannual fluctuations, with drops after pre-monsoon cyclones, measured values of about 36.0–36.6 psu, and strong year-to-year variability [49]. In the southern Arabian Sea, seasonal salinity ranges from around 35 to 36.5 psu [15].

An important winter feature is the development of Arabian Sea High-Salinity Water (ASHSW), associated with winter monsoon conditions and producing salinity maxima (Smax). Moreover, the core depth of ASHSW displays yearly and semi-yearly cycles, and changeability in the strength and vertical range of these Smax values can affect mixed-layer depth during autumnal and winter monsoon periods [15,47].

From the literature, the Arabian Gulf displays a considerably higher level of salinity of 39–50 psu (min–max) (Figure 9) due to evaporation control and limited water exchange, whereas minimum and maximum values of the Gulf of Oman (35–37 psu) and Arabian Sea (35–36.6 psu) showed that they uphold lower and steadier salinity settings affected by marine circulation.

3.10.3. System-Level Variability and Exchange

As previously discussed, circulation in the Arabian Gulf is sustained by wind forcing, density gradients, and limited exchange through the Strait of Hormuz, resulting in comparatively long flushing times of 1.2 to 1.3 years [141]. In comparison, the Gulf of Oman is strongly influenced by monsoon-driven variability and open-ocean coupling, which control upwelling, heat exchange, and stratification [100,133]. The Strait of Hormuz works as a main transition region where seasonal variations in stratification and exchange affect the salinity and temperature fields in both basins [138].

In general, the Arabian Gulf exhibits more pronounced hydrographic changes, whereas the Gulf of Oman displays a stronger monsoon-driven pattern and long-term shifts in stratification. Overall, these patterns emphasize that seasonal forcing dominates hydrographic variability, while long-term warming trends are increasingly varying stratification and circulation dynamics across both basins.

Long-term climate-change effects were assessed through synthesis of published temperature, salinity, stratification, and climate-projection studies rather than through new independent trend calculations. Reported indicators included long-term sea surface temperature (SST) trends, projected warming under Representative Concentration Pathway (RCP) scenarios, changes in thermocline strength, and expected effects on ventilation and oxygen distribution. These indicators were used to evaluate how climate-driven warming may modify stratification and circulation regimes.

Seasonal variability in temperature, salinity, and density can be represented by harmonic fitting of monthly means:

$$X\left(t\right)=\overline{X}+A\cos \left(\omega t+\varphi \right)$$

(14)

where $X\left(t\right)$ is the variable of interest, $\overline{X}$ is the annual mean, $A$ is the seasonal amplitude, $\omega =2\pi /12$ month⁻¹, and $\varphi$ is phase [142]. Vertical hydrographic structure may be approximated by an exponential temperature profile:

$$T\left(z\right)=T_{\mathrm{\infty }}+(T_0-T_{\mathrm{\infty }})e^{-z/H}$$

(15)

where $T\left(z\right)$ is temperature at depth $z$, $T_0$ is surface temperature, $T_{\mathrm{\infty }}$ is deep-water temperature, and $H$ is thermocline depth [143].

The spatial–temporal variability in SST and SSS is usually extracted by means of EOF/PCA analysis from the covariance matrix:

$$\mathrm{C}={\displaystyle \frac{1}{n}}\mathrm{X}{\mathrm{X}}^T$$

(16)

where eigenvectors describe dominant spatial modes, and principal components define temporal evolution [6,29,69,131]. Yearly means and variability are calculated as:

$${\overline{X}}_{yr}={\displaystyle \frac{1}{N}}{\displaystyle {\sum }_{i=1}^N}{X}_i$$

(17)

$$\sigma =\sqrt{{\displaystyle \frac{1}{N-1}}{\displaystyle \sum _{i=1}^N}(X_i-\overline{X}{)}^2}$$

(18)

And linear trends are often assessed using the Theil–Sen estimator for robust long-term change analysis [14]. Annual amplitude is defined as:

$$=X_{\mathrm{m}\mathrm{a}\mathrm{x}}-X_{\mathrm{m}\mathrm{i}\mathrm{n}}$$

(19)

It is widely used to quantify SST and salinity ranges, such as the strong northwest–southeast contrasts in the Arabian Gulf [45]. Where observations are sparse, model outputs (e.g., HYCOM, ORAS4) are used to resolve seasonal and interannual variability in SST and salinity and are validated against existing in situ datasets [92,133].

3.11. Hydrodynamic Exchange and Coastal Aquifer Interactions

The hydrodynamics in both basins are based on hydrodynamic processes that are closely related to groundwater–seawater interactions, such as seawater intrusion (SWI) and submarine groundwater discharge (SGD), that control coastal nutrient and pollutant fluxes. The SGD shows a significant pathway for solute and water exchange and can be improved through wave-driven mixing at the sediment–water interface and by tidal pumping in the arid environment [144,145]. The seawater intrusion, which often extends several kilometers inland, is further exacerbated by evaporation and groundwater abstraction. This affects the aquifer quality and regional water resources [146]. Thus, the interaction between coastal aquifer processes and hydrodynamic exchange affects nutrient transport, water quality, and ecosystem stability. Overall, this highlights the intense coupling among physical circulation and subsurface hydrological systems.

The radar chart (Figure 10) compares important hydrodynamic features of the two basins using normalized indices (1–10 scale). The Arabian Gulf shows low depth (2), high salinity (9), and a long residence time (8), indicating a shallow, retention-dominated system. In contrast, the Gulf of Oman has greater depth (10), higher exchange strength (8), and mixing intensity (8), reflecting stronger circulation and flushing (residence time = 3). The classification of the Arabian Gulf as a hypersaline retention system and the Gulf of Oman as a dynamic exchange system is supported by normalized hydrodynamic indicators, including depth, salinity, exchange strength, mixing intensity, and residence time, as summarized in Supplementary Tables S7–S9 and visualized in the radar chart [6,7,8,38,39,40]. These patterns highlight the gulf as a retention system and the Gulf of Oman as an exchange-driven system. These joined processes highlight the need for integrated hydrodynamic and hydrogeological methods to know and manage coastal systems in arid and semi-arid regions.

The Arabian Gulf is categorized by shallow depth, elevated salinity, and longer residence time, reflecting limited exchange and intense evaporation. In comparison, the Gulf of Oman displays greater depth, stronger circulation, and more intense mixing due to open-ocean connectivity and monsoon forcing. These differences highlights distinct hydrodynamic regimes and their implications for water exchange, nutrient transport, and environmental vulnerability.

3.12. Ecological Consequences of Seasonal and Interannual Variability

Extreme seasonal temperature variation often exceeds 15–20 °C, with high salinity values of more than 40 psu and locally more than 50 psu in the Arabian Gulf, and this causes intense physiological stress on oceanic organisms [45,126]. Moreover, the increasing frequency of thermal extremes further raises ecological vulnerability [45]. Although the Gulf of Oman experiences milder conditions, it is undergoing rapid warming of around 0.58 °C per decade, and intense stratification may decrease oxygen ventilation at depth. Thus, the Omani Gulf is experiencing more moderate conditions but is undergoing rapid warming, with strengthening stratification potentially reducing oxygen ventilation at depth [10,136].

Seasonal stratification strongly influences oxygen circulation and nutrient availability. Summer stratification can restrict vertical exchange and elevate the risk of hypoxia in the Arabian Gulf, whereas winter mixing increases ventilation in the Arabian Gulf [45,116]. In contrast, under warm conditions, the Gulf of Oman intensifies thermocline development and, as a result, may reduce vertical mixing and exacerbate oxygen depletion, especially in areas affected by the Arabian Sea oxygen-depleted zone [10].

Oxygen depletion in the Gulf of Oman is strongly linked to the balance between ventilation, stratification, and the residence time of subsurface waters. Strong mixing and exchange enhance oxygen renewal, whereas persistent stratification restricts vertical ventilation and allows biological oxygen consumption to deplete dissolved oxygen at depth [113]. The export of dense Arabian Gulf water forms a subsurface salinity layer that can modify stratification and influence oxygen distribution [147]. Under future warming, strengthened thermocline development may further reduce ventilation and increase the susceptibility of intermediate waters to hypoxia [148].

Hypersaline conditions in the Arabian Gulf are typically around 40 psu, and exceed 50 psu in some regions, creating a distinctly saline oceanic environment that affects species composition and biogeochemical cycling [46,55]. In the Gulf of Oman, changes in water masses, such as ASHSW, affect mixed-layer depth and efficiency cycles [15]. These processes are further related to cross-basin transfer, in which variations in temperature and salinity gradients control nutrient transfer, mixing, and ecosystem connectivity between the Arabian Gulf and the Gulf of Oman [141].

Overall, the aforementioned factors govern the main ecological processes, such as nutrient cycling, oxygen distribution, and efficiency. The Arabian Gulf is more vulnerable due to its depth and evaporative nature, whereas the Gulf of Oman is more sensitive to monsoon-driven and stratification fluctuations. Taken together, these evolving aspects reveal the strong coupling between hydrodynamics and ecosystem responses in both basins. These results underscore the sensitivity of regional ecosystems to hydrographic variations, particularly amid ongoing climate change and increasing anthropogenic pressures.

The ecological interpretation of oxygen depletion is therefore based on published evidence linking warming, stratification, reduced ventilation, and subsurface oxygen decline. In the Gulf of Oman, stronger thermocline development may restrict vertical exchange and increase the risk of hypoxia, particularly where subsurface waters are already sensitive to limited ventilation.

3.13. Integrated Hydrodynamic–Ecological–Groundwater Interactions

The hydrodynamic processes clearly control ecosystem structure in the Arabian Gulf and the Gulf of Oman. The high salinity and temperature variations in gulf waters impose environmental stress and influence species composition [86,141]. In contrast, as explained earlier, the Omani Sea upholds more modest conditions but supports maximum productivity due to monsoon-driven upwelling and nutrient distribution [24].

The hydrodynamic gradient is reflected in phytoplankton distributions, with flagellate dominance associated with stratified conditions and diatom-dominated blooms linked to mixing and upwelling [13,86]. Moreover, limited flushing has caused eutrophication and more frequent harmful algal blooms as inputs into the Arabian Gulf have increased. This underscores the position of circulation in controlling ecosystem response and water quality [97].

Changes in groundwater also affect the region’s ecology and hydrology. Decreasing freshwater input and decreasing groundwater storage, the satellite-based analysis signifies that yearly water storage amplitude in the Oman and Arabian Gulf regions is about 101 mm, while groundwater storage has declined at a rate of almost 6.1 mm yr⁻¹ during drought periods from 2005 to 2011, showing broader regional water stress, which affects the density gradients, and salinity potentially alters circulation and exchange processes [149]. These land–sea interchanges strengthen the coupling between ocean conditions and terrestrial hydrology, as well as between marine conditions, while the system is continuously modified by anthropogenic pressures.

Hydrodynamic modeling shows that contaminants, such as radionuclides, can accumulate in shallow areas before being progressively exported through the Strait of Hormuz, with circulation patterns governing residence time and spatial dispersion [6]. Furthermore, rapid coastal expansion, desalination expansion, and population growth have increased stress on marine systems, thereby increasing vulnerability to eutrophication, hypoxia, and ecosystem degradation [150,151]. Additional supporting information on physical, ecological, and groundwater interactions across the Arabian Gulf and Gulf of Oman–Arabian Sea system is provided in Supplementary Table S10 [6,7,24,25,38,87,93,149,152,153,154].

Moreover, coastal development and dust deposition influence the distribution of trace metals and nutrients. In contrast, by the Strait of Hormuz, the export and accumulation of pollutants is controlled by hydrodynamic transport [6,97]. Overall, the Arabian Gulf retains contaminants due to partial exchange, while the Gulf of Oman serves as a pathway for their removal. The interactions among hydrodynamics, nutrient dynamics, and groundwater processes are illustrated in Figure 11, highlighting the coupled nature of physical, biogeochemical, and subsurface exchanges in the Arabian Gulf and the Gulf of Oman. The main coupled processes linking hydrodynamics, aquifers, and ecosystem responses are summarized in Supplementary Table S11 [6,7,8,38,40,144,145,155,156,157,158,159].

The following are the fundamental equations used to calculate hydrodynamic, ecological, and groundwater patterns.

i.

Hydrodynamics

Marine circulation is commonly represented by the hydrostatic Boussinesq equations, also referred to as hydrostatic primitive equations formulated under the Boussinesq approximation [160]. In this review, the term hydrostatic Boussinesq equations is used in the ocean-modeling sense, where vertical acceleration is neglected under hydrostatic balance, while density variations are retained in the buoyancy and pressure-gradient terms. This usage is distinct from Boussinesq-type wave models and Boussinesq turbulence closure formulations.

$$\nabla ·\mathrm{u}=0$$

(20)

$${\displaystyle \frac{\partial \mathrm{u}}{\partial t}}+(\mathrm{u}·\nabla )\mathrm{u}=-{\displaystyle \frac{1}{\rho }}\nabla p+\mathrm{g}+f\mathrm{k}\times \mathrm{u}+\nu {\nabla }^2\mathrm{u}-{\mathsf{\tau }}_b$$

(21)

where $\mathrm{u}$ is the velocity, $p$ is the pressure, $\rho$ is the density, $f$ is the Coriolis parameter, $\nu$ is the eddy viscosity, and ${\tau }_b$ is the bed friction [161,162].

For coastal applications, these are depth-averaged to shallow-water form:

$${\displaystyle \frac{\partial h}{\partial t}}+{\displaystyle \frac{\partial \left(hu\right)}{\partial x}}+{\displaystyle \frac{\partial \left(hv\right)}{\partial y}}=0$$

(22)

$${\displaystyle \frac{\partial \left(hu\right)}{\partial t}}+{\displaystyle \frac{\partial \left(h{u}^2\right)}{\partial x}}+{\displaystyle \frac{\partial \left(huv\right)}{\partial y}}=-gh{\displaystyle \frac{\partial \eta }{\partial x}}+fhv-{\tau }_{bx}$$

(23)

$${\displaystyle \frac{\partial \left(hv\right)}{\partial t}}+{\displaystyle \frac{\partial \left(huv\right)}{\partial x}}+{\displaystyle \frac{\partial \left(h{v}^2\right)}{\partial y}}=-gh{\displaystyle \frac{\partial \eta }{\partial y}}-fhu-{\tau }_{by}$$

(24)

where $h$ is water depth and $\eta$ free-surface elevation [111,163,164,165].

ii.

Aquatic Ecology/Transport

A generic transport-reaction equation for ecological tracers is:

$${\displaystyle \frac{\partial C}{\partial t}}+\mathrm{u}·\nabla C=\nabla ·(K\nabla C)-w{\displaystyle \frac{\partial C}{\partial z}}+S(C)$$

(25)

where $C$ is the concentration, $K$ the diffusion/dispersion coefficient, $w$ the settling or swimming velocity, and $S\left(C\right)$ the source–sink terms (growth, grazing, decay, and remineralization) [166,167,168].

In its depth-averaged form [166]:

$${\displaystyle \frac{\partial C}{\partial t}}+{\displaystyle \frac{P}{H}}{\displaystyle \frac{\partial C}{\partial x}}+{\displaystyle \frac{Q}{H}}{\displaystyle \frac{\partial C}{\partial y}}={\displaystyle \frac{\partial }{\partial x}}\left(E_x{\displaystyle \frac{\partial C}{\partial x}}\right)+{\displaystyle \frac{\partial }{\partial y}}\left(E_y{\displaystyle \frac{\partial C}{\partial y}}\right)-w{\displaystyle \frac{\partial C}{\partial z}}+S_c+Q_c$$

(26)

iii.

Groundwater and Coupled Exchange

Groundwater flow is commonly written as:

$$S_s{\displaystyle \frac{\partial h}{\partial t}}=\nabla ·(K\nabla h)+q$$

(27)

where $h$ is the hydraulic head, $S_s$ the specific storage, $K$ the hydraulic conductivity, and $q$ the source/sink flux [169].

At the groundwater–surface water interface, coupling is imposed through continuity of the head and flux, or by first-order exchange:

$$q_e=\alpha (h_s-h_g)$$

(28)

where $q_e$ is the exchange flux, $h_s$ the surface-water head, $h_g$ the groundwater head, and $\alpha$ the exchange coefficient [170].

3.14. Hydrodynamic Modeling and Observational Constraints

Hydrodynamic modeling has advanced significantly in the Arabian Gulf and the Gulf of Oman. The numerical models are high-resolution and can simulate baroclinic circulation, tidal dynamics, pollutant transport, and mesoscale variability. The 3D models (i.e., MITgcm and Delft3D-FLOW) are primarily used to provide information on salinity–temperature variations, thermohaline exchange, and climate-driven changes, while accounting for comprehensive tidal constituents, atmospheric forcing, and bathymetry [23,92]. Moreover, models such as SCHISM and unstructured-grid models enabled higher resolution near complex coastlines and improved analysis of nearshore circulation and tracer transport [6,48]. The WRF and WAVEWATCH III (barotropic tidal models and wind–wave hindcast systems) provide consistent forcing datasets and show good agreement with observations [171].

However, specifically in the Arabian Gulf and the Gulf of Oman, despite advances in models, they are constrained by the scarcity of long-term observational data on currents, heat fluxes, and vertical structure [44]. Interestingly, the climatic datasets show strong agreement with numerical model outputs, which often smooth short-term and mesoscale changes [10]. Moreover, the satellite data increases spatial exposure but cannot sufficiently resolve subsurface processes. At shallow depth, extreme salinity and intense evaporation in the Arabian Gulf cause escalation of the sensitivity to model parameterization and boundary settings that need higher-resolution approaches [15,38,60]. Likewise, in the Arabian Gulf, the partial representation of vital water masses introduces uncertainties into biogeochemical simulations and circulation [172]. Overall, numerical modeling reliability varies strongly with observational help and accurate representation of stratified dynamics, as current models efficiently capture large-scale circulation and exchange processes. A detailed summary of model frameworks, primary domains, applications, validation strengths, and main constraints is provided in Supplementary Table S12 [5,6,8,10,23,38,48,50,51,60,86,171,172,173,174].

Statements regarding the capability of advanced hydrodynamic models are based on their demonstrated use in the reviewed literature to reproduce circulation, tides, salinity structure, temperature fields, and tracer transport. However, model validation approaches differ considerably among studies. Some models report quantitative metrics such as root mean square error (RMSE), correlation coefficients, or agreement indices, whereas others rely mainly on qualitative comparison with available observations. Therefore, this review does not assign a single accuracy ranking to model types; instead, model performance is evaluated according to the validation evidence reported in each study [80,175,176].

Because the reviewed models differ in spatial resolution, vertical discretization, boundary forcing, turbulence closure, and validation periods, their results are not treated as directly interchangeable. The comparison is therefore process-based rather than strictly quantitative. Model outputs are considered more robust when similar hydrodynamic mechanisms—such as inverse estuarine exchange, tidal mixing, wind-driven circulation, and salinity gradients—are reproduced across independent studies using different modeling frameworks. In contrast, results based on limited validation, short-term observations, or site-specific configurations are interpreted with greater caution [80,175].

Model validation in the region remains uneven. Tide gauges, satellite-derived sea surface temperature (SST), altimetry, short-term Acoustic Doppler Current Profiler (ADCP) measurements, and hydrographic profiles provide useful constraints, but they mainly represent surface or short-duration conditions. Fewer studies are validated against long-term moorings, full-depth ADCP transects, or sustained temperature–salinity–oxygen profiles, which limits confidence in simulated subsurface exchange, stratification, deep-water transport, residence time, and vertical mixing [176,177]. Thus, while models can reproduce large-scale circulation patterns reasonably well, uncertainty remains higher for subsurface processes and exchange rates through the Strait of Hormuz.

In this review, the term “limitations” refers to constraints in the application and validation of numerical models rather than a failure of the numerical methods themselves. These constraints mainly arise from sparse subsurface observations, uncertain open-boundary conditions, short validation periods, coarse representation of small-scale mixing, and limited full-depth measurements [178]. Uncertainty was therefore assessed qualitatively by considering the type, duration, vertical coverage, and independence of validation datasets. Higher confidence was assigned to findings supported by multiple observations or by agreement between models and observations, whereas greater uncertainty was assigned to processes relying mainly on surface satellite products, short-term campaigns, or limited spatial coverage. The main validation datasets used in regional hydrodynamic studies and their limitations are summarized in

Table 7. Common validation datasets and limitations in regional hydrodynamic models.

Validation Data Type	Common Use	Strength	Main Limitation	References
Tide gauges	Sea-level and tidal validation	Long time series; useful for tidal amplitude and phase	Limited vertical information; cannot validate subsurface exchange	[32,51,63]
Satellite SST/altimetry	Surface thermal patterns and sea-level variability	Wide spatial coverage; useful where field data are sparse	Cannot validate subsurface exchange, deep currents, or vertical stratification	[10,45,179]
Short-term ADCP/current measurements	Current velocity validation and nearshore circulation assessment	Direct velocity measurements; useful for model calibration	Often short deployment duration and limited spatial coverage	[6,43,48]
CTD/hydrographic profiles	Temperature–salinity structure, stratification, and water-mass validation	Provides vertical water-column information	Sparse temporal coverage; often from cruises or seasonal datasets	[10,79,180]
Long-term moorings	Seasonal/interannual exchange, subsurface variability, and deep circulation	Best dataset for validating long-term subsurface dynamics	Rare in the region; limited full-depth coverage across Hormuz and the Gulf of Oman	[8,44,125,171]

.

3.15. Comparative Interpretation Between the Arabian Gulf and the Gulf of Oman

The Arabian Gulf and the Omani Gulf are both controlled by thermohaline forcing; however, the main difference lies in their circulation patterns, owing to their distinct basin geometries and levels of openness. Thus, the interference between the two basins is bidirectional. The Arabian Gulf transfers the hypersaline water that produces a different subsurface signature in the Omani Sea, whereas the Gulf of Oman delivers comparatively fresher surface inflow that sustains Arabian Gulf circulation [6,40,86]. Moreover, this exchange is modified further by the winds, tides, and anthropogenic effects that typically can change the transport pathways of salt, heat, and pollutants and change the stratification [48,82]. A comparison between the two basins based on density-driven circulation is presented in

Table 8. Main features of density-driven circulation in the Arabian Gulf and the Gulf of Oman.

Feature	Arabian Gulf	Gulf of Oman	References
Bathymetry & openness	Shallow semi-enclosed basin (35–36 m mean depth); restricted exchange via Hormuz	Deep basin (3000–3500 m); open to Arabian Sea/Indian Ocean	[5,6,43,87]
Primary density forcing	Extreme evaporation drives hypersalinity; salinity commonly >39 psu; embayments may exceed 70 psu	Density structure shaped by Indian Ocean influence, plus inflow of dense Persian Gulf Water at subsurface depths	[5,45,86,87]
Circulation type	Inverse estuarine: surface inflow from the Gulf of Oman + deep outflow of dense Gulf water	Thermohaline layering + wind-driven circulation; gyres/eddies and coastal upwelling patterns	[5,6,50,86]
Exchange pathway	Surface inflow typically along the Iranian margin; dense outflow exits at depth through Hormuz	Receives Persian Gulf water forming a subsurface layer (100–300 m)	[5,48,87,165]
Vertical structure	Clear two-layer system; dense outflow often below 40 m; outflow speeds 0.2 m s−1 reported with salinity 39–41 psu	Subsurface Persian Gulf water layer; intrusion and mixing influenced by isopycnal spreading and double diffusion	[5,89]
Main modulators	Tides, Shamal winds, bathymetry, and anthropogenic brine discharge can alter stratification/exchange	Monsoon winds, mesoscale eddies, and climate-driven thermocline variability influence stratification and ventilation	[5,10,82,92]
Seasonality	Strong seasonal changes in inflow intensity; outflow persists year-round	Strong seasonal and interannual variability; thermocline strength and mixing vary with climate forcing	[6,10,48]

. Taken together, these factors highlight the significance of basin geometry and connectivity in formative regional circulation patterns. The Arabian Gulf is a hypersaline retention system, while the Gulf of Oman is a dynamic exchange system, controlled by the Strait of Hormuz.

The coupling between the two basins is not one-directional. Stratification in the Gulf of Oman can indirectly influence Arabian Gulf renewal by modifying the density structure and boundary conditions in the Strait of Hormuz [181]. Although deep-water formation within the Arabian Gulf is primarily controlled by evaporation, cooling, and salinity increase within the gulf, the vertical structure of Gulf of Oman waters affects the stability, depth, and entrainment of the inflowing surface layer and outflowing dense water [84]. Therefore, Gulf of Oman stratification does not directly generate deep-water renewal inside the Arabian Gulf, but it modulates the exchange interface through which renewal and export occur.

The classification of the Arabian Gulf as a hypersaline retention system and the Gulf of Oman as a dynamic exchange system is based on the comparative hydrodynamic indicators summarized in Table 2 and visualized in the normalized radar chart (Figure 10). The Arabian Gulf is characterized by shallow depth, high salinity, strong evaporation, restricted exchange through the Strait of Hormuz, and long water renewal time, which collectively support its interpretation as a retention-dominated basin [83]. In contrast, the Gulf of Oman is deeper, more strongly connected to the open ocean, and characterized by stronger exchange, monsoon-driven mixing, upwelling, and shorter residence time, supporting its interpretation as an exchange-driven system [182]. The radar chart should therefore be interpreted as a normalized comparative synthesis rather than as direct raw measurements. Additional supporting information on hydrodynamic exchange features and their implications is provided in Supplementary Table S2.

4. Future Recommendations

Although hydrodynamic modeling of the Arabian Gulf and the Gulf of Oman system has advanced significantly, there are still limitations that require attention. One of the biggest limitations is its temporary nature and spatial sparsity, particularly for intermediate water variations, deep circulations, and small-scale events such as diapycnal mixing and internal waves. Data mostly come from short-term projects and do not incorporate perspectives on long-term variability arising from variations in hydrographic settings and responses to extreme events. Moreover, biogeochemical budgets and nutrient fluxes are poorly constrained, particularly regarding inflow–outflow differences and the impacts of human activities. A substantial limitation is the lack of cohesive ecohydrodynamic–groundwater modeling frameworks, as current models mostly focus on circulation, tides, and pollutant transport while overlooking groundwater fluxes and ecological feedback mechanisms.

The most important knowledge gap identified by this review is the lack of sustained, multi-depth observational datasets across the Strait of Hormuz and the Gulf of Oman. This gap limits confidence in estimates of exchange rates, subsurface circulation, ventilation, and model validation [82,83]. Addressing this gap is essential for improving predictive capability and for resolving coupled hydrodynamic–biogeochemical processes.

Future research should focus on enhancing long-term observational networks, improving high-resolution field measurements, developing coupled models incorporating hydrodynamics, integrating climate-change projections, and integrating ecosystem and groundwater processes. To address these difficulties, it is essential to advance predictive capacity and support sustainable management in this naturally sensitive region of the Arabian Gulf and the Gulf of Oman.

5. Conclusions

This review provides an integrated understanding of the hydrodynamics and water circulation in the Arabian Gulf and the Gulf of Oman, with a focus on the major differences between a shallow, evaporation-controlled basin and a deeper, monsoon-influenced system. In the Arabian Gulf, density-driven reverse-estuarine circulation results in hypersaline conditions and extended residence times that increase environmental vulnerability. In contrast, the Omani Sea comprises a more dynamic circulation regime driven by monsoon forcing, upwelling, and exchange with the open ocean.

The major finding of this investigation is the Strait of Hormuz’s critical role in linking the Arabian Gulf and the Gulf of Oman. The Strait of Hormuz controls the flow of temperature, salt, nutrients, and contaminants. These factors influence both the hydrodynamic processes and the region’s environmental surroundings. This coupling underscores that changes occurring in either the Arabian Gulf or the Gulf of Oman will affect the other basin, whether governed by anthropogenic pressures or climate variability. Moreover, to understand deep-water processes, groundwater–hydrodynamic interactions and small-scale mixing need to be better understood. Despite advanced observational tools and modeling, key uncertainties remain. These problems require better long-term monitoring, higher-resolution models, and integrated coupled methods that connect physical, ecological, and hydrological processes.

Overall, this review reveals that the two basins should be viewed as a coupled hydrodynamic system, in which the basins’ physical characteristics, atmospheric forcing, and exchange processes together form a circulation and the ecosystem’s dynamics. This integrated approach is needed for future predictions and for integrating sustainable management practices in this climatically and economically significant region.