Intensification Trend and Mechanisms of Oman Upwelling During 1993–2018

Abstract

The long-term trend of coastal upwelling under global warming has been a research focus in recent years. Based on datasets including sea surface temperature (SST), sea surface wind, air–sea heat fluxes, ocean currents, and sea level pressure, this study explores the long-term trend and underlying mechanisms of the Oman coastal upwelling intensity in summer during 1993–2018. The results indicate a persistent decrease in SST within the Oman upwelling region during this period, suggesting an intensification trend of Oman upwelling. This trend is primarily driven by the strengthened positive wind stress curl (WSC), while the enhanced net shortwave radiation flux at the sea surface partially suppresses the SST cooling induced by the strengthened positive WSC, and the effect of horizontal oceanic heat transport is weak. Further analysis revealed that the increasing trend in the positive WSC results from the nonuniform responses of sea level pressure and the associated surface winds to global warming. There is an increasing trend in sea level pressure over the western Arabian Sea, coupled with decreasing atmospheric pressure over the Arabian Peninsula and the Somali Peninsula. This enhances the atmospheric pressure gradient between land and sea, and consequently strengthens the alongshore winds off the Oman coast. However, in the coastal region, wind changes are less pronounced, resulting in an insignificant trend in the alongshore component of surface wind. Consequently, it results in the increasing positive WSC over the Oman upwelling region, and sustains the intensification trend of Oman coastal upwelling.

1. Introduction

Coastal wind-driven upwelling is one of the key dynamic processes in the continental shelf seas. Its variations significantly affect the ecological environment, fishery resource distribution, regional climate, air–sea CO₂ exchange, and global carbon cycling processes in upwelling regions and adjacent seas [1]. Since Bakun (1990) proposed the hypothesis that global warming may strengthen the eastern boundary upwelling systems by enhancing land–sea pressure gradients [2], the long-term trends of coastal upwelling have become a key focus in global climate change research [3,4,5,6,7,8,9]. Previous studies have indicated that, under global warming, the major wind-driven eastern boundary upwelling systems (e.g., the California, Canary, Benguela, and Humboldt Current systems) generally show an intensification trend [5,6,7]. This intensification is attributed mainly to the stronger rate of warming over land relative to the ocean, which increases the thermal gradient between land and sea, subsequently enhancing the coastal wind field in a manner that is favorable for upwelling development [3,10].

In contrast to the relatively well-established understanding of permanent eastern boundary upwelling systems, research on the long-term trend of seasonal coastal upwelling driven by monsoons remains relatively scarce. Recent studies have indicated that the rapid warming in the Arabian Sea over the past six decades has weakened summer monsoon circulation, leading to a decreasing trend in upwelling along the western boundary of Arabian Sea [11,12,13,14]. Xie et al. demonstrated that the Qiongdong upwelling in the northwestern South China Sea exhibited a weakening trend during 1982–2012, dominated by the weakening of the local wind stress curl (WSC) [15]. In contrast to the above findings, some modeling and observational studies have reported intensification trends in the various upwelling systems, including Somalia, Oman, the northern South China Sea, and the Yellow Sea [16,17,18,19]. These different findings highlight the regional complexity of the seasonal upwelling systems’ responses to global warming.

The Oman coastal upwelling (16–22°N, approximately delineated by the red line segment in Figure 1a) is a typical monsoon-driven upwelling system off the western boundary of the Arabian Sea [20,21,22]. Driven by the world’s strongest cross-equatorial monsoon low-level jet (i.e., Findlater Jet) in summer [23], this area develops one of the strongest upwelling systems in the world [24,25,26]. Unlike the stable existence of the upwelling system driven by trade winds along the eastern boundary of the global oceans, Oman upwelling only occurs in summer. During summer, a significant cold center with temperatures 3 °C lower than those of the surrounding waters appears along the Oman coast, indicating the presence of strong upwelling (Figure 1b). Notably, although both the Oman upwelling and Somali upwelling are western boundary upwellings driven by the Indian summer monsoon, their formation mechanisms are different (Figure 1c) [27,28,29,30]. Their difference primarily results from the influence of the Somali Plateau on surface wind patterns. Specifically, the southwesterly wind weakens in the area extending from the Gulf of Aden entrance to the Oman coast (i.e., the leeward side of the Somali Peninsula), whereas it intensifies in the coastal region of the northeastern Somali Peninsula. This creates an elliptical zone of strong winds (>12 m/s) which is bounded by the green curves (Figure 1c). This unique wind pattern results in a positive WSC over the mouth of the Gulf of Aden, the Oman coast, and the northeastern part of Socotra Island, and a negative WSC over the interior of the Gulf of Aden, and the central and southern Arabian Sea. Consequently, unlike Somali upwelling, which is driven primarily by the alongshore wind stress (WS) [16,27], Oman upwelling is jointly driven by alongshore wind stress and the local positive WSC [17,28,29,30]. Thus, Oman upwelling is a unique upwelling system formed by the modulation of the nearshore wind field distribution by land topography. Its formation mechanism clearly demonstrates the significant impact of topographic effects on the dynamics of coastal upwelling processes [31].

Existing studies of the long-term trend of Oman upwelling have differing views. Varela et al. reported no significant intensification trend in the alongshore WS off Oman during 1982–2010 [5]. Under the global warming scenario, in the Arabian Sea, both the coastal and offshore SSTs exhibited warming trends during 1982–2015, with the coastal warming rate much lower than that in offshore regions, and this suggests an intensification of upwelling along the Oman coast [32]. Recent coral observations have revealed that the intensity of upwelling near Oman has been very stable over the past millennium, but a significant decrease has recently occurred [33]. Ajith et al. [34], based on remote sensing data, suggested that enhanced Ekman transport between 1982 and 2015 led to an intensification of coastal upwelling in Oman, a conclusion further confirmed by Lahiri et al. [35]. Numerical model projections by Praveen et al. [17] suggest potential upwelling intensification along the Oman coast in the future due to poleward shifts in the monsoon low-level jet.

The aforementioned studies reached different conclusions over different time periods, reflecting the potential modulation of interdecadal climate variations on the long-term trend of Oman upwelling. In this study, the long-term trend of Oman upwelling and its potential dynamic mechanisms were investigated based on multiple datasets, including SST, surface wind field, ocean current, and air–sea heat fluxes during summer months (i.e., June–August) of the period 1993–2018. The satellite remote sensing data used in this study span 26 years, which helps to minimize the influence of interdecadal variability and provides a reliable trend of Oman upwelling in recent decades.

2. Materials and Methods

2.1. Materials

2.1.1. Sea Surface Temperature

The SST data from the Objectively Analyzed Air–Sea Heat Flux (OAFlux) project were provided by the Woods Hole Oceanographic Institution (WHOI) [36]. These data are derived from a fusion of satellite-retrieved Advanced Very High Resolution Radiometer (AVHRR) [37] observations and model forecasts, with a spatial resolution of 1° × 1°. In this study, monthly OAFlux SST data are used to analyze the long-term trend of Oman upwelling during summer.

The Optimum Interpolation Sea Surface Temperature (OISST) dataset, with a spatial resolution of 0.25° × 0.25°, was obtained from the National Oceanic and Atmospheric Administration (NOAA) [38]. The monthly OISST data are employed in this study to validate the reliability of the OAFlux SST results.

2.1.2. Sea Surface Wind

The monthly 10 m ocean surface wind data from the Cross-Calibrated Multi-Platform (CCMP) product [39], distributed by NASA’s Physical Oceanography Distributed Active Archive Center (PO. DAAC), was obtained from Remote Sensing Systems (RSS, https://www.remss.com/, accessed on 10 April 2025). This high-accuracy dataset combines surface wind observations from multiple microwave scatterometers (including QuickSCAT and ASCAT), microwave radiometers (GMI, SSM/I, and SSMIS), and buoy measurements. The dataset has a horizontal resolution of 0.25°, and data are available from November 1992. In this study, we use monthly mean WS and WSC data to investigate the dynamic mechanisms for the long-term trend of Oman upwelling during summer.

2.1.3. Air–Sea Heat Flux and Sea Level Pressure

The air–sea heat flux data used in this study were from the ERA5 reanalysis provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) [40]. The heat flux components include net shortwave radiation, net longwave radiation, sensible heat flux, and latent heat flux, with a spatial resolution of 0.25° × 0.25°. These data will be employed to investigate potential drivers for the long-term trend of Oman upwelling. Additionally, monthly sea level pressure data from ERA5 with 0.25° horizontal resolution is used to analyze the underlying influence of atmospheric circulation on the long-term trends of Oman upwelling.

2.1.4. Ocean Currents

Ocean current data with spatial resolutions of 1/12° × 1/12° and 50 vertical levels ranging from 0 to 5700 m, including 31 levels in the upper 500 m, were obtained from the Copernicus Marine Environment Monitoring Service (CMEMS, http://marine.copernicus.eu/, accessed on 10 April 2025). These data were used to calculate heat transport into the upwelling region and investigate the long-term relationship between horizontal heat transport and Oman upwelling.

The time span of the abovementioned monthly data used in this study is from January 1993 to December 2018, covering a total of 26 years.

2.2. Methods

2.2.1. Upwelling Index (UI_SST)

Following methods from previous studies [15,31,41,42,43], we quantify the intensity of Oman upwelling by using the upwelling index based on SST (UI_SST), which is defined as follows:

$${ UI}_{SST}={SST}_{off}{-SST}_{upwelling}$$

(1)

where SST_upwelling represents the area-averaged SST in the core upwelling region (Box A in Figure 1b) and SST_off denotes the area-averaged SST in the selected open ocean area without upwelling at the same latitude (Box B in Figure 1b). This index can effectively reflect the upwelling intensity through the temperature gradient between nearshore and offshore areas: higher UI_SST values indicate stronger cooling effects and thus more intense upwelling along the Oman coast.

A series of experiments have been conducted to test the sensitivity of selection areas to UI_SST calculations. We selected the area approximately covering the climatological upwelling SST front in the Oman upwelling region for a comparison with Box A, and confirmed the robustness of UI_SST independent from the selected region. Similarly, we selected two other areas at the same latitude as Box B, and calculated the average UI_SST for these two areas for a comparison. The sensitivity tests also demonstrated the reliability of Box B, which we selected to calculate UI_SST.

2.2.2. Horizontal Heat Transport

To estimate the contributions of horizonal advection to SST changes in the Oman upwelling region, the horizontal heat transport flux ($F_H$) across the Oman upwelling region (i.e., Box A in Figure 1b) is calculated as follow [44]:

$$F_H=\rho C_P{\int }_A(T-T_0)V_{n}dA$$

(2)

where $F_H$ represents the heat transport flux in the Oman upwelling region, $\rho$ represents the seawater density, $C_P$ represents the specific heat capacity of seawater, T represents the temperature, and T₀ represents the reference temperature, defined as the mean water temperature in the study area during upwelling periods. V_n represents the flow velocity normal to the boundary, and dA represents the area of the computational unit.

2.2.3. Wind Stress and Wind Stress Curl

The wind stress τ is calculated using the bulk formula:

$$\overrightarrow{ \tau }={\rho }_a{C}_d\left|\overrightarrow{U_{10}}\right|\overrightarrow{U_{10}}$$

(3)

where ${\rho }_a$ represents the air density, $U_{10}$ represents the horizontal wind vector at a height of 10 m, and $C_d$ represents the drag coefficient, parameterized as follows [45]:

$$\begin{gathered} 1000 C_d=0.29+{\displaystyle \frac{3.1}{U_{10}}}+{\displaystyle \frac{7.7}{U_{10}^2}} (3\le U_{10} \le 6 \mathrm{m}/\mathrm{s}) \\ 1000 C_d=0.60+0.71U_{10} (6\le U_{10} \le 26 \mathrm{m}/\mathrm{s}) \end{gathered}$$

The wind stress curl ${curl}_z\mathsf{\tau }$ is calculated using the following formula:

$${curl}_z\mathsf{\tau }={\displaystyle \frac{\partial {\tau }_x}{\partial y}}-{\displaystyle \frac{\partial {\tau }_y}{\partial x}}$$

(4)

3. Results

3.1. The Long-Term Trend of Oman Upwelling

Figure 2 show time series of SST inside and outside the Oman upwelling region, and the associated upwelling index in summer during 1993–2018. The OAFlux SST indicates that the area outside the Oman upwelling region showed a warming trend with a linear rate of 0.012 °C·year⁻¹, significant at the 95% confidence level, which is consistent with the well-known global ocean warming [11,46,47]. In contrast, the SST in the Oman upwelling region displayed a significant decreasing trend, with a linear rate of −0.024 °C·year⁻¹, indicating an intensification of upwelling during 1993–2018, which is consistent with the OISST result (Figure 2a). This conclusion is further supported by the UI_SST analysis, which reveals a linear increase of 0.036 °C·year⁻¹ (Figure 2b). From the spatial distribution of OAFlux SST trends (Figure 2c), one can see that the significant cooling trend is present in most parts of the Oman upwelling region and extends offshore, which is well reproduced in the OISST data (Figure 2d). These good agreements between OAFlux SST and OISST underpin the fact that the Oman upwelling shows an intensification trend during 1993–2018.

3.2. Possible Mechanisms for the Intensification of Oman Upwelling

Previous studies have indicated that the Oman upwelling is primarily driven by alongshore WS and its curl [5,20,35]. In addition, air–sea heat flux exchange [48,49] and horizontal ocean heat transport processes [50,51] are also important factors that regulate SST variations in the upwelling region, although these processes do not directly affect upwelling intensity. Thus, we quantify the relative contributions of these major physical forces to SST changes in the Oman upwelling region.

3.2.1. Sea Surface WS and Its Curl

Figure 3 examines the statistical relationship between the intensity of Oman upwelling and the alongshore component of WS, as well as the WSC. Based on the geometric characteristics of the Oman coastline, the WS is projected along the linear fitting coastline to obtain the alongshore WS component. Statistical results revealed that, after removing the regression estimate, the correlation coefficient between UI_SST and the mean alongshore WS from 1993 to 2018 was 0.74, significant at the 95% confidence level. Similarly, the UI_SST is closely related to the WSC, with a correlation coefficient (0.57) significant at 95% confidence level (Figure 3). These results underpin the fact that both the southwest WS and the WSC are the key factors regulating the variabilities of Oman upwelling, which is consistent with previous studies [20,28,29,30].

To further explore the impact of WS and WSC on the long-term trend of Oman upwelling, Figure 4 shows the spatial distribution of the long-term trends of WS and WSC. One can see that the long-term trend of WS in the Oman region shows significant spatial heterogeneity (Figure 4a). The southwest WS is intensified in offshore areas of Oman and the western Arabian Sea. In particular, in the northeastern waters of the Somali Peninsula and Socotra Island, the increasing rate exceeded 0.3 N·m⁻²·year⁻¹. In contrast, the WS in the Oman coastal region shows no significant trend, and may even have a weakening tendency in the southeastern coastal region of Oman, and their magnitude was substantially smaller than the offshore enhancement.

As mentioned above, the coastal alongshore WS component dominated the interannual variability of Oman upwelling through the Ekman transport processes. However, it did not significantly increase and even weakened in the southeastern region during our study period. This means that the alongshore WS component is not the cause of the intensification of Oman upwelling.

Figure 4b show the potential impact of the WSC on the Oman upwelling. The spatial heterogeneity of the long-term trend of WS mentioned above (Figure 4a) is characterized by enhanced offshore WS and relatively stable nearshore WS. This spatial pattern is bound to induce a strong positive WSC in the Oman upwelling region (Figure 4b), similar to the climatological WSC distribution (Figure 1c). The enhanced positive WSC tends to bring more cold subsurface waters into the surface layer and thus strengthen the upwelling, resulting in a sustained decrease in SST in the Oman upwelling region. Therefore, the increase in the positive WSC is a key factor in the intensification of Oman upwelling.

3.2.2. Air–Sea Heat Flux

The air–sea heat flux comprises shortwave radiation (SW), longwave radiation (LW), latent heat (LH), and sensible heat (SH) fluxes, all of which significantly influence SST variability [48,49]. Therefore, this study examines the potential effects of these thermal forcing factors on the long-term SST trend in the Oman upwelling region.

Figure 5a indicates a significant increasing trend in the net SW flux in the Oman upwelling region during 1993–2018. This trend reflects an increase in solar radiation absorption in the Oman upwelling region under global warming. Consequently, the enhanced solar radiation results in an increase in net heat flux (Figure 5b). Therefore, this enhanced surface heating, dominated by the SW flux, leads to warming SST, and partly offsets the cooling effect from the strengthened upwelling. In this study, the net surface heat flux is defined as positive downward, indicating heat transfer from the atmosphere to the ocean surface.

Further analysis reveals that summer net heat flux contributes approximately 0.04 °C·year⁻¹ to SST warming in the Oman upwelling region (Figure 5c). This finding is consistent with a previous study [52]. This result demonstrates that although a persistent increase in surface heat in the Oman upwelling region would lead to a rise in SST, the continuously enhanced upwelling of cold water remains the dominant factor causing SST decrease in the upwelling zone.

3.2.3. Oceanic Horizontal Heat Advection

To assess the contribution of horizontal heat transport, this study calculates the horizontal heat flux entering the Oman upwelling region through sections A and B (Figure 6a).

As shown in Figure 6a, under the influence of the southwest monsoon, relatively warm water from the southwestern part of the Oman upwelling region flows into the upwelling zone through section A (i.e., positive heat flux), whereas colder water from the upwelling zone is transported away through section B (i.e., negative heat flux).The total heat transport across section A and section B is negative. Notably, the net horizontal heat transport exhibited no significant trend from 1993 to 2018 (Figure 6b), suggesting that its contribution to the long-term SST trend was negligible.

4. Discussion

According to the analysis in Section 3, the intensification of Oman upwelling is primarily caused by the enhanced positive WSC over the area during this study period. However, why does the positive WSC increase during 1993–2018?

To answer the aforementioned question, Figure 7 show the long-term trends of sea level pressure and surface wind fields in summer during 1993–2018. One can see that sea level pressure exhibited a significant increasing trend over the western Arabian Sea, whereas the land pressure over the Arabian Peninsula and the Somali Peninsula continued to decrease (Figure 7a). This opposite trend in the pressure leads to an increased pressure gradient between the ocean and land, which in turn drives the strengthening of the offshore wind along the Oman coast, whereas the nearshore wind field is less affected by factors such as topography (Figure 7b). This result indicates that the long-term trend of sea level pressure is the dominant factor regulating the long-term trend of the wind field.

This study utilizes OAFlux SST and reanalysis products like ERA5, which are subject to limitations in data spatial resolution and uncertainties in heat flux parameterization schemes. These factors may have introduced biases in the quantitative assessment of key ocean dynamic processes (such as upwelling intensity and heat flux exchange). Although these limitations might affect the detailed interpretation of mechanisms, cross-validation results from multiple data sources support this study’s primary conclusion of intensified upwelling trends.

This study has analyzed and discussed the contributions of wind stress and its curl, surface heat fluxes, and horizontal heat transport to the long-term SST trends in the Oman upwelling region. However, a comprehensive understanding of all drivers influencing climatic variability in this area (e.g., ENSO modulation and air–sea feedback processes) would require integrated multivariate analysis (e.g., EOF analysis) and longer time series of datasets for thorough investigation. These detailed driving mechanisms await further research in future studies.

5. Conclusions

This study investigates the long-term trend and underlying mechanisms of Oman coastal upwelling under global warming, utilizing multisource data including SST, wind stress and its curl, air–sea heat fluxes, ocean currents, and sea level pressure during the summers of 1993–2018. The main findings are as follows:

Oman upwelling exhibited a significant intensification trend during 1993–2018, with a linear rate of 0.036 °C·year⁻¹, leading to continuous SST cooling there. This trend is primarily driven by the sustained increase in the wind stress curl over the upwelling region. In contrast, the impact of horizontal oceanic heat transport on the long-term SST trend is negligible. Additionally, the enhanced surface heating dominated by shortwave radiation flux led to warming SST, and partly offset the cooling effect from the strengthened upwelling.

The aforementioned strengthening positive wind stress curl in the Oman upwelling region during the summers of 1993–2018 is closely related to the uneven response of sea level pressure between the land area and the ocean area to global warming. Specifically, sea level pressure over the western Arabian Sea increased significantly, whereas land pressure over the Arabian and Somali Peninsulas decreased continuously, resulting in an increased atmospheric pressure gradient between the land and sea. This led to a significant increase in the wind stress over the offshore Oman region, with no significant change in the wind stress over the nearshore region, thereby promoting a continuous increase in the positive wind stress curl, which lead to the intensification of Oman upwelling.