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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Oceans and Coastal Zones".

Deadline for manuscript submissions: closed (29 April 2019) | Viewed by 38791

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Special Issue Editors

Dr. Miroslav Gačić

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Guest Editor

Senior Scientist, National Institute of Oceanography and Applied Geophysics - OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico (Trieste), Italy
Interests: coastal oceanography; strait dynamics; long-term variations; winter convection; experimental approach; Mediterranean
Special Issues, Collections and Topics in MDPI journals

Dr. Manuel Bensi

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Guest Editor

National Institute of Oceanography and Experimental Geophysics, Borgo Grotta Gigante 42-C, I-34010 Trieste, Italy
Interests: abyssal circulation; thermohaline properties; mesoscle variability; experimental oceanography; polar oceanography; Mediterranean circulation

Special Issue Information

Dear Colleagues,

Ocean circulation generated by the wind and/or by density gradients contributes to water property exchange between different parts of the ocean or between semi-enclosed seas and adjacent oceanic areas. Different space and time scales characterize ocean exchange and circulation. In addition to the mean circulation, basin-scale, and sub-basin flows, mesoscale eddies and internal processes (e.g., mixing induced by bottom roughness, internal waves, etc.) contribute to re-distribution of ocean properties and energy. Thermohaline oceanic circulation is driven by the winter convection and dense-water formation processes that are thus directly influenced by winter climatic conditions. Long-term and climatic changes in circulation and in the vertical mixing processes directly influence the variability of the biogeochemical properties of the ocean. A special role in trapping and/or transporting the biogeochemical properties of sea water is played by travelling eddies; however, this is yet to be quantified. This issue is open to all papers addressing the processes, which are associated with ocean circulation and mixing in both oceanic areas and semi-enclosed seas. It will also deal with the implications of circulation on biogeochemical properties and marine pollution.

Dr. Miroslav Gacic
Dr. Manuel Bensi
Guest Editors

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Keywords

ocean circulation
long-term variability
vertical mixing
mesoscale eddies
biogeochemical properties

Published Papers (10 papers)

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Editorial

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5 pages, 199 KiB

Open AccessEditorial

Ocean Exchange and Circulation

by Miroslav Gačić and Manuel Bensi

Water 2020, 12(3), 882; https://doi.org/10.3390/w12030882 - 20 Mar 2020

Cited by 1 | Viewed by 1899

Abstract

The great spatial and temporal variability, which characterizes the marine environment, requires a huge effort to be observed and studied properly since changes in circulation and mixing processes directly influence the variability of the physical and biogeochemical properties. A multi-platform approach and a collaborative effort, in addition to optimizing both data collection and quality, is needed to bring the scientific community to more efficient monitoring and predicting of the world ocean processes. This Special Issue consists of nine original scientific articles that address oceanic circulation and water mass exchange. Most of them deal with mean circulation, basin and sub-basin-scale flows, mesoscale eddies, and internal processes (e.g., mixing and internal waves) that contribute to the redistribution of oceanic properties and energy within the ocean. One paper deals with numerical modelling application finalized to evaluate the capacity of coastal vegetated areas to mitigate the impact of a tsunami. The study areas in which these topics are developed include both oceanic areas and semi-enclosed seas such as the Mediterranean Sea, the Norwegian Sea and the Fram Strait, the South China Sea, and the Northwest Pacific. Scientific findings presented in this Special Issue highlight how a combination of various modern observation techniques can improve our understanding of the complex physical and biogeochemical processes in the ocean. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

Research

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26 pages, 6538 KiB

Open AccessArticle

Levantine Intermediate and Levantine Deep Water Formation: An Argo Float Study from 2001 to 2017

by Elisabeth Kubin, Pierre-Marie Poulain, Elena Mauri, Milena Menna and Giulio Notarstefano

Water 2019, 11(9), 1781; https://doi.org/10.3390/w11091781 - 27 Aug 2019

Cited by 21 | Viewed by 5642

Abstract

Levantine intermediate water (LIW) is formed in the Levantine Sea (Eastern Mediterranean) and spreads throughout the Mediterranean at intermediate depths, following the general circulation. The LIW, characterized by high salinity and relatively high temperatures, is one of the main contributors of the Mediterranean Overturning Circulation and influences the mechanisms of deep water formation in the Western and Eastern Mediterranean sub-basins. In this study, the LIW and Levantine deep water (LDW) formation processes are investigated using Argo float data from 2001 to 2017 in the Northwestern Levantine Sea (NWLS), the larger area around Rhodes Gyre (RG). To find pronounced events of LIW and LDW formation, more than 800 Argo profiles were analyzed visually. Events of LIW and LDW formation captured by the Argo float data are compared to buoyancy, heat and freshwater fluxes, sea surface height (SSH), and sea surface temperature (SST). All pronounced events (with a mixed layer depth (MLD) deeper than 250 m) of dense water formation were characterized by low surface temperatures and strongly negative SSH. The formation of intermediate water with typical LIW characteristics (potential temperature > 15 °C, salinity > 39 psu) occurred mainly along the Northern coastline, while LDW formation (13.7 °C < potential temperature < 14.5 °C, 38.8 psu < salinity < 38.9 psu) occurred during strong convection events within temporary and strongly depressed mesoscale eddies in the center of RG. This study reveals and confirms the important contribution of boundary currents in ventilating the interior ocean and therefore underlines the need to rethink the drivers and contributors of the thermohaline circulation of the Mediterranean Sea. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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23 pages, 9789 KiB

Open AccessArticle

On the Variability of the Circulation and Water Mass Properties in the Eastern Levantine Sea between September 2016–August 2017

by Elena Mauri, Lina Sitz, Riccardo Gerin, Pierre-Marie Poulain, Daniel Hayes and Hezi Gildor

Water 2019, 11(9), 1741; https://doi.org/10.3390/w11091741 - 21 Aug 2019

Cited by 26 | Viewed by 3894

Abstract

The surface circulation and the thermohaline properties of the water masses of the eastern Levantine Sea (Mediterranean Sea) were monitored with mobile autonomous systems (surface drifters and gliders) during the period September 2016–August 2017. The drifters provided data for more than a year and revealed complex circulation features at scales ranging from the basin scale to the sub-mesoscale. Three drifters were captured in a semi-permanent gyre (Cyprus Eddy) allowing a quantitative study of its kinematics. During the experiment, three gliders were operated, in two different periods: September to December 2016 and February to March 2017. The autonomous instruments crossed the prevailing sub-basin structures several times. The collected in-situ observations were analyzed and interpreted in concert with remote sensing products (sea surface temperature and altimetry). The evolution of some of the prevailing features confirmed the complexity of the circulation of the basin. The Cyprus Eddy is the most persistent anticyclone, moving its geographical position and sometimes merging with the North Shikmona Eddy in a bigger structure. The gliders sampled this wide anticyclonic feature revealing its vertical structure in the two different periods. In fall, in stratified conditions, a high salinity core is evident below the thermocline. The isopycnals are characterized by an upward bending over the high salinity lens and a downward bending below it, typical of an anticyclonic modewater eddy. In winter, the core disappears following the vertical mixing that, homogenizes the upper Cyprus Eddy water down to 300 m. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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21 pages, 4232 KiB

Open AccessArticle

Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea

by Danijela Šantić, Vedrana Kovačević, Manuel Bensi, Michele Giani, Ana Vrdoljak Tomaš, Marin Ordulj, Chiara Santinelli, Stefanija Šestanović, Mladen Šolić and Branka Grbec

Water 2019, 11(8), 1655; https://doi.org/10.3390/w11081655 - 10 Aug 2019

Cited by 12 | Viewed by 3713

Abstract

Southern Adriatic (Eastern Mediterranean Sea) is a region strongly dominated by large-scale oceanographic processes and local open-ocean dense water formation. In this study, picoplankton biomass, distribution, and activity were examined during two oceanographic cruises and analyzed in relation to environmental parameters and hydrographic conditions comparing pre and post-winter phases (December 2015, April 2016). Picoplankton density with the domination of autotrophic biomasses was higher in the pre-winter phase when significant amounts of picoaoutotrophs were also found in the meso-and bathy-pelagic layers, while Synechococcus dominated the picoautotrophic group. Higher values of bacterial production and domination of High Nucleic Acid content bacteria (HNA bacteria) were found in deep waters, especially during the post-winter phase, suggesting that bacteria can have an active role in the deep-sea environment. Aerobic anoxygenic phototrophic bacteria accounted for a small proportion of total heterotrophic bacteria but contributed up to 4% of bacterial carbon content. Changes in the picoplankton community were mainly driven by nutrient availability, heterotrophic nanoflagellates abundance, and water mass movements and mixing. Our results suggest that autotrophic and heterotrophic members of the picoplankton community are an important carbon source in the food web in the deep-sea, as well as in the epipelagic layer. Besides, viral lysis may affect the activity of the picoplankton community and enrich the water column with dissolved organic carbon. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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19 pages, 7433 KiB

Open AccessArticle

Sea Surface Circulation Structures in the Malta-Sicily Channel from Remote Sensing Data

by Nydia C. Reyes Suarez, Michael S. Cook, Miroslav Gačić, Jeffrey D. Paduan, Aldo Drago and Vanessa Cardin

Water 2019, 11(8), 1589; https://doi.org/10.3390/w11081589 - 31 Jul 2019

Cited by 9 | Viewed by 4686

Abstract

The Malta-Sicily Channel is part of the Sicily Channel system where water and thermohaline properties between the Eastern and Western Mediterranean basins take place. Several mesoscales features are detached from the main circulation due to wind and bathymetric forcing. In this paper, surface circulation structures are studied using different remotely sensed datasets: satellite data (absolute dynamic topography, Cross-Calibrated Multi-Platform wind vector analysis, satellite chlorophyll and sea surface temperature) and high frequency radar data. We identified high frequency motions (at short time scales—hours to days), as well as mesoscale structures fundamental for the understanding of the Malta-Sicily Channel circulation dynamics. One of those is the Malta-Sicily Gyre; an anticyclonic structure trapped between the Sicilian and Maltese coasts, which is poorly studied in the literature and often confused with the Malta Channel Crest and the Ionian Shelf Break Vortex. In order to characterize this gyre, we calculated its kinetic properties taking advantage of the fine-scale temporal and spatial resolution of the high frequency radar data, and thus confirming its presence with an updated version of the surface circulation patterns in the area. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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23 pages, 12529 KiB

Open AccessArticle

New Insights of the Sicily Channel and Southern Tyrrhenian Sea Variability

by Milena Menna, Pierre-Marie Poulain, Daniele Ciani, Andrea Doglioli, Giulio Notarstefano, Riccardo Gerin, Marie-Helene Rio, Rosalia Santoleri, Adam Gauci and Aldo Drago

Water 2019, 11(7), 1355; https://doi.org/10.3390/w11071355 - 29 Jun 2019

Cited by 20 | Viewed by 4113

Abstract

The dynamics of the Sicily Channel and the southern Tyrrhenian Sea are highly influenced by the seasonal variability of the Mediterranean basin-wide circulation, by the interannual variability of the numerous mesoscale structures present in the Channel, and by the decadal variability of the adjacent Ionian Sea. In the present study, all these aspects are investigated using in-situ (Lagrangian drifter trajectories and Argo float profiles) and satellite data (Absolute Dynamic Topography, Sea Level Anomaly, Sea Surface Temperature, wind products) over the period from 1993 to 2018. The availability of long time series of data and high-resolution multi-sensor surface currents allow us to add new details on the circulation features and on their driving mechanisms and to detect new permanent eddies not yet described in literature. The structures prevailing in winter are mainly driven by wind, whereas those prevailing in summer are regulated by topographical forcing on surface currents. The strength of the surface structures located at the western entrance of the Ionian Sea and of the mesoscale activity along the northern Sicily coast is modulated by the large-scale internal variability. The vertical hydrological characteristics of these mesoscale eddies are delineated using the Argo float profiles inside these structures. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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19 pages, 6048 KiB

Open AccessArticle

A Census of the 1993–2016 Complex Mesoscale Eddy Processes in the South China Sea

by Huimeng Wang, Yunyan Du, Fuyuan Liang, Yong Sun and Jiawei Yi

Water 2019, 11(6), 1208; https://doi.org/10.3390/w11061208 - 10 Jun 2019

Cited by 6 | Viewed by 2713

Abstract

Mesoscale eddy process with at least one splitting and/or merging event can be defined as either a complex process or a simple process. Investigation of the difference between these two categories could provide new insights into how different factors, such as the seabed topography, Kuroshio intrusion, and winds, affect the origin, migration, and decay of the mesoscale eddies. This study compared the characteristics of the complex against the simple eddy processes in the South China Sea (SCS) from 1993 to 2016. We comprehensively analyzed the eddy processes with regards to their characteristic points, trajectories, and networks. The simple and complex processes share many similarities but do show significantly different behaviors. Both the simple and complex processes mainly start from the eastern SCS. However, the complex processes mainly vanish in the western SCS whereas the simple processes disappear almost everywhere across the SCS. The complex processes last longer and migrate more than the simple processes. Lastly, the complex processes mainly move westward within the community. The complex processes can be further categorized into complex anticyclonic and cyclonic eddy processes. Spatially, the splitting and merging events mainly occur in the southwest of Taiwan, northwest of the Luzon Island, and the southeast of Vietnam. Temporally, the merging and splitting events mainly occur in the fall. The interaction among the communities reveals the different migration patterns of the complex anticyclonic and cyclonic eddy processes in the SCS. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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20 pages, 9539 KiB

Open AccessArticle

Deep Flow Variability Offshore South-West Svalbard (Fram Strait)

by Manuel Bensi, Vedrana Kovačević, Leonardo Langone, Stefano Aliani, Laura Ursella, Ilona Goszczko, Thomas Soltwedel, Ragnheid Skogseth, Frank Nilsen, Davide Deponte, Paolo Mansutti, Roberto Laterza, Michele Rebesco, Leonardo Rui, Renata Giulia Lucchi, Anna Wåhlin, Angelo Viola, Agnieszka Beszczynska-Möller and Angelo Rubino

Water 2019, 11(4), 683; https://doi.org/10.3390/w11040683 - 02 Apr 2019

Cited by 9 | Viewed by 5186

Abstract

Water mass generation and mixing in the eastern Fram Strait are strongly influenced by the interaction between Atlantic and Arctic waters and by the local atmospheric forcing, which produce dense water that substantially contributes to maintaining the global thermohaline circulation. The West Spitsbergen margin is an ideal area to study such processes. Hence, in order to investigate the deep flow variability on short-term, seasonal, and multiannual timescales, two moorings were deployed at ~1040 m depth on the southwest Spitsbergen continental slope. We present and discuss time series data collected between June 2014 and June 2016. They reveal thermohaline and current fluctuations that were largest from October to April, when the deep layer, typically occupied by Norwegian Sea Deep Water, was perturbed by sporadic intrusions of warmer, saltier, and less dense water. Surprisingly, the observed anomalies occurred quasi-simultaneously at both sites, despite their distance (~170 km). We argue that these anomalies may arise mainly by the effect of topographically trapped waves excited and modulated by atmospheric forcing. Propagation of internal waves causes a change in the vertical distribution of the Atlantic water, which can reach deep layers. During such events, strong currents typically precede thermohaline variations without significant changes in turbidity. However, turbidity increases during April–June in concomitance with enhanced downslope currents. Since prolonged injections of warm water within the deep layer could lead to a progressive reduction of the density of the abyssal water moving toward the Arctic Ocean, understanding the interplay between shelf, slope, and deep waters along the west Spitsbergen margin could be crucial for making projections on future changes in the global thermohaline circulation. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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12 pages, 7078 KiB

Open AccessArticle

The Four Patterns of the East Branch of the Kuroshio Bifurcation in the Luzon Strait

by Ruili Sun, Fangguo Zhai and Yanzhen Gu

Water 2018, 10(12), 1822; https://doi.org/10.3390/w10121822 - 10 Dec 2018

Cited by 4 | Viewed by 3370

Abstract

Based on the self-organizing map (SOM) method, a suite of satellite measurement data, and Hybrid Coordinate Ocean Model (HYCOM) reanalysis data, the east branch of the Kuroshio bifurcation is found to have four coherent patterns associated with mesoscale eddies in the Pacific Ocean: anomalous southward, anomalous eastward, anomalous northward, and anomalous westward. The robust clockwise cycle of the four patterns causes significant intraseasonal variation of 62.2 days for the east branch. Furthermore, the study shows that the four patterns of the east branch of the Kuroshio bifurcation can influence the horizontal and vertical distribution of local sea temperature. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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22 pages, 12594 KiB

Open AccessArticle

Numerical Investigations of Tsunami Run-Up and Flow Structure on Coastal Vegetated Beaches

by Hongxing Zhang, Mingliang Zhang, Tianping Xu and Jun Tang

Water 2018, 10(12), 1776; https://doi.org/10.3390/w10121776 - 03 Dec 2018

Cited by 5 | Viewed by 2849

Abstract

Tsunami waves become hazardous when they reach the coast. In South and Southeast Asian countries, coastal forest is widely utilized as a natural approach to mitigate tsunami damage. In this study, a depth-integrated numerical model was established to simulate wave propagation in a coastal region with and without forest cover. This numerical model was based on a finite volume Roe-type scheme, and was developed to solve the governing equations with the option of treating either a wet or dry wave front boundary. The governing equations were modified by adding a drag force term caused by vegetation. First, the model was validated for the case of solitary wave (breaking and non-breaking) run-up and run-down on a sloping beach, and long periodic wave propagation was investigated on a partially vegetated beach. The simulated results agree well with the measured data. Further, tsunami wave propagation on an actual-scale slope covered by coastal forest Pandanus odoratissimus (P. odoratissimus) and Casuarina equisetifolia (C. equisetifolia) was simulated to elucidate the influence of vegetation on tsunami mitigation with a different forest open gap. The numerical results revealed that coastal vegetation on sloping beach has significant potential to mitigate the impacts from tsunami waves by acting as a buffer zone. Coastal vegetation with open gaps causes the peak flow velocity at the exit of the gap to increase, and reduces the peak flow velocity behind the forest. Compared to a forest with open gaps in a linear arrangement, specific arrangements of gaps in the forest can increase the energy attenuation from tsunami wave. The results also showed that different cost-effective natural strategies in varying forest parameters including vegetation collocations, densities, and growth stages had significant impacts in reducing the severity of tsunami damage. Full article

(This article belongs to the Special Issue Ocean Exchange and Circulation)

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