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Rapid rise of salinity is observed in the Black Sea in recent years, with the largest positive trend (0.07 psμ per 10 years) detected in the pycnocline. We use long-term hydrological measurements for 1985–2019 to show that salinity of pycnocline has intense seasonal and interannual variability modulated by the mechanical and convective mixing. In the warm period of a year, shear turbulence driven by strong winds and intense geostrophic currents causes the penetration of warm waters into the lower density layers. This is accompanied by the rise in their salinity, the source of which is the deep saline waters situated below the halocline. This process is most intense in the areas of downwelling and intensifies in the autumn period, when thermal stratification is relatively weak. Another important reason is the entrainment of salty Mediterranean waters in the upper part of the Black Sea halocline, which is modulated by the deepening of the seasonal thermocline near the Bosphorus strait and mechanical mixing. The increase of salinity is compensated during cold winters, when convective mixing transports fresher water influenced by river discharge into lower density layers of the basin and causes a decrease in pycnocline salinity. This process is most intense in the center of the cyclonic gyres, where pycnocline is located closer to the surface and winter temperature reaches minimal values. Due to the long-term warming of the Black Sea, the process of freshening of deep layers now is observed only in rare cold years. At the same time, an intensification of wind speed, vorticity, and geostrophic circulation processes promote the blurring of the halocline and the rise of the salinity of the Black Sea upper layers. Such rise begins after 2007 in the upper part of Black Sea halocline (depth 50–100 m) and is traced down to 250 m by 2020. • Global warming suppresses the freshening of pycnocline during winter convection. • Shear turbulence in warm period causes the entrainment of salt from deep layers. • Injections of Mediterranean waters in pycnocline intensify in warm conditions. • Intensification of currents and wind, and rise of winter temperature is observed. • All this processes cause rapid rise of pycnocline salinity in the Black Sea. [ABSTRACT FROM AUTHOR]

The Southwestern Atlantic (SWA) is characterized by its large Eddy Kinetic Energy as the result of the confluence of two major western boundary currents, the northward flowing Malvinas Current (MC) and the southward flowing Brazil Current. The SWA study was addressed in the literature based on altimetry data, in situ measurements, regional models and ocean reanalysis. The present study constitutes the first effort to sample a portion of the SWA, with a dense drifter array (N = 62) deployment. The drifters, drogued at 15 m depths, were deployed across the MC and the Argentine Continental Shelf along two zonal transects located at 47°S and 47.25°S, between the 8th and the September 9, 2021. Drifters were set to deliver their position every 10 and 60 min, providing accurate Lagrangian trajectories that provide information on a large range of space and time scales of the surface currents. Three regions are clearly identified based on the analysis of the speed of the drifters, of their trajectories and of the spectral density of their velocities: the continental shelf, the slope and the open ocean. The comparison of the trajectories of the drifters with satellite altimetry images shows that, in general, drifters follow mesoscale features that are detectable in satellite altimetry maps. The analysis of the drifter trajectories also allowed us the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data. Comparison with cloud-free, high-resolution color images, shows that drifter trajectories organized by the mesoscale flow might also locally follow sub-mesoscale features. In frontal regions it was found that drifter velocities double satellite altimetry geostrophic velocities, which suggests that the dynamics at those regions is largely dominated by ageostrophic components. The ageostrophic Ekman component might explain the direction of the drifters when strong winds from a given direction prevail for several days and the drifters are not in a region with large sea surface height (SSH) gradients. The joint analysis of drifters' trajectory and SSH clearly depicts that mesoscale features on the open ocean region control the cross-shelf exchanges between the MC and open ocean regions as well as the strength and width of the MC. Finally, the spatial density distribution of the drifters during the first hours after deployment and within a small eddy also allowed us to characterize the flow in terms of its divergence, vorticity and strain, indicating that the MC is geostrophic and has a jet-like behavior while the eddy is largely ageostrophic and has a dominant vorticity component over strain. We conclude observing that the analysis of a dense array of drifters provides valuable information of the flow that cannot be attained solely based on satellite data. A dedicated experiment based on the deployment of 62 surface drifters, an array of unprecedented dimension for the region, was devised to improve our understanding of the surface circulation of the Argentine Continental Shelf (ACS), the Patagonian Slope and, generally, the Southwestern Atlantic Ocean. The main highlights derived from the analysis are. • Three regions were clearly identified based on the analysis of the speed of the drifters, their trajectories and the spectral density of their velocities: the continental shelf, the slope and the open ocean. • Within the open ocean region, drifters might exceed 1.8 m s−1 when trapped by the Brazil-Malvinas Confluence (BMC), along the northern branch of the Subantarctic Front, or between large mesoscale eddies of different polarities. • Within the slope region, thanks to the deployment strategy adopted, fine scale details of the Malvinas Current (MC) have been unveiled for the first time: large shears between jets of the MC (up to 20 cm s−1 between drifters separated by only 2.5 km) and the clear modulation of the trajectories of the drifters due to tidal currents, despite the dominant MC. • Over the ACS, drifters showed how tides are, as expected, the main forcing of the shelf circulation. However, small scale patterns may also modulate shelf currents. The origin of these patterns should be further investigated. • Clear examples of the accuracy of the geostrophic currents predicted by satellite altimetry in the different regions are provided. In general, currents in the slope and open ocean region are very well represented by the satellite altimetry estimations, at least for features at the resolved mesoscale. • The analysis of drifter trajectories also allowed the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data but are visible in color images. • In frontal regions it was found that the recorded drifter velocities double satellite altimetry geostrophic velocities, suggesting that the dynamics at those regions is largely dominated by unresolved components. • First in situ evidence that mesoscale features on the open ocean region control the cross-shelf exchanges between the slope and open ocean regions as well as the strength and width of the MC, is provided. • The analysis of drifter triplets allowed us to characterize the flow in terms of its divergence, vorticity and strain. The analysis confirms that within the MC strain dominates, indicating that the MC is in geostrophic balance and has a jet-like behavior. Within a small mesoscale cyclonic eddy that trapped three drifters for more than 20 days, negative vorticity prevailed. • The potential role of Ekman transport to promote exchanges between the slope and adjacent regions is discussed. It is proposed that wind may impact drifter trajectories only in regions exhibiting small Sea Surface Height (SSH) gradients. [ABSTRACT FROM AUTHOR]

The self-organizing map (SOM) is implemented to explore various ocean surface current patterns in the Bay of Bengal (BoB) from 1993 – 2015 based on a satellite sensor-derived current product estimated by combining geostrophic, wind-driven, and temperature-driven current components. Four major patterns/groups corresponding to some of the primary current systems in the BoB are obtained (G1, G2, G3, and G4). The start and end of all the patterns drastically varied year to year and are primarily related to positive and negative Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) years. The correlation of ENSO and the following year's transition date from WBC to SMC is 0.70, while -0.76 for IOD and transition date from EICC to WMC. Through temporal analysis and monthly frequency of occurrence measurements, this study reveals that the first significant dominant pattern is observed during the summer monsoon season when the southwest monsoon current (SMC) is visible near the south of Sri Lanka (pattern G1). The following major pattern is the winter monsoon current (WMC) in the south of Sri Lanka (pattern G4), whose formation is contributed by the northeast monsoon and westward propagating Rossby wave. The other two dominating patterns are East India Coastal Current (EICC, G2) and Western Boundary Current (WBC, G3). SOM analysis is then performed on WBC with 9 nodes (3 × 3 , n a m e d P 1 t o P 9) , which captures the meandering nature (P3), and meandering in the presence of SMC (P2), the straight nature (P7) and in the presence of SMC (P4). The spatial and temporal analysis of nine SOM patterns (P1 – P9) suggests that the non-meandering nature of WBC (P4) occurs more frequently than meandering WBC (P2). • Surface Circulation can be efficiently captured by Self-Organizing Maps (SOM). • Southwest monsoon current is a dominant feature with 44.43% occurrence in a year. • Variability of transition of boundary currents are influenced by IOD and ENSO. • The non-meandering WBC is more frequent than the meandering ones. [ABSTRACT FROM AUTHOR]

The Lofoten Vortex (LV), a quasi-permanent anticyclonic eddy in the Lofoten Basin of the Norwegian Sea, is investigated with an eddy-permitting primitive equation model nested into the ECCO2 ocean state estimate. The LV, as simulated by the model, extends from the sea surface to the ocean bottom at about 3000 m and has the subsurface core between 50 m and 1100 m depths. Above and below the vortex core the relative vorticity signal decreases in amplitude while the radius increases by as much as 25–30% relative to the values in the core. Analyzing the model run, we show that the vertical structure of the LV can be casted into four standard configurations, each of which forms a distinct cluster in the parameter space of potential vorticity anomalies in and above the LV core. The stability of the LV for each of the configurations is then studied with three-layer and a two-layer (in winter) quasi-geostrophic (QG) models over a flat bottom as well as over a realistic topography. The QG results show a number of common features with those of the primitive equation model. Thus, among the azimuthal modes dominating the LV instability, both the QG model and the primitive equation model show a major role the 2nd and 3rd modes. In the QG model simulations the LV is the subject of a rather strong dynamic instability, penetrating deep into the core. The results predict 50–95% volume loss from the vortex within 4–5 months. Such a drastic effect is not observed in the primitive equation model, where, for the same intensity of perturbations, only 10–30% volume loss during the same period is detected. Taking into account the gently sloping topography of the central part of the Lofoten basin and the mean flow in the QG model, brings the rate of development of instability close to that in the primitive equation model. Some remaining differences in the two models are discussed. Overall, the LV decay rate obtained in the models is slow enough for eddy mergers and convection to restore the thermodynamic properties of the LV, primarily re-building its potential energy anomaly. This justifies the quasi-permanent presence of the LV in the Lofoten Basin. [ABSTRACT FROM AUTHOR]

This work updates the methods of Lumpkin and Johnson (2013) to obtain an improved near-surface velocity climatology for the global ocean using observations from undrogued and 15-m drogued Global Drifter Program (GDP) drifters. The proposed procedure includes the correction of the slip bias of undrogued drifters, thus recovering about half of the GDP dataset; and a new approach for decomposing Lagrangian data into mean, seasonal and eddy components, which reduces the smoothing of spatial gradients inherent in data binning methods. The sensitivity of the results to method parameters, the method performance relative to other techniques, and the associated estimation errors, are evaluated using statistics calculated for a test dataset consisting of altimeter-derived geostrophic velocities subsampled at the drifter locations, and for the full altimeter-derived geostrophic velocity fields. It is demonstrated that (1) the correction of drifter slip bias produces statistically similar mean velocities for both drogued and undrogued drifter datasets at most latitudes and reduces differences between their variance estimates, (2) the proposed decomposition method produces pseudo-Eulerian mean fields with magnitudes and horizontal scales closer to time-averaged Eulerian observations than other methods, and (3) standard errors calculated for pseudo-Eulerian quantities underestimate the real errors by a factor of almost two. The improved decomposition method and the inclusion of undrogued drifters in the analysis allows resolving details of the time-mean circulation not well defined in the previous version of the climatology, such as the cross-stream structure of western boundary currents, recirculation cells, and zonally-elongated mid-ocean striations. [ABSTRACT FROM AUTHOR]

To understand long-term mean and seasonal variations of the Oyashio transport off the southeastern coast of Hokkaido, time series of estimated transports were generated by combining satellite altimetry and CTD data from the period 1993–2014 along a monitoring transect referred to as the “A-line”. Linear regressions between CTD-derived geostrophic transport and altimetry-derived sea level anomaly differences were calculated by minimizing a cost function newly proposed in this study. The horizontal structure of 21-year mean transport was characterized by a southwestward flow on the continental slope (SW-1) associated with the First Oyashio Intrusion, a northeastward flow (NE-2) likely trapped against the Kuril-Kamchatka trench, a southwestward flow (SW-3) related to the Second Oyashio Intrusion, and a very stable northeastward flow (NE-4) corresponding to a combination of the Oyashio Return Flow and quasi-stationary Jet. Analysis of long-term mean monthly transports revealed four alternating flows that were present throughout all seasons of the year. The boundaries between adjacent pairs of the four flows on the A-line did not change on a seasonal timescale as much as the southernmost latitude of the First Oyashio Intrusion. At a 0-month (1-month) time lag, the sum of the SW-1, NE-2, and SW-3 (SW-1 and NE-2) transports, which corresponds to net Oyashio transport (partial net Oyashio transport related to the southernmost latitude of the First Oyashio Intrusion), was correlated with Sverdrup transport more robustly than was each of the four alternating flows. This result suggests that as the Oyashio crossed the A-line, it responded barotropically to basin-scale wind stress by adjusting transports composed of both southwestward and northeastward flows. [ABSTRACT FROM AUTHOR]

The circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component U ζ and the wind-driven component U τ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO. • Drifter trajectories describe three seasonally dependent circulation patterns in the western tropical Indian Ocean. • The geostrophic component U ζ and wind-driven component U τ predominate the mean state and the soasonal variability of circulation. • The 3-D structures, show seasonal variability associated with monsoon transitions. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean receives the world's most significant tropical ocean rain and river runoff per unit area. The 3-dimensional spreading of Rain and River Water (RRW) entering the tropical Indian Ocean and associated salinity and circulation anomalies are explored for 60 years using ocean reanalysis data tailored to a tracer transport model. Over 60 years, the cumulative impact of RRW entering the tropical Indian Ocean is to freshen the Indian Ocean basin as large as 2.0–0.1 p.s.u from the surface to 500 m via subduction and transport. The RRW propagates to a vast extent of the Indian Ocean and a limited extent of the Atlantic and Pacific Oceans via general circulation pathways. A quasi-equilibrium model of accumulation of RRW over the tropical Indian Ocean suggests that it induces clockwise geostrophic currents from the Bay of Bengal to the Arabian Sea over 0–500 m depths with a net transport tendency of 0.80 ± 0.14 Sv year−1. The study implies that coupled climate models with precipitation biases may miscalculate such salinity and circulation anomalies due to RRW and aggravating biases in simulated climate dynamics. • The spreading of rain and river water entering the tropical Indian Ocean for 60 yr is identified from ocean reanalysis data. • RRW mixes in the surface, subducts and spreads, offering a dilution in salinity and expansion in dynamic heights. • RRW offers a net transport tendency of water from the Bay of Bengal to the Arabian Sea at a rate of 0.80 ± 0.14 Sv year−1. [ABSTRACT FROM AUTHOR]

Mesoscale eddies can elevate turbulent mixing, but the mechanisms have not been fully understood. Based on a field experiment in two mesoscale eddies near the critical latitude of the M 2 internal tides in the northwestern Pacific Ocean, enhanced turbulent mixing was found both in and below the mixed layer. Microstructure observations indicate that enhanced turbulent mixing mainly appeared outside the eddy center in the mixed layer (approximately 117 m), which was speculated to be related to geostrophic current shear. By analyzing the CTD and Argo measurements through a fine-scale parameterization method, enhanced turbulent mixing was found to be concentrated at the southern flank of the anticyclonic eddy below 300 m, which showed little correlation with topographic roughness and local winds. Based on a ray-tracing model, the wind-generated near-inertial waves at higher latitudes were also excluded as the cause of the enhanced turbulent mixing. The enhanced turbulent mixing below 300 m was generally consistent with the features of subthermocline eddies and eddy-modulated parametric subharmonic instability (PSI) of the M 2 internal tides. However, due to the limitation of observations, the eddy-modulated PSI mechanism was not fully validated. • Enhanced turbulent mixing was found in and below mixed layer in mesoscale eddies. • Geostrophic current shear may elevate turbulent mixing in the mixed layer. • Enhanced mixing below 300 m was consistent with features of eddy-modulated PSI. • Subthermocline eddy may also contribute to the enhanced mixing below 300 m. [ABSTRACT FROM AUTHOR]

Spatial and temporal variability of the island mass effect (IME; defined as local increases of phytoplankton associated with the presence of islands) at the Juan Fernández Archipelago (JFA) is analyzed using chlorophyll-a (Chl-a) satellite data, altimetry, sea surface temperature, wind, geostrophic currents and net heat flux over a ten year period (2002–2012). The the JFA islands (Robinson Crusoe-Santa Clara (RC-SC) and Alejandro Selkirk (AS)) present wakes with significant Chl-a increases, mainly during spring time. These wakes can reach Chl-a values of one order of magnitude higher (~1mgm−3) than the surrounding oligotrophic waters (<0.1mgm−3). The wakes are similar to von Kármán vortex streets which have been used to explain the impact of IME on Chl-a increases in numerical models. The wakes are formed from a high productivity area in the lee of the island, extending to the oceanic region as high Chl-a patches associated with submesoscale eddies that are detached from the islands and connected by less-productive zones. This pattern coincides with previous models that predict the effects of island-generated flow perturbations on biological production variability. The IME is a recurrent feature of islands that has even been observed in decadal average fields. In such average fields, the Chl-a values in RC-SC and AS islands can exceed values found in a Control Zone (a zone without islands) by ~50% and 30%, respectively. Seasonal and interannual variability reveals that, as a consequence of the IME, the winter Chl-a maximum associated with the development of winter convection and mesoscale eddies that propagate from the continental zone, promote that the Chl-a maximum extends towards spring. The IME has an impact on the island on both a local as well as a more regional scale that affects an area of ~40,000km2 (1°Latitude×4°Longitude) centered on the islands. The transport of high productivity patches associated with submesoscale eddies may be responsible for IME propagation at a regional scale. Around the islands, the presence of a weak oceanic incident flow and strong and recurrent wind-wakes, suggest that the generation of Chl-a wakes result from a combined effect between both forcings. [ABSTRACT FROM AUTHOR]

Abstract: Integrating the observations gathered by different platforms into a unique physical picture of the environment is a fundamental aspect of networked ocean observing systems. These are constituted by a spatially distributed set of sensors and platforms that simultaneously monitor a given ocean region. Remote sensing from satellites is an integral part of present ocean observing systems. Due to their autonomy, mobility and controllability, underwater gliders are envisioned to play a significant role in the development of networked ocean observatories. Exploiting synergism between remote sensing and underwater gliders is expected to result on a better characterization of the marine environment than using these observational sources individually. This study investigates a methodology to estimate the three dimensional distribution of geostrophic currents resulting from merging satellite altimetry and in situ samples gathered by a fleet of Slocum gliders. Specifically, the approach computes the volumetric or three dimensional distribution of absolute dynamic height (ADH) that minimizes the total energy of the system while being close to in situ observations and matching the absolute dynamic topography (ADT) observed from satellite at the sea surface. A three dimensional finite element technique is employed to solve the minimization problem. The methodology is validated making use of the dataset collected during the field experiment called Rapid Environmental Picture-2010 (REP-10) carried out by the NATO Undersea Research Center-NURC during August 2010. A marine region off-shore La Spezia (northwest coast of Italy) was sampled by a fleet of three coastal Slocum gliders. Results indicate that the geostrophic current field estimated from gliders and altimetry significantly improves the estimates obtained using only the data gathered by the glider fleet. [Copyright &y& Elsevier]

Abstract: Two episodic phytoplankton bloom events were identified in the southeastern and southwestern Bay of Bengal in January 1998 and December 2005, respectively, from SeaWiFS-derived chlorophyll-a concentration data collected between September 1997 and December 2010. Possible causes were examined using time series datasets of satellite-derived sea surface height anomalies, sea surface temperature, wind stress and Ekman pumping velocity data. It was found that strong cyclonic eddies with long residence time transported cold, nutrient-rich water from the subsurface layer to the euphotic zone, and were responsible for sustaining the continuous growth of phytoplankton that led to the two aforementioned phytoplankton blooms. Moreover, regarding the event in December 2005, three tropical cyclones passed through the region of the phytoplankton bloom and could have been additional factors that enhanced the bloom. Regarding the event in January 1998, transportation from the adjacent Andaman Sea by the geostrophic current and an anomalous zonal easterly wind contributed to the phytoplankton bloom in the southeastern Bay of Bengal. In addition, both blooms occurred during Indian Ocean Dipole (IOD) years. The absence of the second downwelling Kelvin waves in positive IOD year of 1997–1998 favouring the long residence of the strong cyclonic eddy in the eastern bay, and the strong wind-induced mixing in negative IOD year of 2005 might contribute to enhancing productivity. [Copyright &y& Elsevier]

Abstract: This paper quantitatively assesses the mesoscale spatial variability in vertical velocity associated with an open ocean eddy dipole. High-resolution, in situ data were collected during a research cruise aboard the NERC research ship RRS Discovery to the Iceland Basin in July/August 2007. A quasi-synoptic SeaSoar spatial survey revealed a southeastward flowing jet with counter-rotating eddies on either side. The anti-cyclonic component was identified as a mode water eddy, characterised by a homogenous core (∼35.5psu and 12°C) centred at a depth of ∼600m. Vertical velocities were calculated by inverting the quasi-geostrophic (QG) Omega equation at each point in a three-dimensional grid encompassing the dipole. The strongest vertical velocities (up to 5mday−1) were found primarily in the central jet between the eddies, as fast flowing water was forced over raised isopycnals associated with the large potential vorticity anomaly of the mode water eddy. Weaker upward (downward) vertical velocity was diagnosed ahead of the cyclonic (mode water) eddy in the direction of propagation, reaching 0.5mday−1 (2.5mday−1) at the depth of maximum potential vorticity (PV) anomaly. The results demonstrate that the mesoscale velocity field cannot be accurately reconstructed from analysis of individual isolated eddy features and that detailed three-dimensional maps of potential vorticity are required to quantify the cumulative effects of their interactions. An examination of potential sources of error associated with the vertical velocity diagnosis is presented, including sampling strategy, quasi-synopticity, sensitivity to interpolation length scale and the unquantified effect of lower boundary conditions. The first three of these errors are quantified as potentially reaching 50%, ∼20% and ∼25% of the calculated vertical velocity, respectively, indicating a potential margin of error in the vertical velocity diagnosis of order one. [Copyright &y& Elsevier]

Abstract: Nearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80TW)), and much larger than the annual average values (O(10–20TW)). [Copyright &y& Elsevier]

Abstract: There are two zonal bands of eminently high eddy kinetic energy (EKE) in the North Pacific Ocean. The highest one is located in the Kuroshio Extension and the second one is in the subtropical area. This paper focuses on the latter. An eddy detection scheme based on velocity vector geometry is applied to the SSHA-derived geostrophic currents to identify and track eddies, and to generate an eddy dataset which includes spatial and temporal information on eddy generation, evolution and termination. Analysis of this dataset allowed the investigation of a broad range of eddy parameters. From 1993 to 2010, about six thousand eddies with lifetime longer than eight weeks were generated within the band. All eddies moved westward, and both cyclonic and anticyclonic eddies deflected northward south of 21°N and southward north of 21°N, respectively. Statistically, three different stages of an eddy''s lifetime can be identified: the first one-fifth of its lifetime corresponds to the growing period; the successive three-fifths after that to its stable stage; the last one-fifth to its decaying period. Observed Argo vertical profiles collected within the detected eddy areas are used to investigate the eddy-induced vertical displacement of the thermocline and the halocline. Frontal intensity derived from the SST data is used to explain the mechanism modulating temporal and spatial eddy variations within the zonal band. [Copyright &y& Elsevier]

Abstract: The surface currents of the Levantine sub-basin (Mediterranean Sea) are described using 18 years (1992–2010) of drifter data and satellite-derived sea level anomalies. The combination of drifter and satellite data allowed to estimate maps of surface geostrophic circulation and to obtain more accurate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of surface currents are investigated with particular focus on the main sub-basin eddies of the eastern Levantine. The mean velocity field depicts the typical patterns of the along-slope and offshore currents and outlines the sub-regions where eddies are generated recurrently (west Egyptian coast, Ierapetra, Mersa-Matruh, south-west of Cyprus, Israel–Lebanon coast, Latakia) or persist steadily (Rhodes Gyre). Highly variable and energetic currents are observed between the Ierapetra and Mersa-Matruh regions, as the result of the interaction of the Mid-Mediterranean Jet meandering in between, and interacting with, the eddies generated by the instability of the coastal current. Seasonal pseudo-Eulerian maps show the current field stronger in summer and weaker in winter, mainly in the western Levantine and in the Cyprus–Syria Passage. The Shikmona Eddy displays a periodic nature with higher intensities during the cold months and an enhanced activity in the period 1998–2005. The Cyprus Eddy has a less periodic nature, characterised by events of high activity and periods in which it dominates as a single enlarged eddy in the southeast Levantine, eventually including the Shikmona Eddy. The Latakia Eddy is mainly cyclonic with higher intensities in summer and fall; occasional weekly or monthly inversions of circulation from cyclonic to anticyclonic are triggered by the interaction between the MMJ and the northward coastal meandering current. [Copyright &y& Elsevier]

Abstract: Variable properties of the Mediterranean outflow and the variability of the Western Alboran Gyre are analyzed by means of 5-year long time series of near bottom potential temperature at Espartel sill in the Strait of Gibraltar and altimetry data in the Alboran Sea. Geostrophic velocity at the southern edge of the gyre and potential temperature at Espartel sill are significantly correlated (correlation coefficient 0.67), suggesting that the intensification of the Alboran Gyre favors the ventilation of Western Mediterranean Deep Water. The analysis of historical temperature profiles shows that Western Mediterranean Deep Water in the Alboran Sea can be suctioned from a layer between 500 and 700m depth for typical changes of the gyre intensification. [Copyright &y& Elsevier]

Abstract: Four repeat sections across the East Greenland shelf and slope south of Denmark Strait are analysed to investigate the components of the boundary current system. The sections were occupied in summer 2001, 2003, 2004 and 2007, and included use of a vessel-mounted acoustic Doppler current profiler, enabling the computation of absolute geostrophic velocities. The components of the boundary current are the East Greenland/Irminger Current (EGIC) in the upper layer, the Deep Western Boundary Current (DWBC) at the base of the continental slope, and the East Greenland spill jet which resides inshore and beneath the EGIC. Special emphasis is placed on the spill jet, a recently discovered feature about which relatively little is known. The spill jet was observed in each occupation, transporting 5.0±2.2Sv equatorward in the mean, which is similar to the DWBC at this latitude (4.9±1.4Sv). The spill jet displayed considerable variability between sections, which appears to be linked to the geographical location of the upper-layer hydrographic front associated with the EGIC. When the front is located near the shelfbreak, the spill jet is confined to the outer shelf/upper slope and its transport is smaller. During these times there is less mixing and the water advected by the jet is generally lighter than that transported by the DWBC. In contrast, when the front is located seaward of the shelfbreak, the spill jet extends farther down the continental slope and its volume flux is larger. At these times, there is stronger mixing and the spill jet can transport water as dense as the Denmark Strait Overflow Water. A vorticity analysis indicates that the jet is susceptible to a variety of instability processes including baroclinic, barotropic and symmetric instability. In addition, it is subject to double diffusive mixing that may influence its downstream evolution. It appears that the spill jet is a permanent feature of the summertime circulation in this region and contributes significantly to the intermediate, and at times deep, limb of the Atlantic Meridional Overturning Circulation. [Copyright &y& Elsevier]

Abstract: Closed loop mesoscale eddies were identified and tracked in the Ulleung Basin of the southwestern Japan/East Sea (JES) using the winding-angle (WA) methodology, for mapping the absolute geostrophic currents into surface streamlines of flow. The geostrophic velocity used here was the sum of the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), time variable velocity and the 1992–2007 mean geostrophic velocity. Local sampling bias was removed using the drifter observations. This WA methodology of deriving the Lagrangian path lines that drifters followed over a 7-day period was validated by individual drifter tracks and it demonstrated closed looping eddy motions. The WA method demonstrated that less than 6% of the closed streamlines appeared when drifters did not show a closed loop in their vicinity, compared to 30% of the excess detection rate by the Okubo–Weiss method of locating closed loop structures. Three groups of eddies were identified: (1) Coastal Cold and Warm Eddies, which appeared in the area between the coast of southern Korea and the East Korean Warm Current (EKWC), when a southward coastal current was present, (2) Frontal Cold and Warm Eddies, which were formed in the region of the seaward extension of the meandering EKWC, north of Ulleung Island and (3) Ulleung Warm Eddies (UWE) and Dok Cold Eddies (DCE), which appeared during meanders of the EKWC, in the Ulleung Basin. No seasonal concentration for eddy generation and eddy population was found. The average radius of eddies was about 38–60km. These were born, moved in an erratic pattern and then died in the vicinity where the EKWC separated from the coast and formed a large meander. The time-mean large meander formed meridionally concentrated bands of positive and negative relative vorticity. The cyclonic (cold) eddies tend to reside within the band of positive time-mean relative vorticity, and the anticyclonic (warm) eddies reside within the bands of negative relative vorticity. Six UWE and four warm eddies, in the Yamato Basin (about 10% of warm eddies), were sustained longer than a year. Because the large meander of the EKWC appeared to be controlled by topography, and the JES is a nearly enclosed basin with rapid flow-out to the east through the narrow Tsugaru Strait, there was little eddy energy propagation to the west. The warm eddies in the southwestern part of the JES appeared to be interacting very locally with the mean flow. [Copyright &y& Elsevier]

Abstract: Seasonal hydrographic conductivity–temperature–depth surveys and moored current meter measurements have been analysed using an inverse approach in order to highlight the main features of the circulation in the western Sicily Strait during 2003. The variational inverse section model combines different types of constraints to seek for a continuous flow field satisfying data and physical assumptions within prescribed prior error bars. It is based on a finite element discretization that allows an appropriate resolution of very irregular topography. The corresponding results, consistent with data and dynamics, are providing new insight into the circulation of the surface and intermediate layers in conjunction with transport and formal error estimates during five hydrographic cruises. In the upper layer, these insights include the southward Atlantic Tunisian Current (ATC) off the Cap Bon Coasts, its high variability at short time-scales and its recirculation during October. For the Levantine Intermediate Water (LIW) regime, a detailed view of the circulation in the western Sicily Strait is given evidencing its recirculation at the western sill during the same period. Transports for both ATC and LIW are computed and found to be maximum in spring and decrease in summer and fall. [Copyright &y& Elsevier]

Abstract: The mean circulation of the surface layer of the southwestern Japan/East Sea (JES) was examined using current measurements collected at 15m by satellite-tracked drifters and merged sea level anomalies from satellite altimeters. The study of circulation patterns in this paper focused on the inflow passing through the western channel of the Korea Strait from the East China Sea. Empirical Orthogonal Function (EOF) analysis of non-seasonal sea level anomalies revealed that significant energy in the circulation pattern of Ulleung Basin was controlled by the inflow conditions through the Korea Strait. Three circulation patterns were identified that depended on the initial relative vorticity of the inflow. When inflow had initially large negative vorticity, the flow gained more negative vorticity due to deepening of the bottom (stretching) and then turned right after entering the JES. The inflow then followed the path of the Tsushima Warm Current along the coast of Japan. When the inflow was strong, with a speed in excess of 55cm/s and with a large positive vorticity, potential vorticity appeared to be conserved. In this case, the EKWC followed isobaths along the coast and then left the coast, following topographic features north of Ulleung-Do. The northward flowing jet developed inertial meandering after leaving the coast, which is a characteristic of many western boundary currents. The regular, bimonthly deployments of drifters in the western portion of the Korea Strait revealed that splitting or branching of the flow through the western channel of the Korea Strait occurred only 15% of the time. And splitting or branching rarely occurred during the fall and winter seasons, when the inflow splitting was previously reported in hydrographic surveys. The time-averaged circulation map of the EKWC and its seaward extension were considerably enhanced by using regularly sampled geostrophic velocities calculated from sea level anomalies to remove biases in the mean velocity that were caused by irregular spatial and temporal drifter observations. The East Korean Warm Current, a mean coastal current along the Korean coast, behaved like the simple model by in which the generation of the Ulleung Warm Eddy and the meandering circulation pattern were well reproduced. [Copyright &y& Elsevier]

Abstract: A field experiment was conducted across the Luzon Strait in July 2007, and a total of 51 profiles covering variables of horizontal velocity, temperature, salinity, and pressure were collected at 11 stations. Using this observation, the volume transport through the Luzon Strait, its differences between July 2007 and October 2005, and the distribution of subtidal flow and geostrophic flow have been investigated. The net transport has a two-layer vertical structure, which is eastward both in the upper layer (<26kgm−3 σ 0), and in the intermediate layer (26–27.3kgm−3 σ 0), while it is westward in the deeper layer (>27.3kgm−3 σ 0), with respective values of 3.0, 4.0, and −1.5Sv. The net transport is eastward, and estimated to be 5.5Sv. The distribution of the subtidal flow in the intermediate layer shows that a westward flow exists in the northern part, countered by an eastward flow existing in the southern part of the strait. This distribution is in direct contrast to the previous results obtained in October 2005, in which a westward flow occurs in the south countered by an eastward flow in the north in the intermediate layer. This suggests that the flow pattern varies greatly from October 2005 to July 2007 not only in the upper layer but also in the intermediate layer. The deep layer, on the other hand, shows few changes between the two observation periods. [Copyright &y& Elsevier]

Abstract: The main components of large-scale circulation of the eastern tropical Pacific were identified in the mid 20th century, but the details of the circulation at length scales of 102 km or less, the mesoscale field, are less well known particularly during summer. The winter circulation is characterized by large mesoscale eddies generated by intense cross-shore wind pulses. These eddies propagate offshore to provide an important source of mesoscale variability for the eastern tropical Pacific. The summer circulation has not commanded similar attention, the main reason being that the frequent generation of hurricanes in the area renders in situ observations difficult. Before the experiment presented here, the large-scale summer circulation of the Gulf of Tehuantepec was thought to be dominated by a poleward flow along the coast. A drifter-deployment experiment carried out in June 2000, supported by satellite altimetry and wind data, was designed to characterize this hypothesized Costa Rica Coastal Current. We present a detailed comparison between altimetry-estimated geostrophic and in situ currents estimated from drifters. Contrary to expectation, no evidence of a coherent poleward coastal flow across the gulf was found. During the 10-week period of observations, we documented a recurrent pattern of circulation within 500km of shore, forced by a combination of local winds and the regional-scale flow. Instead of the Costa Rica Coastal Current, we found a summer eddy field capable of influencing large areas of the eastern tropical Pacific. Even in summer, the cross-isthmus wind jet is capable of inducing eddy formation. [Copyright &y& Elsevier]

Abstract: The geostrophic circulation between the Costa Rica Dome and Central America is described from CTD observations collected in two surveys: (a) The Wet Cruise in September–October 1993, and the Jet Cruise in February–March 1994. Poleward coastal flow was present on both occasions, but the transition from flow around the dome to the poleward Costa Rica Coastal Current flow was quite tortuous because of the presence of mesoscale eddies. In particular, a warm anticyclonic eddy was found off the Gulf of Fonseca during both cruises, at an almost identical position and with similar dimensions (150m deep, 250km in diameter) and surface speed (0.5ms−1). In the Gulf of Panama, poleward flow was also observed, weaker in February–March 1994 than in September–October 1993, when it penetrated to 600m depth and transported 8.5Sv. In September–October 1993, the current between the dome and the coast was mostly ∼100m deep and weak (∼0.15ms−1), although in its southern side it was deeper (∼450m) and faster at 0.3ms−1. The poleward transport between the dome and the coast was ∼7Sv. In February–March 1994 the Costa Rica Dome was a closed ring adjacent to the continental shelf, ∼500km in diameter, at least 400m deep, had geostrophic surface speeds ∼0.25ms−1, and subsurface maximum speed (0.15–0.20ms−1) at ∼180m depth; the associated uplift of the isotherms was ∼150m. The flow in the south part of the dome splits into two branches, the weakest one going around the dome and the strongest one continuing east and turning south before reaching the Gulf of Panama. [Copyright &y& Elsevier]

Abstract: The Atlantic water (AW) layer in the West Spitsbergen Current (WSC) is an important source of heat and salt for the Arctic Ocean and the areas of deep convection in the Greenland Sea. In the southern Fram Strait, the WSC has two main branches, an eastern core on the continental slope and a western one in the Knipovich Ridge area. Here we analyze interannual variability of the heat content and geostrophic flow (level of no motion at 1000m) in the AW layer (temperature above and salinity above 34.9) in both branches of the WSC using summer hydrographic data along a zonal section from to at occupied by R/V Oceania from 1996 to 2003. In the eastern branch, our time series go back to 1991. In particular, we make inferences of correlation with the winter North Atlantic Oscillation (NAO) index. The AW temperature series in the eastern core show two warm periods, one at the beginning of the 1990s and one at the beginning of the 2000s, separated by a cold event in 1997–1998. There is a competition between a lagged and non-lagged response to the NAO. While the cold event lags the spectacular drop of the NAO index in 1996, the temperature extremes in the warm periods are concomitant with the extremes of the NAO index in these periods. However, if we consider the average thickness of the AW layer on the whole section in the 1996–2003 period, we obtain a high correlation () in which the NAO index leads by 1yr. In contrast, the geostrophic flow shows a close relation to the NAO only in limited areas. The most striking result is a nearly perfect correlation for the strength of the western branch of the WSC, which evolves without a lag and in opposite phase to the NAO index (). Possible causes of links between the evolution of the AW layer and the NAO are discussed based on information available from the literature. It is suggested that both the eastern and western branches undergo interannual variations which are coherent from the northern North Atlantic to Fram Strait. A conclusion is drawn that the summer AW thickness in the eastern branch may be a proxy for the annual mean volume transport in the WSC. [Copyright &y& Elsevier]

The results of a fine-resolution hydrological survey conducted in the western Iberia coastal transition zone in February 2000 are used to describe the vertical structure of the density field and geostrophic currents in the Central Water (CW) layer. The vertical density distribution was characterized by the presence of three pycnocline layers: (i) at the surface driven by the buoyancy input of a low-salinity plume, (ii) at about 200 m (σθ=27.0), below the surface mixed layer and, (iii) in the transition layer between the Central and Mediterranean Water at depths close to 600 m (σθ=27.3). It is shown that the density distribution at the depth of the deeper pycnocline controls much of the geopotential anomaly field when the reference level is below the lower Mediterranean Water core (≃1200 m), leading to a high sensitivity in the geostrophic velocity profiles to the choice of the reference level. The use of a reference level above 600 m artificially reduces the flow in the lower CW layer (27.0<σθ<27.3) where significant volume transports are found when a reference level at 1400 dbar is used. In particular, the presence of an equatorward flow at about 400 m in the near-slope region at 41°N was found to be responsible for the volume transport of 0.3 Sv in the direction opposite to the same volume transport in the upper CW layer (σθ<27.0), a feature not reported in previous studies based on field data. In the northernmost sections, the flow in the lower CW layer was directed in the same poleward direction as the upper layer in the near-slope region, leading to a deep signature in the poleward current with a maximum volume transport increase from 0.5 Sv in the upper CW layer to 1.1 Sv in both the layers. Schematic views of the non-divergent circulation in two isopycnal CW layers are used to present the main flow paths and volume transports associated with the mesoscale eddy field. [Copyright &y& Elsevier]

Interdecadal variations of the Western Subarctic Gyre (WSAG) in the North Pacific were examined mainly by analysis of absolute dynamic topography (ADT) during 1993–2017. The ADT was based on altimetry-derived sea level anomalies superimposed on state-of-the-art mean dynamic topography. We specified geostrophic surface streamlines of the WSAG associated with closed isolines of the ADT. The WSAG intensity was the strongest in the late 1990s, when the WSAG was zonally contracted, and it decreased to the mid-2010s in a linear manner with time. The WSAG expanded abruptly toward the east around 2001 in a step function–like manner. After this abrupt shift, the WSAG was connected most strongly to the Alaskan Gyre. This condition persisted even into the late 2010s. The shift to the very elongated pattern was also accompanied by a drastic lowering of the ADT around 170°W, 50°N, where the WSAG and Alaskan Gyre had been distinctly separated by a peak of the ADT during the previous period when the shape of the gyre was less elongated. A reduced-gravity model of wind-induced, long, baroclinic Rossby waves with weak dissipation could account for both the interdecadal weakening of the WSAG and the step function–like lowering of the ADT peak at the time of the shift of the WSAG to the very elongated pattern. In addition to the baroclinic response of the subarctic ocean to basin-scale wind stress, mesoscale clockwise eddies contributed to sea level rise around the center of the WSAG. In particular, there was an interdecadal increase in the frequency of appearance of strong, mesoscale, clockwise eddies that were generated south of the Aleutian Islands at longitudes of 170°E−180°. Those eddies propagated approximately to the west-southwest, and they disappeared around the center of the WSAG. • The Western Subarctic Gyre weakened gradually from the late 1990s to mid-2010s. • A sea level peak between the WSAG and Alaskan Gyre lowered abruptly around 2001. • The WSAG shape expanded abruptly to the east when the sea level peak lowered. • Wind-forced baroclinic Rossby waves caused WSAG weakening and eastward expansion. • Increased disappearance of Aleutian eddies in WSAG after 2005 may affect weakening. [ABSTRACT FROM AUTHOR]

The influence of long, sub-inertial waves travelling poleward on spatiotemporal fluctuations in the hydrography of coastal upwelling areas has widely been discussed theoretically in the literature since the late 1970s. In this paper, in contrast, measured unexpected wave-like changes in the intermediate density field along the Namibian continental slope are presented and suggested to be the result of a geostrophically adjusted southward current with significant on-offshore meanders. Hydrographic measurements had been carried out on board of r/v 'METEOR' along the 200-m isobath above the Namibian continental shelf (19–23 °S) during seven days in October/November 2000. Measured water properties rhythmically change in space between oceanic (on-shore meander, well oxygenated, dominant Eastern South Atlantic Central Water, ESACW) and coastal waters (off-shore meander, poorly oxygenated, dominant South Atlantic Central Water, SACW). The first case is accompanied by downward displacements of the permanent pycnocline, while the second one corresponds to the opposite situation. The distance between these rather periodic along-section changes in stratification is roughly constant and estimated to be about 60 km. At two hydrographic station positions, duplicate measurements enable an estimation of the associated southward propagation speed of the signal. The obtained value of about 0.16 m/s roughly peaks at the pressure level of about 70 dbar, and the corresponding time scale is estimated to be a few days. • New periodic hydrographic structures on the Namibian shelf in the Benguela upwelling zone have been discovered. • Observed structures are suggested here to result from a topographically trapped, southward meander current. [ABSTRACT FROM AUTHOR]

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of "no motion". The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 1 1 2 -layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea. • Mid-depth striations in the North Indian Ocean have been observed by the Argo trajectory data and T/S profiles. • The striations are more prominent in the Arabian Sea while barely observed in the Bay of Bengal. • The meridional scale of those striations is 300 km, which is because of the smaller basin scale in the equatorial zone. • Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea. [ABSTRACT FROM AUTHOR]

The distribution and transport of paralarvae of the complex Sthenoteuthis oualaniensis - Dosidicus gigas (SD complex, Cephalopoda: Ommastrephidae) in the tropical-subtropical convergence off Mexico were analyzed under contrasting interannual environmental conditions. Two El Niño years were sampled (2010 and 2016), in which the SST were up to 3 °C above the average throughout the study area, while during La Niña year (2012) the SST anomalies were up to 2 °C below the average. During El Niño years, a relative shallow thermocline (<60 m depth) was detected around Cabo Corrientes, where the paralarvae were found with mean abundance of 2 org 1000 m−3. During La Niña condition, a shallower thermocline was detected in the entrance of the Gulf of California (~20 m depth) and the paralarvae abundance rose to 84 org 1000 m−3 with high frequency of occurrence. In both El Niño and La Niña conditions the paralarvae were mainly found in the strongly stratified waters (>80 cycles h−1). In addition, a simple dispersion Lagrangian model based on observed satellite geostrophic currents, suggested association of the SD complex paralarvae with mesoscale structures, in which the eggs and hatched paralarvae had an appropriate habitat for a nursery, retaining the paralarvae until their recruitment in a ~100 km radius. But during warm extreme events, with less mesoscale activity, the paralarvae could be transported as far as 500 km southeast of the spawning zone. Even though the interannual ENSO conditions modulate the magnitude of the paralarvae abundance, the shallow and well developed thermocline could help paralarvae to increase their survival chances, while mesoscale activity retains them near the productive coastal waters. • ENSO phases modulate Sthenoteuthis-Dosidicus complex paralarval abundance. • Mesoscale eddies retain paralarvae near coastal waters. • Lack of mesoscale eddy circulation leads to high paralarval dispersion rate. [ABSTRACT FROM AUTHOR]