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• This study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. • The reliability of reconstruction was evaluated by satellite observation, in-situ data, and numerical output. • From three eddy cases, the real-time structures reflect more small fluctuations with a great change comparing with the background structure. • The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution. The internal structures of mesoscale eddies are variable due to different generation mechanism, spatiotemporal scale, and movement characteristics. Based on the principle of data assimilation, this study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. Three eddy cases with different types and locations were selected to validate the construction results. The reliability of construction was evaluated by satellite observation, in-situ comparisons, and contrast with the numerical output. The results showed that the real-time geostrophic current characters were basically consistent with the surface and deep current (observed by satellite and acoustic doppler current profiler, ADCP). The density features exhibited by constructed results (eddy centers, mean radii, and variation tendency) were similar to the expendable conductivity-temperature-depth (XCTD) survey data and numerical output. Compared to the composite structure, the real-time structures represented more small or mesoscale fluctuations. During eddy cases evolution, the mean radii at the same depth first increased and then decreased. The horizontal scale, influence depth, and internal stability of an anticyclonic eddy with low-latitude were more powerful than the other two eddy cases owing to the domination of warm water in the Kuroshio extension region. The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution. [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 Slope Sea is the dynamic ocean region located between the United States and Canadian northeast continental shelves and the northeastward flowing Gulf Stream (GS) located farther offshore. Here we define it as located between the 200-m isobath and the monthly GS sea surface temperature (SST) front from −75° to −55° E. Monthly mean near-surface eddy kinetic energy (EKE) was computed for the Slope Sea using surface geostrophic current anomalies derived from gridded 1993–2016 Copernicus Marine Environment Monitoring Service (CMEMS) sea height anomalies. Long-term, monthly mean Slope Sea EKE anomalies show a robust seasonal cycle with a winter (February) minimum and summer (June) maximum. This agrees with both seasonally-varying density stratification and dissipation and also the seasonal variation in the formation of GS WCRs within the Slope Sea. The RMS of the Slope Sea EKE seasonal cycle generally increased after 2002 by a factor of up to ∼2 relative to prior years. The seasonal cycle of Slope Sea EKE displayed higher EKE in the vicinity of the New England Seamount Chain (NESC) that extends towards the shelf break front from approximately −67° E to −63° E. Interannual variability of annual mean near-surface EKE from individual digitized GS warm core ring (WCR) observations from a Bedford Institute of Oceanography (BIO) WCR database is highly correlated with Slope Sea EKE. However, interannual variability of annual mean near-surface EKE computed from a census of all newly formed WCRs displayed only a weak correlation. Many of the WCRs from both the BIO and WCR census displayed anomalously low EKE values and were observed within the northern Slope Sea away from the GS. Some were located inshore of the position of the climatological mean shelf break front. WCRs with higher EKE were located throughout the Slope Sea, with higher numbers in the vicinity of the NESC. The many observations of the less energetic features located close to or inshore of the mean shelf break front suggest they are important to cross-shelf fluxes of heat, salt, nutrients, shelf biota. They therefore likely impact the shelf ecosystem, similar to the more energetic and typical WCRs impacting the outer shelf as discussed by earlier workers. • Altimeter-derived currents describe eddy kinetic energy changes in the slope sea. • Slope sea eddy kinetic energy displays strong seasonal and inter-annual variation. • Slope sea eddies display large variation in strength and formation location. • Eddy kinetic energy changes do not support the warm-core ring regime shift of 2000. [ABSTRACT FROM AUTHOR]

Mesoscale eddies are major features in ocean dynamics that significantly influence ocean production depending on their polarity, life-stage, location and time of formation. Eddies in the south-west Indian Ocean have been the focus of many studies, especially in the Mozambique Channel where southward-moving cyclonic and anticyclonic eddies feed into the Agulhas Current. However, eddies formed south of Madagascar that move westwards towards the coast of South Africa have been much less studied, despite their significant contribution to the Agulhas Current. This study aims to understand how the plankton communities (i.e. pico-, nano-, phyto-, microzoo- and mesozooplankton) are distributed in a cyclonic eddy generated off southern Madagascar. We also investigate a possible link between the Madagascar shelf and the eddy. Temperature, salinity, fluorescence and plankton abundance were measured at 25 stations spaced at 18.5 km intervals across a young (~ 1 month old) cyclonic eddy, and at three locations on the Madagascar shelf, in July 2013. A patch of enhanced chlorophyll a concentration was found at four stations in the eddy, homogeneously distributed vertically in the upper mixed layer and characterised by a higher proportion of diatoms compared to the other stations. Abundances of picoplankton, nanoplankton, ciliates and dinoflagellates were very similar within and outside the eddy. Zooplankton followed a contrasting pattern with higher biomass at four stations just to the west of this high diatom patch. The mesozooplankton composition at these four stations revealed greater species similarities with the Madagascar shelf than with the other stations, suggesting a link between the shelf and the western side of the eddy. The periphery of the eddy seems to have been primarily influenced by a strong geostrophic current propagating from the southern shelf of Madagascar which then wrapped itself around the eddy. This filament may be the source of the high zooplankton biomass in the eddy periphery, which would account for the similarities in zooplankton community composition between the eddy periphery and the shelf. [ABSTRACT FROM AUTHOR]

Wind-driven geostrophic currents have been considered as one of the major energy sources for stimulating abyssal mixing that shapes the ocean overturning circulation. However, the mechanism governing the energy transfer from surface geostrophic currents to small dissipation scales available for abyssal mixing remains elusive. In this study, we identify a potential energy transfer pathway from surface geostrophic eddies to abyssal small-scale turbulence based on observations and numerical simulations of flows over seamounts in the northwestern Pacific Ocean. Our analyses reveal the generation of strong near-bottom anticyclonic circulations following the passage of surface geostrophic eddies across seamounts. These circulations drive downslope Ekman transports and convective instabilities, leading to enhanced turbulent mixing and thickened bottom mixed layers. Altimetry observations suggest that the energy transfer from surface eddies to deep currents and subsequent turbulence may be a dominant mechanism that accounts for 25–40% energy decay of mesoscale eddies passing seamounts. Strong near-bottom anticyclonic circulations associated with the surface geostrophic eddies are likely common for many seamounts and other topographic slopes in the global ocean, and may therefore play a significant role in the oceanic energy balance and water mass transformation in the abyssal ocean. [ABSTRACT FROM AUTHOR]

We study the production and dissipation of the eddy kinetic energy (EKE) in a submesoscale eddy field forced with downfront winds using the Process Study Ocean Model (PSOM) with a horizontal grid resolution of 0.5 km. We simulate an idealized 100 m deep mixed-layer front initially in geostrophic balance with a jet in a domain that permits eddies within a range of O(1 km–100 km). The vertical eddy viscosities and the dissipation are parameterized using four different subgrid vertical mixing parameterizations: the k − ϵ , the KPP, and two different constant eddy viscosity and diffusivity profiles with a magnitude of O ( 10 − 2 m 2 s − 1 ) in the mixed layer. Our study shows that strong vertical eddy viscosities near the surface reduce the parameterized dissipation, whereas strong vertical eddy diffusivities reduce the lateral buoyancy gradients and consequently the rate of restratification by mixed-layer instabilities (MLI). Our simulations show that near the surface, the spatial variability of the dissipation along the periphery of the eddies depends on the relative alignment of the ageostrophic and geostrophic shear. Analysis of the resolved EKE budgets in the frontal region from the simulations show important similarities between the vertical structure of the EKE budget produced by the k − ϵ and KPP parameterizations, and earlier LES studies. Such an agreement is absent in the simulations using constant eddy-viscosity parameterizations. [ABSTRACT FROM AUTHOR]

Abstract: Rings are marine vortices with a scale of hundreds of kilometers that can last for months, whose associated transport and mixing play an important role in the ocean dynamics. Such features are traditionally treated as a geostrophic flow, but since the centrifugal acceleration is not negligible in the inner core, the cyclo-geostrophic balance is a better approximation for the rings. In the present work, we describe a novel objective technique to identify the ring center, which is used as the origin of a convenient framework to handle rings under the cyclo-geostrophic balance. Furthermore, we correct the velocity field by the translation to isolate the swirl movement, a procedure ignored by other methodologies. We show that the lack of such correction would lead to an error of 30km on the center definition of a ship surveyed North Brazil Current Ring with 160km of radius. Another distinct characteristic of our approach is the flexibility in the spatio-temporal structure of the data, because it allows for ungridded data, an important ability for in situ observations. That also enables the use of a hybrid dataset composed from different instruments. The error on the Monte Carlo experiments to identify the center of the propagating ring is less than 10km, and depends on the level of noise, sampling strategy, and strength of the ring, among other factors. This technique was fully implemented in PyRings, an open Python library with a collection of procedures to handle oceanic rings and mesoscale eddies in general. [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: The study of mesoscale and submesoscale [hereafter (sub)mesoscale] hydrodynamic features is essential for understanding thermal and biogeochemical exchanges between coastal areas and the open ocean. In this context, a glider mission was conducted in August 2008, closely co-located and almost simultaneously launched with a JASON 2 altimetric pass, to fully characterize the currents associated with regional (sub)mesoscale processes regularly observed to the north of Mallorca (Mediterranean Sea). A synoptic view from satellite remote-sensing fields, before and during the glider mission, provided a descriptive picture of the main surface dynamics at the Balearic Basin scale. To quantify the absolute surface geostrophic currents, the coastal altimetry-derived current computation was improved and cross-compared with its equivalent derived from glider measurements. Model simulations were then validated both qualitatively and statistically with the multi-sensor observations. The combined use of modeling and multi-sensor observational data reveals the baroclinic structure of the Balearic Current and the Northern Current and a small-scale anticyclonic eddy observed northeast of the Mallorca coast (current∼15cm/s, <30km in extent and >180m deep). This mesoscale structure, partially intercepted by the glider and along-track altimetric measurements, is marked by relatively strong salinity gradients and not, as is more typical, temperature gradients. Finally, the use of the validated model simulation also shows that the geostrophic component of this small-scale eddy is controlled by sub-surface salinity gradients. We hypothesize that this structure contains recently modified Atlantic water arriving from the strait of Ibiza due to a northerly wind, which strengthens the northward geostrophic circulation. [Copyright &y& Elsevier]

Abstract: Multiscale analysis is used to derive two sets of coupled models, each based on the same distinguished limit, to represent the interaction of the midlatitude oceanic synoptic scale—where coherent features such as jets and rings form—and the mesoscale, defined by the internal deformation scale. The synoptic scale and mesoscale overlap at low and mid latitudes, and are hence synonymous in much of the oceanographic literature; at higher latitudes the synoptic scale can be an order of magnitude larger than the deformation scale, which motivates our asymptotic approach and our nonstandard terminology. In the first model the synoptic dynamics are described by ‘Large Amplitude Geostrophic’ (LAG) equations while the eddy dynamics are quasigeostrophic. This model has order one isopycnal variation on the synoptic scale; the synoptic dynamics respond to an eddy momentum flux while the eddy dynamics respond to the baroclinically unstable synoptic density gradient. The second model assumes small isopycnal variation on the synoptic scale, but allows for a planetary scale background density gradient that may be fixed or evolved on a slower time scale. Here the large-scale equations are just the barotropic quasigeostrophic equations, and the mesoscale is modeled by the baroclinic quasigeostrophic equations. The synoptic dynamics now respond to both eddy momentum and buoyancy fluxes, but the small-scale eddy dynamics are simply advected by the synoptic-scale flow—there is no baroclinic production term in the eddy equations. The energy budget is closed by deriving an equation for the slow evolution of the eddy energy, which ensures that energy gained or lost by the synoptic-scale flow is reflected in a corresponding loss or gain by the eddies. This latter model, aided by the eddy energy equation—a key result of this paper—provides a conceptual basis through which to understand the classic baroclinic turbulence cycle. [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: Flow fluctuations inside an anticyclonic eddy in summertime Funka Bay were examined using three moorings and hydrographic data. The flow pattern above a sharp pycnocline with a concave isopycnal structure during the mooring period was characterized by high mean kinetic energy and relatively low eddy kinetic energy. The ratios of eddy to mean kinetic energy were equal to or less than one, and the mean flow field and isopycnal structure indicated the existence of a stable anticyclonic eddy above the sharp pycnocline under approximate geostrophic balance. Larger flow fluctuations with periods longer than 7 days were dominant inside the eddy. The low-frequency flow fluctuations are accompanied by north to northeastward movement of the eddy with deepening of the pycnocline and spin-up of the anticyclonic circulation. The wind field over Funka Bay is characterized by bay-scale wind fluctuations. The bay-scale winds are greatly influenced by the land topography around Funka Bay, resulting in non-uniform structure with significant wind stress curl. The bay-scale wind fluctuations are termed “locally modified wind” in the present study. The locally modified wind has a negative (positive) wind stress curl in the central–northeastern (southwestern) region of Funka Bay. The north to northeastward movement of the eddy is caused by horizontal non-uniform supply of vorticity from the locally modified wind forcing by the Ekman pumping process. Through this process, the anticyclonic circulation is enhanced (weakened) in the central–northeastern (southwestern) part of the eddy, resulting in the eddy moving north to northeastward with the pycnocline deepening and spin-up of the anticyclonic circulation. The locally modified wind forcing induces low-frequency flow fluctuations through the movement of the eddy in summertime Funka Bay. [Copyright &y& Elsevier]

Abstract: Discussions of the dynamics of the Antarctic Circumpolar Current (ACC) generally focus on eddies. The linear, analytical models are discussed only infrequently, and none of these models has gained widespread acceptance. These models nevertheless exist and exhibit interesting, and often realistic, features. We revisit those models, to understand their dynamics and to assess their relevance to the ACC. We focus specifically on the steady, linear, wind-driven models, and those with either a barotropic or equivalent barotropic vertical structure. The most important feature distinguishing them is their choice of geostrophic contours ( or the equivalent), whether closed or blocked. We first examine the flat bottom models. With closed geostrophic contours, the solutions have a flow which is primarily along the contours and a transport which is inversely proportional to the bottom drag coefficient. For realistic parameters, this transport is excessively large. Solution with blocked contours instead exhibit gyres, with cross-contour flow and western boundary currents. But they also have one or more circumpolar jets, which follow the geostrophic contours over most of the domain. In contrast to the closed contour models, these jets have a transport which asymptotes to a constant value when the bottom drag is vanishingly weak. We then discuss solutions with bottom topography, focusing in particular on the equivalent barotropic solution. The central parameter here is the vertical scale of the current, which determines the extent of the interaction with topography and the degree to which the geostrophic contours are blocked. The solutions with a shallow current are less affected by topography and have closed geostrophic contours and large transports. Solutions with too large vertical extent are overly-controlled by topography and exhibit only weak circumpolar transport. Solutions with an intermediate vertical scale have blocked contours and also reasonable circumpolar transport. Furthermore, these solutions exhibit strikingly realistic surface height fields. As in the flat bottom case, the solutions have an interior in Sverdrup balance and a circumpolar transport which asymptotes with vanishing bottom friction. We demonstrate that this transport can be estimated via a contour integral; the result agrees well with the full equivalent barotropic solution. Both are nevertheless roughly 50% larger than observed in Drake Passage. However, the transport can be reduced to realistic values by adding lateral dissipation to the model. Thus the equivalent barotropic model is successful at capturing the steering of the ACC by topography, given the vertical scale of the current. However, it remains to understand what determines that scale. Thermohaline forcing and lateral mixing by eddies are likely to be important. [Copyright &y& Elsevier]

Abstract: Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100m on the shelf in the survey region is approximately 6.4×103 tonneswetweightday−1 northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100m over Malangsgrunnen was approximately 0.08day−1, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort. [Copyright &y& Elsevier]

• The Transition Domain located between the SAF and SAB is where the warm salty water originating from the Kuroshio current joins the cold freshwater from the Oyashio current. • The Transition Domain is important not only for oceanographic processes such as the inter-gyre exchange of water masses but also for mid-latitude climate and biological processes. • Visualization of how water from the Kuroshio and Oyashio water is transported to the Transition Domain using the Lagrangian approach, such as drifting buoy observations and particle tracking. • Time-variation of the geostrophic field in the the Oyashio Second Branch in addition to the J1′s meander plays an important role in supplying water from the Kuroshio region to the Transition Domain. A peculiar water mass forms as a result of stirring and mixing between water originating from the Kuroshio and Oyashio currents in the Transition Domain, which is a boundary region between the subtropical and subarctic gyres extending zonally at approximately 40°N in the western North Pacific west of the dateline. The domain is important not only for oceanographic processes such as the inter-gyre exchange of water masses but also for mid-latitude climate and biological processes. We investigated how water from the Kuroshio and Oyashio currents is transported to the Transition Domain and then mixed, especially in relation to the geostrophic velocity fields, bottom topographic features, and the local eddy activity in the upstream region of the domain. We have clarified the pathways of the water from the Kuroshio and Oyashio currents from a Lagrangian point of view by conducting drifting buoy observations in 2015 and 2017, as well as by particle tracking using geostrophic surface flow fields. Herein, we visualize how the water pathways within and around the Transition Domain are closely tied to low-rise bottom topographic features. Time-variation of the geostrophic field in the upstream region of the Transition Domain also likely plays an important role in supplying water from the Kuroshio region to the Transition Domain; if a steady climatological flow field is used, the Transition Domain will be occupied solely by water from the Oyashio current. Once the water from the Kuroshio region enters a quasi-stationary jet in the western North Pacific (the so-called western Isoguchi Jet; J1) due to the eddies at the Oyashio Second Branch in addition to eddies at another region of the J1′s meander, it is further transported northeastward in the subarctic gyre via the Transition Domain. The subtropical–subarctic exchange through the Transition Domain could essentially be driven by the eddies in these local regions. [ABSTRACT FROM AUTHOR]

Abstract: An eddy-resolving numerical simulation for the Peru–Chile system between 1993 and 2000 is analyzed, mainly for the 1997–1998 El Niño. Atmospheric and lateral oceanic forcings are realistic and contain a wide range of scales from days to interannual. The solution is validated against altimetric observations and the few in situ observations available. The simulated 1997–1998 El Niño closely resembles the real 1997–1998 El Niño in its time sequence of events. The two well-marked, sea-level peaks in May–June and November–December 1997 are reproduced with amplitudes close to those observed. Other sub-periods of the El Niño seem to be captured adequately. Simple dynamical analyses are performed to explain the 1997–1998 evolution of the upwelling in the model. The intensity of the upwelling appears to be determined by an interplay between alongshore, poleward advection (related to coastal trapped waves) and wind intensity, but also by the cross-shore geostrophic flow and distribution of the water masses on a scale of 1000km or more (involving Rossby waves westward propagation and advection from equatorial currents). In particular, the delay of upwelling recovery until fall 1998 (i.e., well after the second El Niño peak) is partly due to the persistent advection of offshore stratified water toward the coast of Peru. Altimetry data suggest that these interpretations of the numerical solution also apply to the real ocean. [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]

A pair of 12.5-year (July 2002–December 2014) HYbrid Coordinate Ocean Model (HYCOM) simulations that only differ in the wind stress formulation are used to investigate the effect of ocean surface currents on global monthly eddy kinetic energy (EKE) variation. The model results (2004–2014) show that the global monthly mean EKE is reduced by 37%, from 1.76 EJ (1018 J) to 1.10 EJ after ocean surface currents are included in the wind stress formulation. The monthly EKE budget indicates that the shear production and buoyancy work are positive (energy source) and the eddy wind work on the geostrophic currents is negative (energy sink) in the steady state (2004–2014) for both simulations. All of these three terms are reduced in the steady state when the ocean currents are included in the wind stress formulation. The global integral of the EKE difference budget suggests that the EKE reduction is primarily due to the reduction of the buoyancy work, followed by the reduction of the wind work on the geostrophic currents and the shear production. To our knowledge this is the first study to separate the eddy wind work into the geostrophic and ageostrophic components to investigate the impact of ocean surface currents on global and depth integrated EKE via the wind stress formulation using HYCOM simulations. • Including ocean surface currents in the wind stress formulation reduces global eddy kinetic energy. • Eddy component of wind work has a much larger magnitude than shear production and buoyancy work. • The reduction of global eddy kinetic energy is primarily due to the reduction of the buoyancy work. • Reduction of the wind work on geostrophic currents also contributes to the reduction of the global eddy kinetic energy. • Reduction of the shear production also contributes to the reduction of the global eddy kinetic energy. [ABSTRACT FROM AUTHOR]