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Under the influence of buoyancy and the earth rotation, river outflows leaving an estuary typically have a two‐part structure, including a recirculating bulge near the river mouth and a coastal current propagating downstream. A continuous river outflow causes the bulge to expand in size and to accumulate freshwater, with the bulge eventually becoming unstable due to baroclinic instability. We conducted a series of laboratory experiments on a rotating tank to simulate an idealized river plume under various Coriolis frequencies, density differences, and shelf slopes. The horizontal velocity structures of the river plume were qutantified using particle imaginary velocimetry (PIV). We designed a velocity‐based vortex identification and tracking algorithm to capture anticyclonic eddies (ACEs) at the bulge center, cyclonic eddies (CEs) on the plume front, and coastal cyclonic return flow (CCRF) at the corner between estuary and coastal wall. Our results suggest a linear relationship between bulge instability parameter and bulge wavenumber, which can be used to predict the bulge instability patterns that are classified according to vortex stretching, splitting, and squeezing. Finally, we estimated the eddy kinetic energy contained within ACEs, CEs, and CCRF to explore the cross‐scale energy transfer and dissipation. Our results show that the bugle is more unstable in gentle slope cases, and its instability decreases with the inflow Rossby number and increases with the Froude number. The generation of CEs on the plume front may extract the energy from the larger scale anticyclonic core, which plays an important role in mass transport and frontal mixing of river plumes. Plain Language Summary: When the river flows into the coastal ocean, it forms a river plume and serves as an important source of land‐borne materials. Due to the density differences between river and ocean water, the river plume stays on top of the ocean water and forms a circular bore that rotates counter‐clockwise in the northern hemisphere. As the riverine water continuously flows into the ocean, this circular bore grows in size with the accumulation of riverine water and finally becomes unstable. To investigate this phenomenon, we carried out a series of experiments on the rotating tank in the laboratory and changed three parameters: rotation periods, fluid density differences, and the angle of continental shelf slopes. Our experimental results showed that a gentler slope, shorter rotation period, and less fluid density difference led to a more unstable river plume with more small‐scale vortices at the edge of the plume. We categorized the river plumes into three types based on their stability status and proposed an empirical relationship between easy‐obtain plume parameters and their stability status. We believe that the relationship can be utilized to predict the river plume stability based on satellite images or field observations. Key Points: Small scale eddies induced by baroclinic instabilities in river plume are triggered by gentler slopes, shorter rotation periods and weaker fluid density differencesBaroclinic instability parameter is proposed to estimate bulge wavenumber and thereby to predict plume instabilityEKE is more distributed in submesoscale processes along the bulge front in an unstable river plume system [ABSTRACT FROM AUTHOR]

Broadened width of high chlorophyll concentration band with wavy structures, patches, and filaments are often observed along the western coastal next to the Pearl River Estuary over the northern South China Sea shelf during the transition period from winter to summer monsoon. Whereas, there is no such wide band in other seasons. By using a high‐resolution numerical model, we reveal that the complex structure and wider band of high coastal chlorophyll concentration results from the smaller scale eddies (about 20–50 km in diameter) associated with buoyant plume‐induced salinity front and density fronts, which are roughly along the 30 and 50 m isobaths, respectively. Two trains of eddies are formed along the fronts by the baroclinic instability triggered by frequently alternating winds over the fronts during the period of monsoon transition. The influences of these two trains of eddies are extended in the cross‐shelf direction by their interactions, and they can temporally enhance the cross‐shelf flow and material exchange. They serve as an efficient pathway to link the inner shelf toward the continental slope. Plain Language Summary: Cross‐shelf flow is a major agent of promoting connectivity in coastal oceans and plays a dominant role in the onshore/offshore delivery and exchange of nutrients, pollutants, biomass, and other materials. Coastal oceans are often influenced by buoyant water which forms fronts between nearshore and offshore. The front‐associated eddies and filaments can facilitate particles in overcoming topographic barriers and therefore could transport materials in the cross‐shelf direction. The shelf region in the northern South China Sea influenced by the discharge of Pearl River is an ideal testbed to illustrate and investigate how instabilities of coastal fronts help to enhance cross‐shelf exchange. Using high‐resolution simulations and Lagrangian drifter experiments, we reveal that the instability eddies generated at the fronts can significantly extend the cross‐shelf connectivity of the inner shelf to the slope, and serve as an effective path for exchange coastal water and materials in the cross‐shelf direction. Our findings advance the knowledge of how coastal fronts boost the cross‐shelf connectivity with implications for marine tracer distribution and ecological environments. Key Points: Eddies are generated along the edge of the river plume over the northern South China Sea shelf during the monsoon transition periodBaroclinic instability of the fronts triggered by the frequent change of monsoon wind plays an important role for eddy generationThe interaction of the eddies enhances the cross‐shelf exchange and serves as a pathway to link the coast toward the continental slope [ABSTRACT FROM AUTHOR]

Intraseasonal variabilities (ISVs) of the western boundary currents (WBCs) east of Luzon Island were explored using acoustic Doppler current profiler (ADCP) measurements from three moorings at 18°N during 2018–2020. Besides the traditionally known surface‐intensified ISV, subsurface‐intensified ISV with a typical period of approximately 60 days was also detected in the currents, and the strongest signal appeared between 400 and 800 m. Further analysis indicates that they are highly associated with subsurface eddies. Based on their lifespan, subsurface eddies are classified into two categories: short‐lived and medium‐to long‐lived eddies. The short‐lived eddies are primarily generated locally near the eastern coast of Luzon Island, whereas the medium‐to long‐lived eddies are mainly generated away from the western boundary, in the region west of 135°E. Additional energy diagnosis suggests that baroclinic instability induced by the velocity shear of the North Equatorial Current (NEC)/subtropical countercurrent (STCC) system dominates the generation of medium‐to long‐lived subsurface eddies in the interior ocean, while barotropic instability and baroclinic instability play a comparable role in the generation of short‐lived eddies near the eastern coast of Luzon Island. Plain Language Summary: The Kuroshio and Luzon Undercurrent (LUC) have a far‐reaching impact on regional climate change as a crucial part of the western boundary current (WBC) system. In situ measurements east of Luzon Island have revealed that the currents exhibit significant intraseasonal variabilities (ISVs). However, the vertical difference of the ISVs is unclear, and we still do not know if subsurface ISVs exist in this region. In this work, we found the currents east of Luzon Island are characterized by two types of ISVs with a period of approximately 60 days: surface‐intensified and subsurface‐intensified. The two types of ISVs are linked to the surface and subsurface mesoscale eddies, respectively. Previous studies have revealed that baroclinic instability associated with the vertical shear of velocity in the North Equatorial Current/subtropical countercurrent (NEC/STCC) system is the primary generation mechanism of surface eddies. This study suggests that baroclinic instability of the NEC/STCC system also dominates the generation of subsurface eddies in the interior ocean, while barotropic instability associated with the horizontal shear of currents near the Luzon coast is non‐negligible in the generation of coastal subsurface eddies. These results significantly deepen our understanding of the ISVs of the currents in this region. Key Points: Quasi‐60‐day intraseasonal variability in the currents east of Luzon Island exhibits surface‐ and subsurface‐intensified vertical featuresSubsurface‐intensified eddies are revealed appearing east of Luzon Island and modulating the western boundary currentsBaroclinic instability dominates the generation of subsurface eddies east of Luzon while coastal barotropic instability is non‐negligible [ABSTRACT FROM AUTHOR]

In order to quantify pelagic‐benthic coupling on high‐latitude shelves, it is imperative to identify the different physical mechanisms by which phytoplankton are exported to the sediments. In June–July 2023, a field program documented the evolution of an under‐ice phytoplankton bloom on the northeast Chukchi shelf. Here, we use in situ data from the cruise, a simple numerical model, historical water column data, and ocean reanalysis fields to characterize the physical setting and describe the dynamically driven vertical export of chlorophyll associated with the bloom. A water mass front separating cold, high‐nutrient winter water in the north and warmer summer waters to the south—roughly coincident with the ice edge—supported a baroclinic jet which is part of the Central Channel flow branch that veers eastward toward Barrow Canyon. A plume of high chlorophyll fluorescence extending from the near‐surface bloom in the winter water downwards along the front was measured throughout the cruise. Using a passive tracer to represent phytoplankton in the model, it was demonstrated that the plume is the result of subduction due to baroclinic instability of the frontal jet. This process, in concert with the gravitational sinking, pumps the chlorophyll downwards an order of magnitude faster than gravitational sinking alone. Particle tracking using the ocean reanalysis fields reveals that a substantial portion of the chlorophyll away from the front is advected off of the northeast Chukchi shelf before reaching the bottom. This highlights the importance of the frontal subduction process for delivering carbon to the sea floor. Plain Language Summary: The Chukchi Sea shelf north of Bering Strait is known to experience some of the largest phytoplankton blooms in the Arctic Ocean. In 2023, a field program was carried out to quantify aspects of the early summer bloom, with an emphasis on characterizing how the phytoplankton biomass from the bloom is exported to the sea floor. A large bloom was measured under the pack ice in very cold, high‐nutrient water, just north of warmer, ice‐free waters. The front separating the warm and cold waters supported a current flowing eastward, which is one of the main flow pathways on the Chukchi shelf. A plume of high chlorophyll fluorescence extending from the near‐surface bloom downwards along the front was measured throughout the cruise. We demonstrate that this vertical pumping was due to a dynamical process associated with the current which resulted in much faster downward export of phytoplankton than gravitational sinking alone. Tracking the fate of particles on the northeast Chukchi shelf using an ocean simulation revealed that much of the phytoplankton biomass away from the front is carried off the shelf before reaching the bottom. This highlights the importance of the frontal process for delivering chlorophyll to the sea floor. Key Points: An under‐ice phytoplankton bloom developed during June–July in the northeast Chukchi Sea within the Central Channel flow branchA plume of chlorophyll fluorescence extending downwards from the bloom along the current's water mass front was continually presentA simple numerical model demonstrates that the plume is the result of baroclinic instability of the frontal jet [ABSTRACT FROM AUTHOR]

Eddy activity is particularly prominent in the Southern Ocean due to the instabilities of the Antarctic Circumpolar Current, which plays a critical role in energy transport of the global ocean. In this study, a systematic energetics analysis framework is employed to investigate notable anomalies of an intensified Eddy Kinetic Energy (EKE) event observed in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017, utilizing a reanalysis product. The EKE anomalies, presenting across all depths, emerged in April, peaked during the austral winter, and persisted into the subsequent summer. Energetics analysis indicates that the pronounced EKE anomalies are primarily determined by baroclinic instability, with distinct governing mechanisms at the surface and in the internal ocean. The anomalous intrusion of warm Circumpolar Deep Water intensified the baroclinic energy conversion in the subsurface, contributing significantly to the EKE anomalies. Moreover, strong anomalous wind‐induced Ekman pumping served to amplify the lifting of isopycnals, which enhanced the baroclinic instability and subsequently intensified the EKE anomalies. This study sheds new light on underlying mechanisms governing local polar dynamics and provides insights into the intricate interaction between ocean dynamics and energy distribution in the Antarctic region. Plain Language Summary: The Indian sector of the Southern Ocean is known for its dynamic variability, often manifested as jets and eddies. Eddy Kinetic Energy (EKE) is widely used to measure the kinetic energy (KE) as the difference between the total KE and the KE of mean currents. This study found an anomalous event of significant increases in EKE in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017. We applied a systematic energetics analysis framework to a reanalysis product to investigate the processes responsible for the observed anomalous event. The results suggest that the main cause was the anomalous intrusion of warm water masses in the upper and deeper ocean layers, which led to the increases in density gradients and then intensified the energy conversion from available potential energy to EKE. Moreover, changes in wind patterns have an impact on the variations of EKE in the upper ocean. This study enhances the understanding of the energy conversions and underlying mechanisms for EKE in polar regimes. Key Points: The subpolar region to the southwest of the Kerguelen Plateau was characterized by anomalous strong Eddy Kinetic Energy (EKE) in 2017The strong regional anomalies in EKE can be primarily attributed to baroclinic instability, with inverse barotropic energy conversionBaroclinic instability is mainly caused by anomalous intrusion of Circumpolar Deep Water, particularly at depths between 500 and 2,000 m [ABSTRACT FROM AUTHOR]

In this study, the variability of eddy kinetic energy (EKE) in the Kuroshio large meander (LM) region is investigated using both satellite sea surface height observation and high‐resolution ocean reanalysis data. The results show that the EKE has a remarkable interannual variability and it is negatively leading correlated with the change in the Kuroshio latitudinal position at 140°E. The mechanism analysis suggests that the baroclinic instability and advection processes are responsible for the EKE interannual variability and its effect on the Kuroshio latitudinal position at 140°E. Specifically, before the high EKE level occurs, a cyclonic eddy generates at the east of Kii Peninsula in the background field. The rapid development of this eddy and its eastward movement to the LM region induce the isopycnal inclinations there and the associated horizontal density gradient, which leads to the strong baroclinic instability and promotes the evolution of the eddy field and the formation of the high EKE level. The developed strong eddies, especially the cyclonic eddies, move downstream to 140°E, which pushes the Kuroshio off the shore and causes the southerly Kuroshio latitudinal position at 140°E. On the contrary, when the cyclonic eddies do not appear in the LM region, the isopycnals are relatively flat, which is not conducive to the generation of baroclinic instability. Consequently, the EKE level is low and only weak eddies are advected to 140°E, which does not substantially shift the Kuroshio southward and thus results in the northerly Kuroshio position. Plain Language Summary: Ocean mesoscale eddies have important effects on physical and biological processes. The formation area of the Kuroshio large meander (LM) is one of the most active areas of mesoscale eddies in the middle latitudes of the western North Pacific. It is, therefore, of significance to investigate the variability of mesoscale eddy activities represented by eddy kinetic energy (EKE) in the LM region. Our results show that the EKE level is high in some years, while it is low in other years and this kind of EKE variability has a leading negative correlation with the Kuroshio latitudinal position at 140°E. The mechanism analysis indicates that the existence of cyclonic eddies in the LM region in the background field causes differences in the isopycnal inclinations and the related baroclinic instabilities, which eventually lead to different EKE levels. Subsequently, when the EKE level is high, strong cyclonic eddies move downstream to 140°E, which pushes the Kuroshio off the shore and causes the southerly Kuroshio latitudinal position at 140°E. Contrarily, when the EKE level is low, only weak eddies are advected to 140°E, which does not substantially shift the Kuroshio southward and thus results in the northerly Kuroshio position there. Key Points: Eddy kinetic energy(EKE) in the Kuroshio large meander (LM) region has a remarkable interannual variabilityEKE interannual variability in the LM area has a leading correlation with that of the Kuroshio latitudinal position at 140°EBaroclinic instability and advection are responsible for the EKE variability and its relationship to the Kuroshio position at 140°E [ABSTRACT FROM AUTHOR]

Due to the high nonlinearity, the accurate prediction of the Antarctic circumpolar current (ACC) transport is still challenging. Using the eddy‐permitting regional ocean modeling system and the conditional nonlinear optimal perturbation approach, the sudden shift in the ACC transport through Drake Passage (DP) is investigated by exploring its optimal precursor (OPR). Here, the sudden shift in the ACC transport is defined as a fluctuation exceeding double STD (∼16 Sv; 1 Sv = 106 m3 s−1) within 30 days. The results indicate that the OPRs for all three cases exhibit specific structures in the middle DP (58°–62°S, 72°–64°W) at the depth of 1,000–3,000 m, implying that the ACC transport is most sensitive to the initial perturbations there. The OPRs' evolutions show similar features: the OPR for each case triggers an eddy‐like dipole perturbation with a northern cyclone and a southern anticyclone, leading to a sudden reduction in the ACC transport of ∼40 Sv. It is verified that the density components in the OPRs determine such evolution processes. Furthermore, baroclinic instability is diagnosed to play a dominant role in the development of OPRs by analyzing the perturbed kinetic energy budget. This study suggests that the deep‐layer density perturbations in the Southern Ocean should be carefully monitored when considering the short‐range prediction of the ACC transport. Plain Language Summary: As the strongest ocean current in the world, the Antarctic circumpolar current (ACC) flows powerfully eastward around Antarctica in the Southern Ocean, playing critical roles in the global ocean and climate. It is observed that the ACC transport can fluctuate intensively in a short‐range period, which makes it challenging to be forecasted accurately. This study uses a high‐resolution regional ocean model and optimization framework to explore one kind of initial perturbation (called optimal precursor; OPR) causing the largest variations of the ACC transport within 30 days in the Drake Passage (DP). It is found that the OPRs are mainly located in the middle DP (58°–62°S, 72°–64°W) at the depth of 1,000–3,000 m. By generating northern clockwise and southern anticlockwise perturbed flows across the ACC jets, the OPRs for all cases can lead to sharp reductions in the ACC transport eventually. Furthermore, in the evolving processes of the OPRs, density components play the dominant roles and baroclinic instability is responsible for the evolutions. The specific structures of OPRs provide the most key regions for the ACC variations, which will conduce to designing the observation system for the short‐range forecast of the ACC transport. Key Points: The perturbations concentrated at 1,000–3,000 m in the middle Drake Passage reduce the ACC transport near 40 Sv after 30 daysThe OPR perturbs the upper‐layer circulation by triggering an eddy‐like dipole modeBaroclinic instability is responsible for the rapid growth of the OPR [ABSTRACT FROM AUTHOR]

Strong eddy activities exist in the Leeuwin Current (LC) and the South Indian Countercurrent (SICC) systems. Through a detailed investigation based on satellite observations, we find a cyclonic preference for eddies with more frequent genesis, longer lifespan, smaller size and stronger intensity in both systems. The evolutions of amplitudes, radii and total vorticities of eddies can be classified as the generation, stable, and decay stages; however, those of the EKE and relative vorticities lack the stable stage, due to planetary vorticity changes related to the meridional deflections, as eddies drift westward. Eddy properties exhibit significant seasonal and interannual variations. In the LC system, more and stronger eddies tend to be generated in winter and in La Niña years; while in the SICC system, eddy properties generally reach their peaks in spring and in years when the negative Southern Annular Mode (SAM) occurs. And eddy intensities in the SICC system are also modulated by ENSO with about a one‐year phase lag. Through instability analysis, we find that baroclinic instability is the most dominant contributor to the development of eddies. By contrast, barotropic instability mainly acts to dampen eddies. Advection of EKE makes almost no net contribution when spatially averaged, although it is locally significant. Pressure work and dissipation may be nonnegligible EKE sinks in the LC system. The EKE variations in these two systems have close connections through the westward‐propagating baroclinic instability anomalies, which may explain the phase lags between these two systems. Plain Language Summary: The southeast Indian Ocean is an eastern basin but hosts active eddies. Eddies are frequently observed in the Leeuwin Current and South Indian Countercurrent systems, and they play important roles in heat and momentum balances. However, the spatial distributions of eddy characteristics and their temporal variations are not well documented. In this study, by using the satellite observations, detailed eddy characteristics are presented in the southeast Indian Ocean. Moreover, by using high‐resolution model outputs, we deduce that the baroclinic instability associated with vertical shears of horizontal currents tends to force the generation of eddies, whereas the barotropic instability tends to dampen eddies. The westward propagation of baroclinic instability anomalies also modulates the eddy activities. Hopefully, these results will give a better assessment of the roles of eddies on ocean thermodynamics, ecosystems and climate change. Key Points: The properties of eddies exhibit significant seasonal and interannual variations and show some preferences depending on eddy polarityEddy generations are mainly dominated by baroclinic instability, whereas barotropic instability tends to dampen eddiesThe westward propagation of baroclinic instability anomalies may contribute to the phase lags between eddy activities in the Leeuwin Current and the South Indian Countercurrent systems [ABSTRACT FROM AUTHOR]

This study investigates basin and regional‐scale eddy (turbulent)‐induced heat and salt transport associated with mesoscale eddies of the Bay of Bengal (BOB). A high level of eddy‐induced transport is found in four subregions: the western boundary of the northern BOB (NB), the western BOB (WB), the southwestern BOB near Sri Lanka (SWB), and the southeastern BOB (SEB). The main seasons for eddy‐induced heat transport in the NB and SWB are identified as the presummer (March to April) and summer monsoon (May to September) seasons, whereas the postsummer monsoon season (October to November) is the main season for the WB and SEB. It is found that not only meridional but also zonal eddy‐induced transport is significant, due to the semienclosed nature of the BOB, which restricts poleward transport in the northern part of the bay. An analysis of upper‐layer eddy energetics reveals that the main contributor to the increased eddy kinetic energy and thus the transport is baroclinic instability in the NB, SWB, and WB, while both barotropic and baroclinic instability are important in the SEB. Using a 2.5‐layer reduced gravity model, baroclinic instability in the NB, SWB, and SEB is further investigated. The results reveal that strong vertical velocity shear and weak stratification are essential for generating baroclinic instability, which enhances the seasonal eddy‐induced transport in the respective subregions. Plain Language Summary: We estimate eddy‐induced heat and salt transport in the Bay of Bengal basin to understand the general characteristics and the key process determining their spatial and seasonal distribution. A linear stability analysis of eddy energetics demonstrates that weak stratification and strong vertical velocity shear generated by the strengthening of mean zonal currents intensify baroclinic instability and enhance the eddy‐induced transport. In addition, it is found that the local wind‐stress curl and remote forcing from the equator partly contribute to the seasonal modulation of eddy‐induced heat transport. Key Points: Baroclinic instability controls the seasonal variation of eddy kinetic energy in the western boundary of the Bay of BengalThe strengthening of mean currents intensifies baroclinic instability and enhances seasonal eddy‐induced transportsEddies associated with local and remote forcing play an important role in the heat and salt transport in the Bay of Bengal [ABSTRACT FROM AUTHOR]

This study attempts to identify the equilibration mechanisms of baroclinic instability and investigate the effects of the orientation of the background flow and topography on eddy‐induced transport. The analysis is based on growth rate balance theory, which assumes that nonlinear equilibration occurs when the growth rate of the primary baroclinic instability becomes comparable to that of the secondary instabilities of amplifying modes. Two‐layer quasi‐geostrophic numerical simulations are performed and compared to growth rate balance theory in order to analytically predict the cross‐stream fluxes of potential vorticity. The model performs remarkably well in predicting the effects of variation in zonal topographic slope and background flow orientation. We find that meridional topographic slopes affect the baroclinic instability in an inherently nonlinear way. A predictive model based on conservation of potential vorticity is developed for the optimal slope that maximizes the transport characteristics of the baroclinic instability. Plain Language Summary: The ocean is full of vortices on the scale of 10–100 km which play a major role in dispersing heat, salt, nutrients, and pollutants. However, the precise equilibration mechanisms controlling the production of these vortices remain a mystery. Physically, this process begins with the creation of north‐south oriented streams which themselves result in a secondary stream formation oriented in an east‐west direction. A new theory has found that when the north‐south streams balance the east‐west streams, the system achieves an equilibrium that allows us to characterize the heat transport by vortices through this system, which is critical to understanding global climate. Some key parts of this mystery are the effects of the ocean floor topography and the direction of the current on the production of these vortices. The proposed theory predicts the fluxes of heat as a function of ocean floor slopes and current direction. Key Points: A semianalytical theory predicts eddy‐induced transport in baroclinically unstable ocean regionsEffects of zonal topographic slope and orientation of the basic flow compare well to growth rate balance theoryWe present a predictive model that determines the meridional slope that maximizes cross‐stream flux of potential vorticity [ABSTRACT FROM AUTHOR]

Mesoscale eddies are important for regulating oceanic energy. Variation in eddy shapes leads to uncertainty in calculations of heat or energy content. In this study, we investigated the deformation of a warm eddy in the northern South China Sea from April to June 2018 and elucidated the mechanism governing the deformation. Satellite altimetry images showed that the warm eddy originated from Luzon Strait (LS eddy), migrated westward, and then moved along 500‐m isobaths until it approached the east of Hainan Island. Thereafter, the LS eddy deformed, moved southward, merged with other eddies, and finally dissipated. Using ship and virtual‐mooring Chinese underwater glider observations, we examined the three‐dimensional structure of the LS eddy. The warm eddy had a low‐density core that reached a depth of 250 m. The LS eddy gave rise to a front along its eastern edge, and two associate submesoscale eddies with horizontal radii of approximately 10 km were found at the front. The warm eddy was circular before deformation but morphed into an egg shape after deformation. This shape change allowed the eddy to entrain additional water mass (approximately 1014 kg). The deformation event was able to be forecasted by the vorticity and deformation index, as the eddy deformed by leaking into the zone with a high vorticity and deformation index. We used modeling and energy transformation calculations to analyze the mechanism of the warm eddy deformation. Our results revealed that baroclinic instability played a primary role in the deformation event. Key Points: Deformation of a warm eddy was observed using satellite and in situ data in spring 2018Submesoscale structures were found at the front side of the warm eddyBaroclinic process makes a primary contribution on the deformation of the warm eddy [ABSTRACT FROM AUTHOR]

Mooring data from September 2011 to July 2013 on the Iceland slope north of Denmark Strait are analyzed to better understand the structure and variability of the North Icelandic Jet (NIJ). Three basic configurations of the flow were identified: (1) a strong separated East Greenland Current (EGC) on the mid‐Iceland slope coincident with a weak NIJ on the upper slope, (2) a merged separated EGC and NIJ, and (3) a strong NIJ located at its climatological mean position, coincident with a weak signature of the separated EGC at the base of the Iceland slope. Our study reveals that the NIJ‐dominant scenario was present during different times of the year for the two successive mooring deployments—appearing mainly from September to February the first year and from January to July the second year. Furthermore, when this scenario was active it varied on short timescales. An energetics analysis demonstrates that the high‐frequency variability is driven by mean‐to‐eddy baroclinic conversion at the shoreward edge of the NIJ, consistent with previous modeling work. The seasonal timing of the NIJ dominant scenario is investigated in relation to the atmospheric forcing upstream of Denmark Strait. The resulting lagged correlations imply that strong turbulent heat fluxes in a localized region on the continental slope of Iceland, south of the Spar Fracture zone, lead to a stronger NIJ dominant state with a two‐month lag. This can be explained dynamically in terms of previous modeling work addressing the circulation response to dense water formation near an island. Plain Language Summary: The dense water flowing southward through Denmark Strait represents the largest contribution to the deep limb of the Atlantic Meridional Overturning Circulation, the large‐scale flow pattern that helps regulate Earth's climate. Three different currents feed the overflow in the strait: the shelfbreak East Greenland Current adjacent to Greenland, the separated East Greenland Current located farther offshore, and the North Icelandic Jet (NIJ) on the Iceland continental slope. The latter, which was recently discovered, supplies the deepest and densest overflow water to the strait. To understand the structure and variability of the NIJ, we analyze two years of mooring data on the Iceland slope roughly 200 km northeast of Denmark strait. We present the spatial and temporal variability of three configurations of the flow and focus on the NIJ‐dominant scenario. The rapid (less than 1 week) variability is found to arise from instability associated with the current's density structure. We then address the monthly variation, which we argue is driven by air‐sea heat fluxes in a localized region on the continental slope north of Iceland. The ocean responds to the fluxes in this region by exciting waves that propagate clockwise around the island, which in turn impact the strength of the NIJ. Key Points: Three different configurations of the flow on the Iceland slope north of Denmark Strait are identified using two years of mooring dataThe configuration where the North Icelandic Jet (NIJ) is dominant is baroclinically unstable and occurs during different times during the two yearsWe seek to explain the seasonal timing when the NIJ is dominant on Iceland slope to upstream atmospheric forcing [ABSTRACT FROM AUTHOR]

Abstract: Baroclinic instability of lateral density gradients gives way to lateral buoyancy transport, which often results in convergence of buoyancy transport. Along a sloping bottom, the induced convergence can force upward extension of bottom water. Eddy transport induced convergence at the bottom and the consequent suspended layers of bottom properties are investigated using a three‐dimensional idealized model. Motivated by the distinct characteristics of intrusions over the Texas‐Louisiana shelf, a series of configurations are performed with the purpose of identifying parameter impacts on the intensity of eddy transport. This study uses the “horizontal slope Burger number” as the predominant parameter; the parameter is functioned with S H = S R i − 1 / 2 = δ / R i to identify formation of baroclinic instability, where S is the slope Burger number, δ is the slope parameter, and Ri is the Richardson number, previously shown to be the parameter that predicts the intensity of baroclinic instability on the shelf. Intrusion spreads into the interior abutting a layer that is characterized by degraded vertical stratification; a thickening in the bottom boundary layer colocates with the intrusion, which usually thins at either edge of the intrusion because of a density barrier in association with concentrated isopycnals. The intensity of convergence degrades and bottom tracer fluxes reduce linearly with increased SH on logarithmic scales, and the characteristics of bottom boundary layer behavior and the reversal in alongshore current tend to vanish. [ABSTRACT FROM AUTHOR]

Abstract: Interannual variability of eddy kinetic energy (EKE) in the subtropical southeast Indian Ocean (SEIO) is investigated using satellite observations in three regions in the 20°S–35°S latitude band: R1 (108°E–115°E), R2 (100°E–108°E), and R3 (60°E–100°E). The El Niño‐Southern Oscillation (ENSO) plays an important role in modulating the interannual variability of EKE in the SEIO. EKE in the three regions shows negative correlations with the Nino3.4, lagging Nino3.4 by 2, 14, and 22 months, respectively. In R1, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of meridional velocity shear between the upper poleward Leeuwin Current (LC) and the underlying equatorward Leeuwin Undercurrent (LUC). During La Niña (El Niño) year, both the poleward LC and equatorward LUC strengthened (weakened) due to the high (low) sea level anomaly (SLA) propagating from the Pacific Ocean as Rossby wave under geostrophic equilibrium, and baroclinic instability of vertical shear enhanced (slackened), further induced the strong (weak) EKE in the eastern boundary. In R2, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the central and southern branches of the upper eastward South Indian Countercurrent (SICC) and the lower extending westward South Equatorial Current (SEC). In R3, both the ENSO and the Southern Annual Mode modulate the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the SICC and SEC. The interannual variability of EKE in the interior SEIO could be influenced by westward propagation of EKE originated from eastern boundary. [ABSTRACT FROM AUTHOR]

Abstract: Mesoscale eddies are known to transport heat and biogeochemical properties from Arctic Ocean boundary currents to basin interiors. Previous hydrographic surveys and model results suggest that eddy formation may be common in the Atlantic Water (AW) inflow area north of Svalbard, but no quantitative eddy survey has yet been done for the region. Here vorticity and water property signatures are used to identify and track AW eddies in an eddy‐resolving sea ice‐ocean model. The boundary current sheds AW eddies along most of the length of the continental slope considered, from the western Yermak Plateau to 40°E, though eddies forming east of 20°E are likely more important for slope‐to‐basin transport. Eddy formation seasonality reflects seasonal stability properties of the boundary current in the eastern portion of the study domain, but on and immediately east of the Yermak Plateau enhanced eddy formation during summer merits further investigation. AW eddies tend to be anticyclonic, have radii close to the local deformation radius, and be centered in the halocline. They transport roughly 0.16 Sv of AW and, due to their warm cores, 1.0 TW away from the boundary current. These findings suggest eddies may be important for halocline ventilation in the Eurasian Basin, as has been shown for Pacific Water eddies in the Canadian Basin. [ABSTRACT FROM AUTHOR]