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The Southeast Tropical Indian Ocean (SETIO), dominated by the Indian Ocean monsoon, is an important source region for strong mesoscale eddies. To date, the impacts of the Indian Ocean monsoon on mesoscale eddies have not been clarified. Here we report on the dipole response of mesoscale eddy formation to monsoon transition in the SETIO, using satellite and reanalysis data sets. During the summer monsoon season, anticyclonic eddies are mainly concentrated north of 12°S, while cyclonic eddies are south of 12°S. This situation reverses during the winter monsoon season. We attribute this dipole feature to the oceanic perturbations and current shear during the different monsoon periods. A geographical boundary along 12°S aligns with meridional changes in eddy potential energy, which delineates the generation and direction of the newly‐formed eddies. The hot spot region, rich in eddy energy properties, tends to promote eddy formation and endurance during the monsoon periods. Plain Language Summary The Southeast Tropical Indian Ocean (SETIO) is a typical region of strong mesoscale (∼10–100 km) eddy generation. Eddies are circular currents that are important in moving heat, nutrients, and marine life around the ocean. The SETIO is also dominated by the Indian Ocean monsoon, which is a seasonal weather pattern that typically occurs in two main phases: the southwest monsoon from June to September, and the northeast monsoon from December to March. To date, the impacts of the Indian Ocean monsoon on the mesoscale eddies remain unclear. Based on satellite and reanalysis data sets, we found that there is a natural latitudinal change in the direction of eddies (anticlockwise/clockwise) formed north/south of 12°S in the summer monsoon, and that this pattern switches in the winter monsoon. The monsoon transition and associated changes to the ocean and its currents drives the dual‐pattern. The geographical boundary along 12°S occurs because it aligns with latitudinal changes in the energy stored in the eddies, which delineates a change in the direction of the newly‐formed eddies. This hot spot region, rich in eddy energy properties, promotes eddies formation and endurance during the monsoon periods. Key Points Strong mesoscale eddies in the Southeast Tropical Indian Ocean are generated in a clear seasonal cycle The eddies present a distinct dipole response to the monsoon transition in the region Changes in oceanic perturbation and current shear modulated by monsoon transition is responsible for this dipole response of eddies

Freshwater input from Greenland ice sheet melt has been increasing in the past decades from warming temperatures. To identify the impacts from enhanced meltwater input into the subpolar North Atlantic from 1997 to 2021, we use output from two nearly identical simulations in the eddy‐rich model VIKING20X (1/20°) only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other with climatologically (1961–2000) continued runoff. The majority of the additional freshwater remains within the boundary current enhancing the density gradient toward the warm and salty interior waters yielding increased current velocities. The accelerated boundary current shows a tendency to enhanced, upstream shifted eddy shedding into the Labrador Sea interior. Further, the experiments allow to attribute higher stratification and shallower mixed layers southwest of Greenland and deeper mixed layers in the Irminger Sea, particularly in 2015–2018, to the runoff increase in the early 2000s. Plain Language Summary Global warming has accelerated the melting of the Greenland ice sheet over the past few decades resulting in enhanced freshwater input into the North Atlantic. The additional freshwater can potentially inhibit deep water formation and have future implications on ocean circulation. To determine the influence from Greenland melt, we compare two high‐resolution model experiments all with the same forcing but differing input of Greenland freshwater fluxes from 1997 to 2021. We find that in the experiment with realistically increasing Greenland meltwater, the water becomes fresher and cooler along the continental shelf and boundary of the subpolar gyre. The density difference between the shelf and interior increases with more freshwater, resulting in faster West Greenland Current speeds and enhanced eddy formation. Deeper mixed layers are found in the eastern Irminger Sea, particularly in 2015–2018. From 2009 to 2013, there were shallower mixed layers in the Labrador Sea where less Greenland meltwater was mixed downwards and spread eastward, causing mixed layers to deepen in the Irminger Sea. Key Points The West Greenland Current (WGC) freshens and cools with the observed recent increase in meltwater runoff from Greenland The density gradient across the boundary current intensifies, strengthening the WGC and increasing local eddy formation Enhanced meltwater runoff contributed to an eastward shift in deep convection towards the Irminger Sea (2015–2018)

Automated methods are important for the identification of mesoscale eddies in the large volume of oceanic data provided by altimetric measurements and numerical simulations. This paper presents an optimized algorithm for detecting and tracking eddies from two-dimensional velocity fields. This eddy identification uses a hybrid methodology based on physical parameters and geometrical properties of the velocity field, and it can be applied to various fields having different spatial resolutions without a specific fine-tuning of the parameters. The efficiency and the robustness of the angular momentum eddy detection and tracking algorithm (AMEDA) was tested with three different types of input data: the 1/8° Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO) geostrophic velocity fields available for the Mediterranean Sea; the output of the idealized Regional Ocean Modeling System numerical model; and the surface velocity field obtained from particle imagery on a rotating tank experiment. All these datasets describe the dynamical evolution of mesoscale eddies generated by the instability of a coastal current. The main advantages of AMEDA are as follows: the algorithm is robust to the grid resolution, it uses a minimal number of tunable parameters, the dynamical features of the detected eddies are quantified, and the tracking procedure identifies the merging and splitting events. The proposed method provides a complete dynamical evolution of the detected eddies during their lifetime. This allows for identifying precisely the formation areas of long-lived eddies, the region where eddy splitting or merging occurs frequently, and the interaction between eddies and oceanic currents.

Eddies generated off the west Greenland coast modulate the deep convection in the Labrador Sea, while there are still open questions related to their formation mechanisms. Using 11 years (2008–18) of output from a NEMO model configured with a 1/60° nest in the Labrador Sea, we present the patterns of baroclinic and barotropic instability off the west Greenland coast. We highlight the generation of Irminger Rings at Cape Desolation and boundary current eddies at the location of the Overturning in the Subpolar North Atlantic Program (OSNAP) West section. In between these formation sites, eddy energy attenuation occurs along the West Greenland Current (WGC). Overall, baroclinic instability dominates in the upper 1000 m and is twice as strong as the barotropic instability. Seasonally, the instabilities are generally twice as strong in winter compared to summer. Interannually from 2008 to 2018, the instabilities generally show a strengthening trend, with values in 2018 two to three times as strong as those in 2008. We found that on an interannual time scale, the strengthening of WGC and the steepening of its velocity contours enhance the barotropic instability, and the intrusion of the upper Irminger Sea Intermediate Water (uISIW) on the Irminger Water enhances the baroclinic instability by increasing the horizontal density gradient. On a seasonal time scale, variability of the eddy momentum and density fluxes modulate the barotropic and baroclinic instability, respectively. From observation-based datasets, we also found that the downstream eddy kinetic energy is highly correlated with the uISIW transports, suggesting that the amount of uISIW affects the eddy formation. Using a very high-resolution numerical model, our study provides new insight into the variability and mechanisms of eddy formation along the west Greenland coast.

The mean vertical structure of mesoscale eddies in the Peru‐Chile Current System is investigated by combining the historical records of Argo float profiles and satellite altimetry data. A composite average of 420 (526) profiles acquired by Argo floats that surfaced into cyclonic (anticyclonic) mesoscale eddies allowed constructing the mean three‐dimensional eddy structure of the eastern South Pacific Ocean. Key differences in their thermohaline vertical structure were revealed. The core of cyclonic eddies (CEs) is centered at ∼150 m depth within the 25.2–26.0 kg m−3 potential density layer corresponding to the thermocline. In contrast, the core of the anticyclonic eddies (AEs) is located below the thermocline at ∼400 m depth impacting the 26.0–26.8 kg m−3 density layer. This difference was attributed to the mechanisms involved in the eddy formation. While intrathermocline CEs would be formed by instabilities of the surface equatorward coastal currents, the subthermocline AEs are likely to be shed by the subsurface poleward Peru‐Chile Undercurrent. In the eddy core, maximum temperature and salinity anomalies are of ±1°C and ±0.1, with positive (negative) values for AEs (CEs). This study also provides new insight into the potential impact of mesoscale eddies for the cross‐shore transport of heat and salt in the eastern South Pacific. Considering only the fraction of the water column associated with the fluid trapped within the eddies, each CE and AE has a typical volume anomaly flux of ∼0.1 Sv and yields to a heat and salt transport anomaly of ±1–3 × 1011 W and ±3–8 × 103 kg s−1, respectively. Key Points A new methodology is proposed to assess the eddy vertical structure from ARGO profiles Cyclonic and anticyclonic eddies show clear distinct vertical structures They impact differently on heat and salt transports of the thermo‐ and subthermocline

An array of moorings deployed off the coast of Palau is used to characterize submesoscale vorticity generated by broadband upper-ocean flows around the island. Palau is a steep-sided archipelago lying in the path of strong zonal geostrophic currents, but tides and inertial oscillations are energetic as well. Vorticity is correspondingly broadband, with both mean and variance O ( f ) in a surface and subsurface layer (where f is the local Coriolis frequency). However, while subinertial vorticity is linearly related to the incident subinertial current, the relationship between superinertial velocity and superinertial vorticity is weak. Instead, there is a strong nonlinear relationship between subinertial velocity and superinertial vorticity. A key observation of this study is that during periods of strong westward flow, vorticity in the tidal bands increases by an order of magnitude. Empirical orthogonal functions (EOFs) of velocity show this nonstationary, superinertial vorticity variance is due to eddy motion at the scale of the array. Comparison of kinetic energy and vorticity time series suggest that lateral shear against the island varies with the subinertial flow, while tidal currents lead to flow reversals inshore of the recirculating wake and possibly eddy shedding. This is a departure from the idealized analog typically drawn on in island wake studies: a cylinder in a steady flow. In that case, eddy formation occurs at a frequency dependent on the scale of the obstacle and strength of the flow alone. The observed tidal formation frequency likely modulates the strength of submesoscale wake eddies and thus their dynamic relationship to the mesoscale wake downstream of Palau.

There has been a steady increase in interest in mining of deep-sea minerals in the Clarion–Clipperton Zone (CCZ) in the eastern Pacific Ocean during the last decade. This region is known to be one of the most eddy-rich regions in the world ocean. Typically, mesoscale eddies are generated by intense wind bursts channeled through gaps in the Sierra Madre mountains in Central America. Here, we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability in the region of potential future mining and eddy kinetic energy (EKE) in the vicinity of gap winds. A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years, from 1993 to 2016, in order to analyze the main characteristic parameters and the spatiotemporal variability of mesoscale eddies in the northeast tropical Pacific Ocean (NETP). Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anticyclonic) were found. For eddies with lifetimes longer than 1 d, cyclonic polarity is more common than anticyclonic rotation. However, anticyclonic eddies are larger in size, show stronger vorticity, and survive longer in the ocean than cyclonic eddies (often 90 d or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration when investigating the impacted deep-ocean region as we found eddies originating from the Tehuantepec (TT) gap winds lasting longer in the ocean and traveling farther distances in a different direction compared to eddies produced by the Papagayo (PP) gap winds. Long-lived anticyclonic eddies generated by the TT gap winds are observed to travel distances up to 4500 km offshore, i.e., as far as west of 110∘ W. EKE anomalies observed in the surface of the central ocean at distances of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5–6 months. A significant seasonal variability of deep-ocean current velocities at water depths of 4100 m was observed in multiple-year time series data, likely reflecting the energy transfer of the surface EKE generated by the gap winds to the deep ocean. Furthermore, the influence of mesoscale eddies on deep-ocean currents is examined by analyzing the deep-ocean current measurements when an anticyclonic eddy crosses the study region. Our findings suggest that despite the significant modulation of dominant current directions driven by the bottom-reaching eddy, the current magnitude intensification was not strong enough to trigger local sediment resuspension in this region. A better insight into the annual variability of ocean surface mesoscale activity in the CCZ and its effects on deep-ocean current variability can be of great help to mitigate the impact of future potential deep-sea mining activities on the benthic ecosystem. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El Niño–Southern Oscillation (ENSO) was found, whereas a weaker correlation was detected for anticyclonic eddies.

Submesoscale lenses of water with anomalous hydrographic properties have previously been observed in the East Australian Current (EAC) system, embedded within the thermocline of mesoscale anticyclonic eddies. The waters within these lenses have high oxygen content and temperature–salinity properties that signify a surface origin. However, it is not known how these lenses form. This study presents field observations that provide insight into a possible generation mechanism via subduction at upper-ocean fronts. High-resolution hydrographic and velocity measurements of submesoscale activity were taken across a front between a mesoscale eddy dipole downstream of the EAC separation point. The front had O (1) Rossby number, strong vertical shear, and flow conducive to symmetric instability. Frontogenesis was measured in conjunction with subduction of an anticyclonic water parcel, indicative of intrathermocline eddy formation. Twenty-five years of satellite imagery reveals the existence of strong mesoscale strain coupled with strong temperature fronts in this region and indicates the conditions that led to frontal subduction observed here are a persistent feature. These processes impact the vertical export of tracers from the surface and dissipation of mesoscale kinetic energy, implicating their importance for understanding regional ocean circulation and biological productivity.

Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial‐interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15N/14N ratio (δ15N) of mixed‐layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ15N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N2 fixation that occurs in the Indian Ocean. Foraminifera δ15N may have been lowered further during eddy migration by in situ N2 fixation and/or recycling of low‐δ15N ammonium. The absence of low‐δ15N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ15N. The low δ15N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer‐bound δ15N from the region could record variations in past Agulhas leakage. Plain Language Summary “Agulhas leakage,” the flow of seawater from the Indian to the Atlantic Ocean, is a key component of global ocean circulation and predominantly takes the form of eddies. Identifying past changes in leakage can provide insights into the relationship between Atlantic Ocean circulation and climate changes. Here, we investigated whether the ratio of the nitrogen isotopes, 15N and 14N, in organic matter preserved in the shells of fossil foraminifera could be used to investigate past Agulhas leakage. Thermocline (i.e., sub‐surface) nitrate has a lower 15N‐to‐14N ratio in the Agulhas Current region, the source region for Agulhas leakage, than in the South Atlantic. To determine whether foraminifera inhabiting Agulhas leakage reflect this trend, we collected living specimens from inside and outside of a well‐developed Agulhas eddy in the southeastern Atlantic. We found that the nitrogen in foraminifera biomass and shells is isotopically lower in the eddy than in the “background” southeastern Atlantic. This signal can be explained by the lower 15N‐to‐14N ratio of the original (Agulhas‐sourced) nitrate, potentially augmented by internal nitrogen cycle processes occurring within the eddy. Our results strongly suggest that the nitrogen isotopes of fossil foraminifera could be used as an indicator of past variations in Agulhas leakage. Key Points Nitrogen isotope ratios of mixed‐layer nitrate, zooplankton, and foraminifera in an Agulhas eddy are lower than in the southeast Atlantic Deeper‐dwelling, symbiont‐barren foraminifera record the nitrogen isotope ratio of thermocline nitrate Foraminifer‐bound nitrogen isotopes in the Cape Basin sediment record could be used to reconstruct past variations in Agulhas leakage

In the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) m s−1, and average core anomalies of potential vorticity 65% below (125% above) ambient values. Anticyclones contained an order of magnitude greater available heat and salt anomaly relative to background conditions than cyclones on average. Circumstantial evidence of eddy decay through lateral intrusions was found although this was not observed consistently. Observed eddy properties and the geometry of flow over the continental slope were consistent with eddy formation due to frictional torque acting on the CU. Loss of heat and salt from the CU due to subthermocline eddies is estimated to account for 44% of the freshening and cooling of the CU as it flows poleward.