Anticyclonic Eddies Research Articles

Eddy-Induced Subsurface Spiciness Anomalies in the Kuroshio Extension Region

Abstract While mesoscale eddy-induced temperature and salinity (T and S) variations at depth levels were widely reported, those on isopycnal surfaces have been largely unexplored so far. This study investigates temperature and salinity anomalies (T′ and S′; dubbed “spiciness anomalies”) on isopycnal surfaces induced by mesoscale eddies in the Kuroshio Extension (KET) region, with a focus on the North Pacific Intermediate Water (NPIW) layer of 26.3–26.7σθ. Cyclonic eddies (CEs) and anticyclonic eddies (AEs) tend to cluster on the northern and southern flanks of the KET jet, respectively. These eddies are characterized by a large radius (CEs: 61.94 km; AEs: 68.05 km), limited zonal movement, and a tendency of meridional movement (CEs: 0.35 cm s−1 southward; AEs: 0.66 cm s−1 northward). The average eddy-induced T′ and S′ are −0.25°C (0.06°C) and −0.05 psu (0.01 psu) for CEs (AEs) in the 26.3–26.7σθ layer. We propose two mechanisms for the generation of subsurface spiciness anomalies, respectively, for moving eddies that travel over long distances with trapped waters and quasi-stationary meander eddies that are generated by the meanders of the KET front. The T′ and S′ induced by moving eddies cumulatively drive cross-front water exchanges. Meander eddies shift the position of the front and induce redistribution of properties. However, these anomalies do not contribute to heat and salt exchanges between water masses. This work provides a useful benchmark for model simulations of mesoscale isopycnal variability in subsurface waters.

Spatiotemporal Variability of the Characteristics of the Black Sea Eddies Based on the Results of Their Three-Dimensional Identification Using NEMO Model Simulation

The vertical variability of the geometric, dynamic, and thermohaline characteristics of Black Sea eddies is studied using three-dimensional identification of closed streamlines based on the results of simulations of the NEMO model in 2006–2016. The vertical extent of eddies depends linearly on their orbital velocity and significantly affects the speed of their translation motion. The influence of the background current on the upper part of the eddy leads to the mean tilt of the vertical axis of anticyclones in the cyclonic direction and this tilt is maximum in the zone of the Black Sea Rim Current. The eddies occupying the upper layer (up to 150 m) move relatively quickly at a speed of 0.08–0.14 m/s, since they are advected by the Rim Current. The most intense eddy dynamics is observed over the continental slope in the upper 20–70 m layer, where the probability of observing eddies exceeds 30%. Several maxima appear in the surface layer on the shallow northwestern shelf near the mouths of the Danube, Southern Bug, and Dnieper rivers. The relationship between the orbital velocity of eddies, their thermohaline structure, and stratification were estimated. Stability decreases in anticyclonic eddies within the layer 40–100 m due to the lowering of the pycnocline, and in cyclones it increases due to its elevation; in the underlying layers a compensating change of the opposite sign is observed. However, as the orbital velocity increases in the eddies of both signs, a decrease in stratification occurs in the upper 0–100 m layer, which is probably associated with an increase in the vertical shear of the current velocity. Three-dimensional identification of eddies makes it possible to track in detail changes in the individual characteristics of an eddy during its evolution. Using the example of an eddy off the Anatolian coast, it is shown that intensification of anticyclones is associated with the processes of entrainment of shelf waters, which can be one of the important sources of potential energy for anticyclones in the Black Sea.

Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean

AbstractThe Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient‐limited. Episodic mixing events characteristic of WBC systems can temporarily alleviate nutrient scarcity by vertically entraining deep nutrients into surface waters. However, our understanding of these nutrient fluxes is lacking because they are spatio‐temporally limited, and once they enter the sunlit layer, the nutrients are rapidly consumed by phytoplankton. Here, we use a novel application of nitrate Δ(15–18), the difference between the nitrogen and oxygen isotope ratios of nitrate, to characterize three (sub)mesoscale events of upward nitrate supply across the Agulhas Current in winter: (1) mixing at the edges of an anticyclonic eddy, (2) inshore upwelling associated with a submesoscale meander of the Agulhas Current, and (3) overturning at the edge of the current core driven by submesoscale instabilities. All three events manifest as upward injections of high‐Δ(15–18) nitrate into the thermocline and surface where nitrate Δ(15–18) is otherwise low; these entrainment events are not always apparent in the other co‐collected data. The dynamics driving the nitrate supply events are common to all WBCs, implying that nutrient entrainment facilitated by WBCs is quantitatively significant and supports productivity in otherwise oligotrophic subtropical surface waters. A future rise in energy across WBC systems may increase these nutrient fluxes, partly offsetting the predicted stratification‐induced decrease in subtropical ocean fertility.

Multiscale spatial patterns and environmental drivers of seamount and island slope megafaunal assemblages along the Mozambique channel

Seamounts are vulnerable ecosystems targeted by fishing and potentially by future mineral exploitation. Their abundance, widespread distribution, and heterogeneity of faunal and abiotic components require integrated studies at multiscale to describe spatial patterns and identify environmental drivers needed by conservation plans. There is also a lack of knowledge on seamount benthic ecosystems in some regions, such as the Indian Ocean. These gaps, in the context of Marine Protected Areas establishment in the region, have motivated the present study focusing on the Mozambique Channel Eparses islands and flat top seamounts, along a 10-degree latitude gradient. These structures are characterized by complex volcanic and carbonate geomorphologies at multiscale and are distributed along a highly dynamic turbulent ocean circulation area with large anticyclonic eddies. For the first time, we analysed, from seabed image transects obtained by towed-camera on four seamounts, and two volcanic islands - Bassas da India and Mayotte - external slopes, and from multiscale environmental data, how benthic communities respond to this high habitat heterogeneity at regional, and local scales. This study reveals high discrepancies of benthic megafauna richness, density, and beta diversity among seamounts and among slopes of the same islands. Moreover, at similar latitude, seamounts display higher densities than island slopes. The highest densities found on a seamount of the Glorieuses archipelago are explained by strong currents and flat homogeneous geomorphology. Except on this seamount, the beta diversity is high, despite the quite limited depth range explored (84–734 m) and is the highest on island slopes and Hall Bank, driven by the diversity and hardness of the substrate. Beta diversity is mainly due to taxa turnover, with high contribution of the habitat-forming sponges and cnidarians, together with a few mobile taxa. We identified from biogeographic network analysis 12 dominant faunal assemblages, displaying a patchy distribution, with variability in composition both among and within sites. Currents and primary productivity explain ∼15% of the observed assemblage structure along the channel, while geomorphology (km scale), topography (60–500 m scale) and substrate (60-m units) explain together 24% of the faunal spatial patterns. Analysis of spatial structures along island slopes detected some small (100–200 m), medium (∼1 km) and large scale (∼2–6 km) megabenthic community structures, partly explained by topography, substrate, depth, and slope. Despite limited taxonomic identifications for this poorly sampled area, this study reveals an outstanding heterogeneity of megabenthic assemblages at multispatial scales in the Mozambique Channel seamounts and island slopes, in response to the complex hydrography and geology of the area. Further characterization of environmental drivers with greater focus at local scales including hydrographic variables are therefore needed to improve predictions of suitable habitats of vulnerable marine ecosystems.

Impact of surface and subsurface-intensified eddies on sea surface temperature and chlorophyll a in the northern Indian Ocean utilizing deep learning

Abstract. Mesoscale eddies, including surface-intensified eddies (SEs) and subsurface-intensified eddies (SSEs), significantly influence phytoplankton distribution in the ocean. Nevertheless, due to the sparse in situ data, understanding of the characteristics of SSEs and their influence on chlorophyll a (Chl a) concentration is still unclear. Consequently, the study utilized a deep learning model to extract SEs and SSEs in the northern Indian Ocean (NIO) from 2000 to 2015, using satellite-derived sea surface height (SSH) and sea surface temperature (SST) data. The analysis revealed that SSEs accounted for 39 % of the total eddies in the NIO, and their SST signatures exhibited opposite behaviour compared to SEs. Furthermore, by integrating ocean colour remote-sensing data, the study investigated the contrasting impacts of SEs and SSEs on Chl a concentration in two basins of the NIO, the Arabian Sea (AS) and the Bay of Bengal (BoB), known for their disparate biological productivity. In the AS, SEs induced Chl a anomalies that were 2 to 3 times higher than those caused by SSEs. Notably, there were no significant differences in Chl a anomalies induced by the same type of eddies between summer and winter. In contrast, the BoB exhibited distinct seasonal variations, where SEs induced slightly higher Chl a anomalies than SSEs during the summer, while substantial differences were observed during the winter. Specifically, subsurface-intensified anticyclonic eddies (SSAEs) led to positive Chl a anomalies, contrasting the negative anomalies induced by surface-intensified anticyclonic eddies (SAEs) with comparable magnitudes. Moreover, while both subsurface-intensified cyclonic eddies (SSCEs) and surface-intensified cyclonic eddies (SCEs) resulted in positive Chl a anomalies during winter in the BoB, the magnitude of SSCEs was only one-third of that induced by SCEs. Besides, subsurface Chl a induced by SSAEs (SSCEs) is ∼0.1 mg m−3 greater (less) than that caused by SAEs (SCEs) in the upper 30 (50) m using Biogeochemical Argo profiles. The distinct Chl a between SEs and SSEs can be attributed to their contrasting subsurface structures revealed by Argo profiles. Compared to SAEs (SCEs), SSAEs (SSCEs) enhance (decrease) production via the convex (concave) of the isopycnals that occur around the mixed layer. The study provides a valuable approach to investigating subsurface eddies and contributes to a comprehensive understanding of their influence on chlorophyll concentration.

Assessment of oceanographic conditions during the North Atlantic EXport processes in the ocean from RemoTe sensing (EXPORTS) field campaign

This manuscript presents an overview of NASA’s EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA) and provides quantitative and dynamical descriptions of the physical processes modulating water transformations during the study. A major programmatic goal was to conduct the sampling in a Lagrangian mode so that ocean ecological and biogeochemical changes can be observed independent from physical advective processes. To accomplish this goal, EXPORTS NA conducted a multi-ship, multi-asset field sampling program within a retentive, anticyclonic mode water eddy. Beneath depths of ∼ 100 m, Lagrangian sampling assets remained within the eddy core waters (ECWs) throughout the experiment, demonstrating that the ECWs within the mode water eddy were retentive. However, strong westerly winds from four storm events deepened the mixed layer (ML) of the surface core waters (SCWs) above the eddy’s mode water core by 25–40 m and exchanged some of the SCWs with surface waters outside of the eddy via Ekman transport. Estimates of flushing times ranged from 5 to 8 days, with surface exchange fractions ranging from 20 to 75 % and were consistent with particle tracking advected by combined geostrophic and Ekman velocities. The relative contributions of horizontal and vertical advection on changes in ECW tracers depended on the horizontal and vertical gradients of that tracer. For example, horizontal advection played a large role in ECW salinity fluxes, while vertical entrainment played a larger role in the fluxes of nutrients into SCW ML. Each storm injected nutrients and low oxygen waters into the ML, after which the surface ocean ecosystem responded by reducing nutrient concentrations and increasing %O2 saturation levels. Overall, ECW values of chlorophyll and POC were the largest at the onset of the field program and decreased throughout the campaign. The analysis presented provides a physical oceanographic context for the many measurements made during the EXPORTS NA field campaign while illustrating the many challenges of conducting a production-flux experiment, even in a Lagrangian frame, and the inherent uncertainties of interpreting biological carbon pump observations that were collected in a Eulerian frame of reference.

Enhanced Mixing Induced by Near-Inertial Waves Inferred by Glider Observation in the Northern South China Sea

Enhanced turbulence triggered by near-inertial wave (NIW) trapping by a mesoscale anticyclone and typhoon “Kompasu” was observed in the northern South China Sea. The observations provide evidence for the trapping of NIW packets of amplitude ~0.2 m/s near the base of an anticyclonic eddy, and of ~0.3 m/s after the passage of typhoon “Kompasu”. The wave energy was amplified in a layer located near the base of the anticyclonic eddy, between 150 and 300 m, while stronger NIWs triggered by the typhoon extended to depths > 500 m. Diffusivity was calculated by a fine-scale parameterization. A diffusivity elevated by one order of magnitude, the occurrence of high near-inertial velocity shears, and the low (≤1) Richardson numbers were consistent with turbulence production and mixing from the base of the anticyclonic eddy and following the passage of the typhoon, and were associated with the trapped NIWs. This study showed that, by serving as a bridge between mesoscale eddies and small-scale motion, NIWs are an important pathway for ocean energy transmission. Mesoscale-NIW interactions represent a significant source of NIWs as well as a sink of mesoscale energy.

Vertical variation of bacterial production and potential role in oxygen loss in the southern Bay of Bengal.

Marine environments wherein long-term microbial oxygen consumption exceeds oxygen replenishment can be associated with oxygen minimum zones (OMZ). The Bay of Bengal OMZ (BOB-OMZ) is one of the most intense OMZs globally. To assess the contribution of bacterial oxygen consumption to oxygen loss in BOB-OMZ, we measured bacterial production (BP), temperature, salinity, and dissolved oxygen (DO) in the whole water column. We then compared the estimated bacterial oxygen demand (BOD) with diapycnal oxygen supply (DOS) at depths of 50-200 m in the southern BOB in January 2020. The average BP was 3.53 ± 3.15 μmol C m-3 h-1 in the upper 200 m of four stations, which was lower than those reported in other tropical waters. The vertical distribution of BP differed between the open ocean and nearshore areas. In the open ocean, temperature and DO were the most important predictors for BP in the whole water column. In the nearshore areas, when DO increased sharply from the suboxic state, extremely high BP occurred at 200 m. The average estimated BOD/DOS could reach up to 153% at depths of 50-200 m, indicating advection and anticyclonic eddies probably are important DO replenishment pathways in the BOB.

Diagnostic Analysis of the Response of Volume Transport through the Tsushima Strait to the Eddy-Induced Variations in the Kuroshio Region

Abstract The Tsushima Strait (TS) is the sole passage for volume transport from the East China Sea to the Sea of Japan. To date, the process underlying the interannual variability in volume transport remains unclear. In this study, 27-yr (1993–2019) reanalysis data from the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2M) system and in situ/satellite observations were employed to understand this process. The results suggest that TS transport was generally high in 1999, 2003/04, and 2010 and low in 1995/96, 2005/06, 2008, and 2014/15. The sea level anomaly (SLA) outside the entrance of the TS, that is, the upstream TS forcing, dominates the interannual TS transport variation. A high SLA pumps more water into the Sea of Japan via the TS, and vice versa. By synthesizing JCOPE2M reanalysis data and satellite observations, further analysis revealed that cyclonic mesoscale eddies from the subtropical countercurrent (STCC) could be responsible for this high SLA by reducing Kuroshio transport, enhancing Kuroshio intrusion across the shelf, and increasing the SLA around the upstream TS region. The reverse was true for anticyclonic STCC eddies. Variability in the Kuroshio intrusion southwest of Kyushu induces variations in the TS transport on an interannual time scale.

Systematic Bias Correction in Ocean Mesoscale Forecasting Using Machine Learning

AbstractThe ocean circulation is modulated by meandering currents and eddies. Forecasting their evolution is a key target of operational models, but their forecast skill remains limited. We propose a machine learning approach that improves the output of an ocean circulation model by learning and predicting its systematic biases. This method can be applied a priori to any region, and is tested in the Gulf of Mexico, where the Loop Current (LC) and the large anticyclonic eddies that detach from it are major forecasting targets. The LC dynamics are recurrent and lie on a low‐dimensional dynamical attractor. Building upon the information gained analyzing this low dimensional attractor, we improve the representation of sea surface anomalies in model outputs through information from satellite altimeter data using a Sequence‐to‐Sequence model, which is a special class of Recurrent Neural Network. Building upon the HYCOM‐NCODA analysis system, we deliver a correction to the forecast at the observation resolution. For at least 15 days the proposed method learns to forecast the systematic bias in the HYCOM‐NCODA, outperforming persistence, and improving the forecast. This data‐driven approach is fast and can be implemented as an added step to any dynamical hindcasting or forecasting model. It offers an interesting avenue for further developing hybrid modeling tools. In these tools, fundamental physical conservations are preserved through the integration of partial differential equations which obey them. In addition, the method highlights specific deficiencies of the hindcast system that deserve further investigation in the future.

Mesoscale Eddy Modulation of Subsurface Chlorophyll Maximum Layers in the South China Sea

AbstractSubsurface chlorophyll maximum (SCM) layers contribute considerably to the integrated biomass of the water column and can be strongly modulated by mesoscale eddies that are ubiquitous in the global ocean. The mechanisms of eddy‐induced surface chlorophyll concentration have been extensively examined in the South China Sea (SCS). However, the potential impact of mesoscale eddies on SCM layers remains unclear. We examined the influence of mesoscale eddies on the depth, thickness and magnitude of SCM layers in the SCS using output from an eddy‐permitting biological–physical coupled model over a 22‐year period. Our study shows that nutrient distribution is largely driven by eddy dynamics, with cyclonic eddies enhancing the supply of inorganic nutrients in the upper layers by uplifting the thermocline, and downward displacement of the thermocline in anticyclonic eddies, reducing the nutrient supply into the euphotic zone from the depth. We found that anticyclonic (cyclonic) eddies are responsible for increased (decreased) SCM depth and decreased (increased) SCM magnitude; SCM thickness decreased in cyclonic eddies but slightly increased in anticyclonic eddies. The effects of mesoscale eddies depend on eddy amplitude. Maximal anomalies in depth, thickness and magnitude always occur near the center of eddies. Phytoplankton community structure at SCM layers is also affected by eddies, with more diatoms in cyclonic eddies and more coccolithophores in anticyclonic eddies. Our study will advance our understanding of mesoscale physical–biogeochemical interactions.

Response of oceanic subsurface chlorophyll maxima to environmental drivers in the Northern Indian Ocean

Subsurface Chlorophyll Maxima (SCM) contributes a significant proportion to depth-integrated ocean primary production, making it important to understand its spatiotemporal variability in changing environmental conditions. Based on field observations and in situ data, we studied SCM characteristics in four distinct environmental settings across Northern Indian Ocean: SEAS-south eastern Arabian Sea (coastal upwelling zone), SAS-Southern Arabian Sea (Arabian Sea mini warm pool-ASMWP), SBOB-Southern Bay of Bengal (presence of mesoscale eddies) and ANS-Andaman Sea (region of active volcanoes). SCM displayed significant spatial variability: ZSCM (SCM depth) ranged between 25 and 88 m (mean = 59.5 m), Chlmax (SCM magnitude) ranged between 0.07 and 0.2 mg m−3 (mean = 0.12 mg m−3) while TSCM (SCM thickness) ranged between 33 and 100 m (mean = 69 m). Major factors affecting the ZSCM were light and nutrients, since ZSCM was closely related to Zeu (euphotic depth) and ZN (nitracline depth). Positive relation between Chlmax and micro phytoplankton suggests micro phytoplankton to be a major contributor in increasing Chlmax. TSCM was associated to ZT (thermocline depth) and nutrient concentration. Increase in stratification and oligotrophy, resulted in deeper, thicker peaks of lower magnitude, and such nature of peaks could increase in continued warming scenarios. Dinoflagellates increased in warmer oligotrophic environments. Nanophytoplankton were higher at deeper ZSCM, possibly due to their adaptability to low light. Upwelling caused shoaling of SCM with higher Chlmax in SEAS. SCM deepened in SAS and SBOB, dominated by picophytoplankton due to the influence of ASMWP (in SAS) and anticyclonic eddy (in SBOB). Ammonia and nutrient inputs from submarine vents in ANS, caused shoaling of ZN and ZSCM. Globally, multiple physical processes operate at short spatiotemporal scale, causing SCM variability, and the same should not be overlooked while estimating primary production or carbon export to deep ocean, through generalisations established at larger scale in the world ocean.

Multi‐Stage Development Within Anisotropy Insight of an Anticyclone Eddy in Northwestern South China Sea in 2021

AbstractMesoscale eddies help regulate ocean energy cascades. Eddies deformation influences barotropic instability, which represents kinetic energy transfer between scales; however, the barotropic instability structure has not been well studied. We investigated an intra‐thermocline eddy (ITE) and developed a novel anisotropic method to examine the horizontal barotropic instability. The development of the ITE was monitored using a state‐of‐the‐art autonomous underwater vehicle from May to July. The ITE became trapped in June and moved eastward in July. Based on anisotropic theory, the barotropic instability was separated into isotropic and anisotropic productions. The anisotropy contained information regarding shape and mean flow feedback of the eddy. Barotropic instability was the main source for ITE eastward propagation and was dominated by anisotropic production. Following a shape and anisotropy change, the ITE gained the mean‐flow kinetic energy by the anisotropy shear production in June and by the anisotropy stretch production when moving eastward in July.

Opportunity observation of an Algerian Eddy to the south of Cape Palos (southwestern Mediterranean Sea)

Large anticyclonic eddies can detach from the Algerian Current, forming open-sea Algerian Eddies. These mesoscale structures have been intensively studied by means of sea surface temperature and altimetry data, and using numerical models. However, few studies describe an in situ sampling of their whole vertical structure. Furthermore, the area extending from Cape La Nao (western edge of the Balearic Channels) to the Almería-Orán Front has received very little attention, and it could be considered that there is a gap in our present oceanographic knowledge of this part of the western Mediterranean. An Algerian Eddy lasting for several months was detected in December 2021 to the south of Cape Palos. In order to analyse this eddy, an opportunity sampling was designed taking advantage of the periodic monitoring campaign RADMED 0222. This sampling revealed that the eddy had a baroclinic character, affecting the whole water column. These results suggest that this eddy was generated at the Algerian Current, finally affecting an area close to the eastern Spanish coast. The presence of these structures in this region of the western Mediterranean could alter the southward progression of the Northern Current and even the presence and structure of the Almería-Orán Front.

Local Topographic Rossby Modes Observed in the Abyssal Sea of Japan

Abstract The short-period current fluctuations (topographic wave fluctuations, TWFs) on the southern rim slope of the abyssal Sea of Japan were investigated using current meter datasets from closely spaced mooring arrays. The TWFs occurred almost continuously throughout the year with short periods in a narrow band (1.5–5 days), showing a seasonal modulation in their amplitude. The TWFs were attributable to alternate passage of cyclonic and anticyclonic eddies on the rim slope, which propagated eastward at a speed of 0.15–0.23 m s−1. In addition, the TWFs showed a bottom-intensified characteristic, along with the two-layer structure consisting of an almost barotropic lower layer and a marginally baroclinic upper layer. The lowest topographic Rossby mode, which is a normal mode of the topographic Rossby waves prescribed by the two ridges on the rim slope, was considered as a cause of the TWFs because of its eastward-propagating eddy train structure along the rim slope and the eigenperiod (3–5 days) near the TWF band. In addition, the local time-dependent Sverdrup balance was considered as a mechanism of the TWF generation, since the TWFs significantly correlated with the wind stress curl variations over the observation area with time lags. That is, the current fluctuations near the eigenperiod were selectively amplified via the resonance between the lowest topographic Rossby mode and the Ekman pumping variations induced by the TWF-band wind stress curl. We concluded that the observed TWFs were a manifestation of the wind-induced lowest topographic Rossby mode prescribed by the bottom topography. Significance Statement The dispersion relation teaches us that short-period (<10 days) Rossby waves have a very long wavelength (>103 km). However, as atmospheric forcing with both such period and wavelength is absent, the short-period Rossby waves excited by a local forcing generally dissipate quickly in a limited area. Nevertheless, we observed short-period (1.5–5 days) current fluctuations occurring continuously throughout the year in the abyssal (>1000 m) Sea of Japan. The deep current fluctuations were attributable to the propagation of cyclonic and anticyclonic eddy trains on the zonally extended slope. This is the wind-induced lowest topographic Rossby normal mode prescribed by the bottom topography. This study suggests that short-period current fluctuations can occur everywhere if appropriate topographic and atmospheric conditions were established.

Enhanced Dissipation of Internal Tides in a Mesoscale Baroclinic Eddy

Abstract The dissipation of low-mode internal tides as they propagate through mesoscale baroclinic eddies is examined using a series of numerical simulations, complemented by three-dimensional ray tracing calculations. The incident mode-1 internal tide is refracted into convergent energy beams, resulting in a zone of reduced energy flux in the lee of the eddy. The dissipation of internal tides is significantly enhanced in the upper water column within strongly baroclinic (anticyclonic) eddies, exhibiting a spatially asymmetric pattern, due to trapped high-mode internal tides. Where the eddy velocity opposes the internal tide propagation velocity, high-mode waves can be trapped within the eddy, whereas high modes can freely propagate away from regions where eddy and internal wave velocities are in the same direction. The trapped high modes with large vertical shear are then dissipated, with the asymmetric distribution of trapping leading to the asymmetric distribution of dissipation. Three-dimensional ray tracing solutions further illustrate the importance of the baroclinic current for wave trapping. Similar enhancement of dissipation is also found for a baroclinic cyclonic eddy. However, a barotropic eddy is incapable of facilitating robust high modes and thus cannot generate significant dissipation of internal tides, despite its strong velocities. Both energy transfer from low to high modes in the baroclinic eddy structure and trapping of those high modes by the eddy velocity field are therefore necessary to produce internal wave dissipation, a conclusion confirmed by examining the sensitivity of the internal tide dissipation to eddy radius, vorticity, and vertical scale. Significance Statement The oceanic tides drive underwater waves at the tidal frequency known as internal tides. When these waves break, or dissipate, they can lead to mixing of oceanic heat and salt which impacts the ocean circulation and climate. Accurate climate predictions require computer models that correctly represent the distribution of this mixing. Here we explore how an oceanic eddy, a swirling vortex of order 100–400 km across, can locally enhance the dissipation of oceanic internal tides. We find that strong ocean eddies can be hotspots for internal tide dissipation, for both clockwise and anticlockwise rotating vortices, and surface-enhanced eddies are most effective at internal tide dissipation. These results can improve climate model representations of tidally driven mixing, leading to more credible future predictions.

Near-Inertial Waves Reaching the Deep Basin in the South China Sea after Typhoon Mangkhut (2018)

Abstract Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north. Significance Statement Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.

Mesoscale Anticyclonic Eddies in the Primorye Current System of the Japan Sea in Summer

Mesoscale Anticyclonic Eddies in the Primorye Current System of the Japan Sea in Summer

Spatiotemporal Variation of Anticyclonic Eddies in the South China Sea during 1993–2019

Based on the absolute dynamic topography data from the Copernicus Marine Environment Monitoring Service, this paper applies the Topographic Position Index to develop a new approach for mapping the anticyclonic eddies in the South China Sea (SCS). The results show that anticyclonic eddies are active in the deep basin of SCS, and the five selected parameters (number or frequency, lifetime, kinetic energy, amplitude, and area or radius) of anticyclonic eddies have a similar temporal variation and a similar spatial distribution pattern. (1) As for monthly variations, anticyclonic eddies are active in late spring and most active in summer. (2) The El Niño–Southern Oscillation had a stronger impact on the inter-annual variations of anticyclonic eddies in the SCS before 2013, resulting in a significant transition of inter-annual variations of these five parameters in around 2004. After 2013, most of these five parameters had a minimum in 2015 and a maximum in 2017. (3) Analyses show that the eddy activities in the SCS are significantly influenced by the monsoon wind and the western boundary current like Kuroshio. Therefore, the areas southwest of Taiwan Island and east of Vietnam are the two areas where the anticyclonic eddies are most active, with much larger eddy kinetic energy and much higher eddy amplitude.

Enhancing impacts of mesoscale eddies on Southern Ocean temperature variability and extremes

As a major sink of anthropogenic heat and carbon, the Southern Ocean experienced pronounced warming with increasing extreme temperature events over the past decades. Mesoscale eddies that strongly influence the uptake, redistribution, and storage of heat in the ocean are expected to play important roles in these changes, yet observational evidence remains limited. Here, we employ a comprehensive analysis of over 500,000 historical hydrographic profile measurements combined with satellite-based eddy observations to show enhanced thermal eddy imprints in the Southern Ocean. Our observations reveal that anticyclonic (cyclonic) eddies are responsible for nearly half of the subsurface high (low)-temperature extremes detected, although only 10% of the profiles are located in eddy interiors. Over the past decade (2006 to 2019), both mean and extreme temperature anomalies within eddies in the Antarctic Circumpolar Current increased significantly, promoting the rise in subsurface ocean temperature variability. This enhanced role of eddies is likely a result of enhanced eddy pumping due to the increase in eddy intensity and ocean stratification caused by ocean warming. Our analysis underscores the crucial role of eddies in amplifying ocean temperature variability and extremes, with their effects expected to be even more pronounced as global warming persists.