Structure Of Mesoscale Eddies Research Articles

Temperature Structure Inversion of Mesoscale Eddies in the South China Sea Based on Deep Learning

Mesoscale eddies are common in global oceans, playing crucial roles in ocean dynamics, ocean circulation, and heat transport, and their vertical structures can affect the water layers from tens to thousands of meters. In this study, we integrated sea surface height and sea surface temperature data into deep learning methods to study the mesoscale eddy subsurface temperature structure and to explore the relationship between sea surface data and eddy subsurface layers. In this study, we introduce Dual_EddyNet, a deep learning algorithm designed to invert the subsurface temperature structure of mesoscale eddies. Using this algorithm, we explore the impact of the sea surface height and sea surface temperature on the subsurface temperature structure inversion of mesoscale eddies. Furthermore, we compare different data fusion strategies, namely single-stream neural networks and dual-stream neural networks, to validate the effectiveness of the dual-stream model. To capture the interrelations among surface data and integrate feature information across various dimensions, we introduce the Triplet Attention Mechanism. The experimental results demonstrate that the proposed Dual_EddyNet performs well in reconstructing the three-dimensional structure of mesoscale eddies in the South China Sea (within a depth of 1000 m), with an inversion accuracy of 91.44% for cyclonic eddies and 95.25% for anticyclonic eddies. This algorithm provides a new method for inverting the subsurface temperatures of mesoscale eddies, and can not only be directly deployed in systems, embedded in ship moving platforms, etc., but can also provide a data reference for assimilations and numerical simulations, demonstrating its rich application potential.

The role of surface potential vorticity in the vertical structure of mesoscale eddies in wind-driven ocean circulations

Abstract The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extra-tropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasi-geostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.

On the Vertical Structure of Mesoscale Eddies in the Kuroshio‐Oyashio Extension

AbstractVertical structure of mesoscale eddies is key to the eddy‐induced heat/material transport that further affects the climate and marine ecosystem. This study explores the vertical structure of mesoscale eddies in the Kuroshio‐Oyashio Extension region (KOE) and its underlying dynamics. By applying the hierarchical ascending classification to the observational and reanalysis data sets, we classify mesoscale eddies with three distinct kinds of vertical structures. Each kind of eddies exhibits clear spatial aggregation along a distinct zonal band. Eddies have core depths of 100–300 m in the northern part of the KOE and core depths of 300–500 m and 0–100 m in the southern. The eddy splitting or merging does not introduce new kind of eddy vertical structure but causes large intra‐kind variability. The different kinds of eddy vertical structures can be partially accounted for by the baroclinic instabilities at the eddy generation sites and local adjustment process.

High‐Resolution Gridded Temperature and Salinity Fields From Argo Floats Based on a Spatiotemporal Four‐Dimensional Multigrid Analysis Method

AbstractThe shortage of in situ ocean observations greatly limits the development of gridded ocean subsurface environmental analysis products. Fully using the correlated information of the observations in time and space is an appropriate approach to this problem. Here we adopt a method of spatiotemporal four‐dimensional multigrid analysis. A weekly gridded Argo product is also constructed in the Northwest Pacific Ocean (0°–40°N, 120°–152°E) from 2005 to 2020, with a horizontal resolution of 0.5° × 0.5° and 65 layers in the upper 1,500 m. Then, the gridded Argo product is evaluated by using in situ Argo floats, satellite altimeter data, the Tropical Atmosphere Ocean/Triangle Trans‐Ocean Buoy Network (TAO/TRITON), high‐resolution shipboard observations and some global gridded Argo products. Compared with in situ Argo floats, the reconstructed fields retain the spatiotemporal information of Argo floats well. The maximum vertical root mean square errors of temperature and salinity between the gridded Argo product and in situ Argo floats are less than 0.47°C and 0.06 psu respectively. In addition, the product agrees well with independent observations of satellite altimeter data, TAO/TRITON. Compared with shipboard observations, the product can well reflect the structure of mesoscale eddies and detect some small‐scale water masses. Compared with global gridded Argo products, our product retains more mesoscale features and has spatiotemporal correlation.

Glider Network Observation Unraveling A Cyclonic Eddy in the Northwestern Pacific Subtropical Countercurrent

The North Pacific Subtropical Countercurrent (STCC) is a zonal current consisting of a weak eastward current near the surface and a westward current (north equatorial current, NEC) below. Previous studies have proved that this region is an extremely active region of the North Pacific eddy. The activity of mesoscale eddy in this region can directly or indirectly affect the periodic variations of the Kuroshio and the Chinese coastal waters. In order to study the structure of mesoscale eddies in this region and their impact on the upper ocean, five Petrel-II underwater gliders were conducted as a network to collect hydrographic properties from December 09 2020 to February 05, 2021. It is found that a cyclonic eddy passed through the glider network from east to west during the observation period, and the glider grid almost covers the whole eddy. The radius, amplitude and propagation velocity of the eddy were analyzed. The underwater gliders were equipped with CTD sensors, collected about 1200 full thermohaline profiles. The observation results indicated that the cyclonic eddy can influence over 400 m in the vertical, which caused the isotherm rising up to 40 m. Analysis based on satellite sea surface temperature (SST) and sea level anomaly (SLA) data suggest that the cyclonic eddy is probably locally generated.

Three-dimensional structure of mesoscale eddies and their interaction with Kuroshio based on observations from a CPIES array

Mesoscale eddies are widespread in the oceans; however, tracking and observing them remains a challenge. Additionally, there are few studies of eddy–Kuroshio interactions based on large-scale in situ data. Data from a large array of 39 CPIESs deployed around the Luzon Strait in 2018–2019 (12 deployed east of Taiwan within a 3 × grid) were used to reveal the three-dimensional structure of mesoscale eddies and their interaction with Kuroshio. The results show a strengthening (weakening) of the tilt of the pycnocline, which has also caused a deeper (shallower) pycnocline in eastern Taiwan following an anticyclonic (cyclonic) interaction with Kuroshio, which resulted in a strengthening (weakening) of the Kuroshio. During the anticyclonic eddy impinging Kuroshio, the downward (upward) movement at the center of the anticyclonic eddy (Kuroshio) results in a positive (negative) anomaly structure for temperature, with corresponding positive (negative) anomalies for salinity above 600 m in the intermediate layer and negative (positive) anomalies below 600 m. In contrast, when the cyclonic eddies interact with the Kuroshio, the temperature and salinity anomalies change simultaneously, with the temperature showing an overall negative anomaly and the salinity showing a negative and positive anomaly above and below 600 m, respectively. The research in this study provides the basis for further research on energy exchange during eddy–Kuroshio interactions.

Inversion of the three-dimensional temperature structure of mesoscale eddies in the Northwest Pacific based on deep learning

Inversion of the three-dimensional temperature structure of mesoscale eddies in the Northwest Pacific based on deep learning

Investigating mesoscale eddy characteristics in the Luzon Strait region using altimetry

The circulation in the northern South China Sea (SCS) strongly responds to anticyclonic eddies that shed from the Kuroshio intruding across Luzon Strait. An eddy tracking algorithm was applied to 26 years of altimetry data to better quantify the surface characteristics of eddies in and near the Luzon Strait with an emphasis on eddies with significant westward propagation across the northern SCS. Spatial mean eddy characteristics revealed that the most robust anticyclonic eddies are in the region of 20–22° N and 118–120° E (where eddies have been observed to shed from the Kuroshio into the northern SCS) and in the North Pacific Gyre east of the Luzon Strait (18–22° N, 121–124° E). The composite structure of mesoscale eddies revealed stronger, more high-amplitude eddies east of the Kuroshio for both circulation types, which was true in all seasons but the starkest contrast was during the summer monsoon. We then isolated the ten farthest westward-propagating eddies that generated in the Luzon Strait region. The majority of these long-propagating eddies occurred during the boreal fall and winter when the cyclonic basin-scale circulation was conducive to eddy propagation along the Chinese shelf break. Using the Kuroshio South China Sea Index (KSI), we find that a highly viable condition for generation of North Pacific Gyre eddies that propagated westward across the Luzon Strait was found when the Kuroshio bifurcates with one branch into the northern SCS and the other northward along eastern Taiwan. In this state, an anticyclonic eddy can generate east of the Kuroshio and be advected into the SCS.

Revisit the Vertical Structure of the Eddies and Eddy‐Induced Transport in the Leeuwin Current System

AbstractThe vertical structure of eddies in the Leeuwin Current system affects the eddy volume, heat, and salt transport, even the ecosystem. However, the understanding of eddy vertical structure and eddy‐induced transport here are still very limited. In this study, satellite observed sea surface heights were combined with decade‐long in situ measurements of Argo floats to study the vertical structure of mesoscale eddies in the LC system and their volume, heat, and salt transport. A novel eddy reconstruction method, which considers the influences of both the eddy and background flow, is devised to study the three‐dimensional structure of eddies. Result shows that, in LC system, anticyclonic eddies (AEs) are usually surface‐intensified, with the geostrophic velocity decreasing sharply below the mixed layer, while cyclonic eddies (CEs) are subsurface‐intensified, with a maximum speed at 240 m. The density anomaly core of the average AE (CE) is at a depth of 130 m (650 m) with a density anomaly of −0.51 (0.24) kg/m3. The volume‐integrated eddy kinetic energy and available potential energy of the average CE are much larger than those of the average AE. The average lifespan of CEs is significantly longer than that of AEs, which can be explained by the deeper vertical scale of CEs. The offshore volume transport by eddy drift across the coastal (107°E) section is 9.05 Sv (12.5 Sv). The heat and salt onshore transport by eddy drift across the coastal (107°E) section are, respectively, 10.6 Tw and 143.1 ton/s (17.1 Tw and 241.0 ton/s).

Four Types of Baroclinic Instability Waves in the Global Oceans and the Implications for the Vertical Structure of Mesoscale Eddies

AbstractLinear stability analysis is re‐conducted to fully understand the geostrophic distribution of the different types of baroclinic instability (BCI) in the global oceans, their correspondence to the different vertical structures of the observed mesoscale eddies, and the properties and formation mechanisms of the instability waves. Four principal vertical types of BCI are identified, which are found to exhibit large‐scale patterns in the global ocean. The surface‐ and bottom‐intensified type (called the Eady type hereafter) is mainly located in the Antarctic Circumpolar Current (ACC) region, locations of the bottom‐intensified type (Charney_b type) are scattered around the Eady type, the surface‐intensified type (Charney_s type) primarily occurs in the subtropics (10°–35°), and the interior‐intensified type (Phillips type) occurs primarily between 5° and 20° in both hemispheres. More specifically, both geostrophic locations and the depths of the maximum perturbation velocities of the Phillips type BCIs match those of observed subsurface eddies. Moreover, the BCI waves show regions of uniform propagation properties: eastward in the ACC and the mid‐latitudes (25°–45°), and westward in the low latitudes (30°S–30°N) of both hemispheres and in the high latitudes of the Northern Hemisphere (>50°N). These waves resemble normal mode Rossby waves in structure (i.e., first baroclinic, second baroclinic, and topographic Rossby waves), but their propagation speeds are found to be Doppler shifted by the mean flows relevant for the corresponding BCI type. Propagating signals with the same dispersion relationships as the BCI waves are captured with numerical ocean general circulation models.

Vliyaniye techeniy i atmosfernoy tsirkulyatsii na effektivnost' promysla khamsy v period yeye zimovki u Chernomorskogo poberezh'ya Krasnodarskogo kraya

Russian fishermen harvest European anchovy primarily off the Black Sea coast of the Krasnodar Territory during its wintering and wintering migrations. At wintering grounds, temperature conditions become a secondary factor in determining the behaviour of commercial concentration of European anchovy, with wind and currents being the primary factors. Therefore, the aim of this work is to determine the potential use of daily data on water circulation and local atmospheric transport in short-term (1–7 days) forecasting of European anchovy fishing in the Black Sea. The research used the European anchovy fishery monitoring materials for January – March 2019, as well as daily maps of the Black and Azov Seas level anomalies (from satellite altimetry data) and surface atmospheric pressure and temperature in Europe (analysis) for the mentioned period. The dynamics of the catch rate and its relation to altimetry and atmospheric transport indicators in the north-eastern part of the Black Sea were investigated using graphical and correlation methods. This analysis showed that the main factor contributing to increased catches is intensification of northwest currents in the coastal 60-km zone. The effect of atmospheric transport on fishing efficiency depends on the mesoscale eddy structure of the nearshore current field. In the presence of an intense northwest current in the fishing area, southwest atmospheric transports have a positive effect on fishing, while in the presence of an anticyclonic meander of currents, northeast atmospheric transports become effective. The presence of maximum significant relationships when the determinants of fishing performance are shifted by 1–7 days allows making short-term predictions of fishing efficiency.

Projecting Three‐Dimensional Ocean Thermohaline Structure in the North Indian Ocean From the Satellite Sea Surface Data Based on a Variational Method

AbstractThis study uses a variational method combined with satellite observations to reconstruct three‐dimensional temperature and salinity profiles for the Northern Indian Ocean. Sensitivity experiments show that sea surface temperature dominantly improve the temperature reconstruction of upper 100 m; sea surface salinity determines salinity estimation in the upper 100 m; sea surface height anomaly dominates the reconstruction of thermocline. The reconstructed temperature fields can be greatly improved in the thermocline by removing barotropic signal from the altimeter sea surface height data through a linear regression method. Ocean reanalysis and in situ temperature and salinity data are used to evaluate the results of reconstruction. Comparing with Simple Ocean Data Assimilation (SODA) in 2016, the spectral correlation between the reconstruction and the SODA density anomalies show that the reconstruction fields can retrieve mesoscale and large‐scale signals better. Moreover, the reconstruction salinity is much more accurate than SODA salinity in the upper ocean over the Bay of Bengal (BoB). Compared with CTD section observations, the reconstruction fields can capture the mesoscale eddy structure in the Arabian Sea and BoB well, respectively. The long time series of reconstruction along Argo trajectory shows that the reconstruction fields can better reproduce the observed intraseasonal oscillations of thermocline/halocline in the BoB. Compared with the World Ocean Atlas 2013 climatology, the reconstruction fields can better characterize upper ocean water mass variability.

The 3‐D Structure of Mesoscale Eddies in the Lofoten Basin of the Norwegian Sea: A Composite Analysis From Altimetry and In Situ Data

AbstractIn this research temperature and salinity profiles in eddy‐centered coordinates obtained from satellite altimetry (eddy data set distributed by Archiving, Validation and Interpretation of Satellite Oceanographic data) are combined to document the mean three‐dimensional structures of cyclonic (CEs) and anticyclonic (AEs) eddies in the Lofoten Basin. For eddies of both polarities, significant eddy‐induced anomalies are concentrated within the zero vorticity radius and vertically to the depth of ∼900–1,000 m. The thermohaline vertical structures of CEs and AEs differ in terms of salinity and temperature anomalies. Horizontal structure of the mesoscale eddies showed warmer and saltier anomalies for AEs from the southwest to the northeast side, as well as colder and less salty anomalies from their southeast side for the CEs. This reflects the main features of the basin‐scale temperature and salinity gradients, strongly affected by the Norwegian Atlantic Slope Current. Mean zonal eddy‐induced transport of volume, heat, and salt is generally westward, consistent with the key role played by eddies generated by the Norwegian Atlantic Slope Current. The obtained results highlight the significant role played by mesoscale eddies in the oceanic circulation of the Lofoten Basin, as well as on heat and salt budgets of a key region for air‐sea exchanges, water mass transformation, and climate.

Demonstration of the refined three-dimensional structure of mesoscale eddies and computational error estimates via Lagrangian analysis

In previous studies, Lagrangian analyses were used to assess large-scale ocean circulation, and the Lagrangian coherent structure could also reveal the evolution of the two-dimensional structure of the mesoscale eddies. However, few studies have demonstrated the three-dimensional structure of the mesoscale eddies via Lagrangian analysis. Compared with previous studies, which investigated the eddy structure via a Eulerian view, we used a Lagrangian view to provide a different perspective to study the eddy structure. An idealized cyclonic mesoscale eddy is built up over a seamount, and it presents downwelling inside the eddy and upwelling alongside the eddy formed within a closed circulation system. This structure is difficult to display via a Eulerian analysis. However, the trajectories of particles can well demonstrate the full cycle: the fluid sank and rotated inside the eddies, converged to the upwelling zone of the bottom layer and returned to the surface through upwelling. We also applied a Lagrangian analysis to a realistic simulation. As a significant phenomenon in the South China Sea, the dipole structure of the anticyclonic eddy (AE)/cyclonic eddy (CE) pair off of central Vietnam has been well studied but mainly at the sea surface. With a Lagrangian analysis, we illustrate the three-dimensional structure of the eddy pair: the fluid sank (rose) and rotated inside the AE (CE). More importantly, the trajectories of the particles suggested that there was no fluid exchange between the two eddies since the strong boundary jet separates them from each other. All the conclusions above have been verified and are supported by the computational error estimate. With a selected time step and integral period, the computational errors always present small values, although they increase with strong divergent and vertical diffusive flow.

Constructing the Three-Dimensional Structure of an Anticyclonic Eddy in the South China Sea Using Multiple Underwater Gliders

AbstractMesoscale eddies have great influence on heat and material transport in the ocean and thus play an important role in modulating the global climate variability. However, our understanding of their fine three-dimensional (3D) thermohaline and biogeochemical structure remains incomplete because of the scarcity of high-resolution measurements. This research aims to construct the fine 3D structure of an anticyclonic eddy in the northern South China Sea (NSCS) to validate the effectiveness of a glider network for observing mesoscale eddies. Twelve Petrel gliders were deployed in NSCS during August 2017 to gather fine information of an anticyclonic eddy. By combining the high-resolution in situ glider data and the sea level anomaly data, we have constructed a detailed 3D structure of the eddy. The analysis results of the absolute dynamic topography map and the water mass comparison imply that the anticyclonic eddy may generate from eddy shedding of the Kuroshio loop current. The maximum potential temperature anomaly (over 3°C) appears at the thermocline, and the maximum salinity anomaly (~0.8 psu) exists at ~50 m. The maximum value of the dissolved oxygen concentration (~7.5 mg L−1) appears at 50–80 m. The maximum chlorophyll concentration (~1.2 μg L−1) lies at 80–120 m, just below that of the dissolved oxygen concentration. The colored dissolved organic matter concentration increases with depth, with the mean value being less than 1.23 ppb above 50 m and beyond 2.2 ppb at 500–800 m. The results verify the capability of the glider network to observe a fine-scaled 3D structure of mesoscale eddies and will provide a useful guide for future eddy observations.

Anatomy of a Cyclonic Eddy in the Kuroshio Extension Based on High-Resolution Observations

Mesoscale eddies are common in the ocean and their surface characteristics have been well revealed based on altimetric observations. Comparatively, the knowledge of the three-dimensional (3D) structure of mesoscale eddies is scarce, especially in the open ocean. In the present study, high-resolution field observations of a cyclonic eddy in the Kuroshio Extension have been carried out and the anatomy of the observed eddy is conducted. The temperature anomaly exhibits a vertical monopole cone structure with a maximum of −7.3 °C located in the main thermocline. The salinity anomaly shows a vertical dipole structure with a fresh anomaly in the main thermocline and a saline anomaly in the North Pacific Intermediate Water (NPIW). The cyclonic flow displays an equivalent barotropic structure. The mixed layer is deep in the center of the eddy and thin in the periphery. The seasonal thermocline is intensified and the permanent thermocline is upward domed by 350 m. The subtropical mode water (STMW) straddled between the seasonal and permanent thermoclines weakens and dissipates in the eddy center. The salinity of NPIW distributed along the isopycnals shows no significant difference inside and outside the eddy. The geostrophic relation is approximately set up in the eddy. The nonlinearity—defined as the ratio between the rotational speed to the translational speed—is 12.5 and decreases with depth. The eddy-wind interaction is examined by high resolution satellite observations. The results show that the cold eddy induces wind stress aloft with positive divergence and negative curl. The wind induced upwelling process is responsible for the formation of the horizontal monopole pattern of salinity, while the horizontal transport results in the horizontal dipole structure of temperature in the mixed layer.

Horizontal velocity structure of mesoscale eddies in the South China Sea

The horizontal velocity structure of mesoscale eddies is investigated using data from 18-year altimetry data in the South China Sea (SCS). A total of 2,433 cyclonic eddies (CEs) and 3,132 anticyclonic eddies (AEs) are detected in this data set. This analysis shows that the eddy velocity structure is strongly linked to the eddy amplitude, with the band of maximum velocity moving closer to the eddy core as the eddy amplitude increases. By combining the records of Argo float profiles and satellite altimetry data, the eddy feature found in sea level anomaly (SLA) data can also be seen at the subsurface. This feature is related to sub-mesoscale processes accompanying with water exchange and energy cascade at the outer edge of mesoscale eddy. A modified Taylor vortex model is built including this new finding to describe the eddy velocity structure. This vortex model is found to be useful as it better describes the eddy’s life history, the knowledge of which is important for better understanding physical, biological and chemical processes in the ocean.

A new automatic oceanic mesoscale eddy detection method using satellite altimeter data based on density clustering

The mesoscale eddy is a typical mesoscale oceanic phenomenon that transfers ocean energy. The detection and extraction of mesoscale eddies is an important aspect of physical oceanography, and automatic mesoscale eddy detection algorithms are the most fundamental tools for detecting and analyzing mesoscale eddies. The main data used in mesoscale eddy detection are sea level anomaly (SLA) data merged by multi-satellite altimeters’ data. These data objectively describe the state of the sea surface height. The mesoscale eddy can be represented by a local equivalent region surrounded by an SLA closed contour, and the detection process requires the extraction of a stable closed contour structure from SLA maps. In consideration of the characteristics of mesoscale eddy detection based on SLA data, this paper proposes a new automatic mesoscale eddy detection algorithm based on clustering. The mesoscale eddy structure can be extracted by separating and filtering SLA data sets to separate a mesoscale eddy region and non-eddy region and then establishing relationships among eddy regions and mapping them on SLA maps. This paper overcomes the problem of the sensitivity of parameter setting that affects the traditional detection algorithm and does not require a sensitivity test. The proposed algorithm is thus more adaptable. An eddy discrimination mechanism is added to the algorithm to ensure the stability of the detected eddy structure and to improve the detection accuracy. On this basis, the paper selects the Northwest Pacific Ocean and the South China Sea to carry out a mesoscale eddy detection experiment. Experimental results show that the proposed algorithm is more efficient than the traditional algorithm and the results of the algorithm remain stable. The proposed algorithm detects not only stable single-core eddies but also stable multi-core eddy structures.

Mesoscale SST Dynamics in the Kuroshio–Oyashio Extension Region

AbstractMesoscale eddies have been extensively studied based on the sea surface height anomaly (SSHA). However, it is the sea surface temperature anomaly (SSTA) that is vital to the mesoscale eddy–atmosphere interactions. In this study, we analyze the amplitude relationship between SSHA and SSTA (referred to as the H-T amplitude relationship) in the Kuroshio–Oyashio extension (KOE) region using both observational and reanalysis data. It is found that the spatial distribution of mesoscale SSHA variance is not coincident with mesoscale SSTA variance. The former peaks in the Kuroshio extension around 35°N whereas the latter is strongest in the Oyashio extension around 40°N. Regression analyses indicate that the rate of SSTA change per SSHA change is 1.8°C m−1 in the Kuroshio extension (145°–160°E, 34°–36°N) but increases drastically by a factor of 3–4 to 6.2°C m−1 in the Oyashio extension (145°–160°E, 39°–41°N). A theoretical expression for the H-T amplitude relationship is derived. Analyzing this expression suggests that the stronger H-T amplitude relationship in the Oyashio extension than the Kuroshio extension is mainly attributed to 1) the smaller thermal expansion coefficient due to the colder background SST, 2) the stronger salinity compensation effect that works against the contribution of SSTA change to sea surface density anomaly (SSDA) change, and 3) the shallower vertical structure of mesoscale eddies. The second factor is ascribed to the strong surface salinity front in the Oyashio extension, while the third factor is found to be qualitatively consistent with the shallower baroclinically unstable modes due to the shallower density front there.

Seasonal characteristics and formation mechanism of the thermohaline structure of mesoscale eddy in the South China Sea

The seasonal characteristics and formation mechanism of the thermohaline structure of mesoscale eddy in the South China Sea are investigated using the latest eddy dataset and ARMOR3D data. Eddy-centric composites reveal that the horizontal distribution of temperature anomaly associated with eddy in winter is more of a dipole pattern in upper 50 m and tends to be centrosymmetric below 50 m, while in summer the distribution pattern is centrosymmetric in the entire water column. The horizontal distribution of eddy-induced salinity anomaly exhibits similar seasonal characteristics, except that the asymmetry of the salinity anomaly is weaker. The vertical distribution of temperature anomaly associated with eddy shows a monolayer structure, while the salinity anomaly demonstrates a triple-layer structure. Further analysis indicates that the vertical distribution of the anomalies is related to the vertical structure of background temperature and salinity fields, and the asymmetry of the anomalies in upper 50 m is mainly caused by the horizontal advection of background temperature and salinity.