Subthermocline eddies carrying the Indonesian Throughflow water observed in the southeastern tropical Indian Ocean

We observed a subthermocline eddy(STE)with a cold and fresh core during an observation cruise along a transect of 10°S in the southeastern tropical Indian Ocean(SETIO)in December 2017.The vertical scale,speed radius,and maximum swirl velocity of the STE were about 200 m,55 km,and 0.5 m/s,respectively.The mean Rossby number and Burger number of the STE were then estimated to be about−0.7 and 2.4,indicating the STE was a submesoscale coherent vortex.The STE core water had characteristics of the Indonesian Throughflow(ITF)water and was distinct from that of surrounding areas.By examining Argo float data,another STE was well captured by five successive profiles of the same Argo float.Both STEs showed significant temperature and salinity anomalies at theσ0=26.0-26.5 kg/m3 surfaces.With the assumption that the low-salinity ITF water parcels could be carried only by surface eddies and the STEs,the Argo profiles,which detected low-salinity ITF water and were located outside a surface eddy,were believed to be inside an STE and were used to analyze the distribution,origin,and generation mechanism of the STE.The results suggested that the STEs carrying ITF water may be generated under topography-current interaction at the eastern coastal waters or under front-induced subduction in the area away from coastal waters.Those STEs may be widely distributed in the SETIO and may play a role in ITF water parcel transport.

Three-Dimensional Climatological Structures of the Arabian Sea Eddies and Eddy-Induced Flux

This study explores the spatial structure and transport characteristics of eddies in the Arabian Sea(AS)using Argo profiles and satellite measurements.The majority of eddies occur in the northern AS,especially along its northeastern boundary.In contrast,the western AS had a relatively higher eddy kinetic energy compared to the eastern part.Particularly,the strongest energetic eddies were present in the Somali Current system.The composite results revealed the evident thermohaline anomalies caused by cyclonic eddies(CEs)and anticyclonic eddies(AEs)in the upper 300m layers.The anomalous temperature structure within CEs and AEs showed a dominant dipole structure in the near-surface layer and a monopole structure below,with maximum temperature anomalies of approximately−0.8℃and+1.0℃located at depths of 100–150m,respectively.The composited salinity structures for CEs and AEs exhibited monopole vertical structures and sandwich-like patterns.For AEs,large positive salinity anomalies occurred at subsurface layers of 60–180 m with a peak value of about 0.07,and weak negative values were observed above 60m and below 180 m.A similar vertical structure but with an opposite sign operates for CEs.The composited CE and AE caused an equatorward salt flux with values of−8.1×10^(4)and−2.2×10^(4)kg s^(−1),respectively.CEs caused an equatorward heat flux of−7.7×10^(11)W,and AEs induced a poleward flux of 1.5×1011 W.

Influence of mesoscale eddies on primary production in the South China Sea during spring inter-monsoon period

Mesoscale eddies have been suggested to have an impact on biological carbon fixation in the South China Sea （SCS）. However, their overall contribution to primary production during the spring inter-monsoon pe riod is still unknown. Based on large-scale biological and environmental in situ observations and synchro nous remote sensing data, the distribution patterns of phytoplankton biomass and the primary production, and the role of mesoscale eddies in regulating primary production in different eddy-controlled waters were investigated. The results suggested that the surface chlorophyll a concentrations and water column inte grated primary production （IPP） are significantly higher in cyclonic eddies and lower in the anticyclonic eddies as compared to that in non-eddy waters. Although eddies could affect various environmental factors, such as nutrients, temperature and light availability, nutrient supply is suggested to be the most important one through which mesoscale eddies regulated the distribution patterns of phytoplankton biomass and pri mary production. The estimated IPP in cyclonic and anticyclonic eddies are about 29.5% higher and 16.6% lower than the total average in the whole study area, respectively, indicating that the promotion effect of mesoscale cold eddies on the primary production was much stronger than the inhibition effect of the warm eddies per unit area. Overall, mesoscale eddies are crucial physical processes that affect the biological car bon fixation and the distribution pattern of primary production in the SCS open sea, especially during the spring inter-monsoon period.

Mesoscale eddies in the South China Sea and their impact on temperature profiles

Some life history statistics of the mesoscale eddies of the South China Sea (SCS) derived from altimetry data will be further discussed according their different formation periods. A total of three ATLAS (autonomous temperature line acquisition system)mooring buoys data will be analyzed to discuss eddies' impact on temperature profiles.They identify that the intraseasonal variation of SCS thermocline is partly controlled by mesoscale eddies.

The Impact of Meso-Scale Eddies on the Subtropical Mode Water in the Western North Pacific

Based on the temperature and salinity from the Argo profiling floats and altimeter-derived geostrophic velocity anomaly (GVA) data in the western North Pacific during 2002-2011, the North Pacific Subtropical Mode Water (NPSTMW) distribution is investigated and cyclonic and anti-cyclonic eddies (CEs and AEs) are constructed to study the influence of their vertical structures on maintaining NPSTMW. Combining eddies identified by the GVA data and Argo profiling float data, it is found that the average NPSTMW thickness of AEs is about 60 dbar, which is thicker than that of CEs. The NPSTMW thicker than 150 dbar in AEs accounts for 18%, whereas that in CEs accounts for only 1%. About 3377 (3517) profiles, which located within one diameter of the nearest CEs (AEs) are used to construct the CE (AE). The composite AE traps low-PV water in the center and with a convex shape in the vertical section. The 'trapped depth' of the composite CE (AE) is 300 m (550 m) where the rotational velocity exceeds the transitional velocity. The present study suggests that the anticyclonic eddies are not only likely to form larger amounts of NPSTMW, but also trap more NPSTMW than cyclonic eddies.

Seasonal variability of mesoscale eddies in the Banda Sea inferred from altimeter data

Using the mesoscale eddy trajectory atlas product derived from satellite altimeter data from 1993 to 2016,this study analyzes statistical characteristics and seasonal variability of mesoscale eddies in the Banda Sea of the Indonesian seas.The results show that there were 147 mesoscale eddies that occurred in the Banda Sea,of which 137 eddies were locally generated and 10 originated from outside.The total numbers of cyclonic eddies(CEs,clockwise)and anticyclonic eddies(AEs,anticlockwise)are 76 and 71,respectively.Seasonally,the number of CEs(AEs)is twice larger than the number of AEs(CEs)in winter(summer).In winter,CEs are distributed in the southern and AEs in the northern basins,respectively,but the opposite thing occurs in summer,i.e.,the polarities of mesoscale eddies observed at the same location reverse seasonally.The mechanisms of polarity distribution reversal(PDR)of mesoscale eddies are examined with reanalysis data of ocean currents and winds.The results indicate that the basin-scale vorticity,wind stress curl,and the meridional shear of zonal current reverse seasonally,which are favorable to the PDR of mesoscale eddies.The possible generation mechanisms of mesoscale eddies include direct wind forcing,barotropic and baroclinic instabilities,of which the direct wind forcing should play the dominant role.

Mesoscale oceanic eddies in the South China Sea from 1992 to 2012:evolution processes and statistical analysis

Automated identification and tracking of mesoscale ocean eddies has recently become one research hotspot in physical oceanography. Several methods have been developed and applied to survey the general kinetic and geometric characteristics of the ocean eddies in the South China Sea（SCS）. However, very few studies attempt to examine eddies＇ internal evolution processes. In this study, we reported a hybrid method to trace eddies＇ propagation in the SCS based on their internal structures, which are characterized by eddy centers, footprint borders, and composite borders. Eddy identification and tracking results were represented by a GIS-based spatiotemporal model. Information on instant states, dynamic evolution processes, and events of disappearance, reappearance, split, and mergence is stored in a GIS database. Results were validated by comparing against the ten Dongsha Cyclonic Eddies（DCEs） and the three long-lived anticyclonic eddies（ACEs） in the northern SCS, which were reported in previous literature. Our study confirmed the development of these eddies. Furthermore, we found more DCE-like and ACE-like eddies in these areas from 2005 to 2012 in our database. Spatial distribution analysis of disappearing, reappearing, splitting, and merging activities shows that eddies in the SCS tend to cluster to the northwest of Luzon Island, southwest of Luzon Strait, and around the marginal sea of Vietnam. Kuroshio intrusions and the complex sea floor topography in these areas are the possible factors that lead to these spatial clusters.

Mean properties of mesoscale eddies in the Kuroshio recirculation region

Using a 19-year altimetric dataset, the mean properties and spatiotemporal variations of eddies in the Kuroshio recirculation region are examined. A total of 2 001 cyclonic tracks and 1 847 anticyclonic tracks were identifi ed using a geometry-based eddy detection method. The mean radius was 57 km for cyclonic eddies and was 61 km for anticyclonic eddies, respectively, and the mean lifetime was about 10 weeks for both type eddies. There were asymmetric spatial distributions for eddy generation and eddy termination, which were domain-dependent. Mean eddy generation rates were 2.0 per week for cyclonic eddies and were 1.9 per week for anticyclonic eddies. Both type eddies tended to deform during their lifetime and had different propagation characteristics, which mainly propagated westward and southwestward with velocities 4.0–9.9 cm/s, in the Kuroshio recirculation region. Further discussion illustrates that the eddy westward speed maybe infl uenced by the combined effect of vertical shear of horizontal currents and nonlinearity of eddy. To better understand the evolution of eddy tracks, a total of 134 long-lived tracks(lifetime ≥20 weeks) were examined. Comparison between short-span eddies(lifetime ≥4 weeks and <20 weeks) and long-lived eddies is also conducted and the result shows that the short-span and long-lived eddies have similar time evolution. Finally, eddy seasonal variations and interannual changes are discussed. Correlation analysis shows that eddy activity is sensitive to the wind stress curl and meridional gradient of sea surface temperature on interannual timescales. Besides, the strength and orientation of background fl ows also have impacts on the eddy genesis.

Influence of warm eddies on sound propagation in the Gulf of Mexico

An automatic detection method is employed to identify and track eddies in the Gulf of Mexico. The physical parameters of the eddies, such as lifespan, radius, and distribution position are first examined and used to determine the spatio–temporal evolution of a strong warm eddy separated from the Mexico current. Then, the influence of this strong warm eddy on sound propagation during its lifespan are comprehensively analyzed with the parabolic equation and explained by using the normal mode and ray theories. Additionally, the influence of mesoscale eddies on the redistribution of total depth-integrated energy among the normal modes in the deep water is also discussed. The variation of arrival angle is investigated to explain the spreading acoustic energy caused by eddies. Overall, the results show that warm eddies can change the propagation paths and cause the convergence zone to broaden and approach the sound source. Moreover,the warm eddy can disperse sound energy and cause the total depth-integrated energy to incline to a lower normal mode.Throughout the whole of these three periods(eddy generating, eddy maturing, and eddy terminating), the fluctuation in the transmission loss is up to 30 dB(depending on the relative location of eddy center to the source).

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

Mesoscale eddies,which are mainly caused by baroclinic effects in the ocean,are common oceanic phenomena in the Northwest Pacific Ocean and play very important roles in ocean circulation,ocean dynamics and material energy transport.The temperature structure of mesoscale eddies will lead to variations in oceanic baroclinity,which can be reflected in the sea level anomaly(SLA).Deep learning can automatically extract different features of data at multiple levels without human intervention,and find the hidden relations of data.Therefore,combining satellite SLA data with deep learning is a good way to invert the temperature structure inside eddies.This paper proposes a deep learning algorithm,eddy convolution neural network(ECN),which can train the relationship between mesoscale eddy temperature anomalies and sea level anomalies(SLAs),relying on the powerful feature extraction and learning abilities of convolutional neural networks.After obtaining the temperature structure model through ECN,according to climatic temperature data,the temperature structure of mesoscale eddies in the Northwest Pacific is retrieved with a spatial resolution of 0.25°at depths of 0–1000 m.The overall accuracy of the ECN temperature structure is verified using Argo profiles at the locations of cyclonic and anticyclonic eddies during 2015–2016.Taking 10%error as the acceptable threshold of accuracy,89.64%and 87.25%of the cyclonic and anticyclonic eddy temperature structures obtained by ECN met the threshold,respectively.

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea（SCS） is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic（warm-core anticyclonic） eddies tend to cool（warm） the overlying atmosphere and cause surface winds to decelerate（accelerate）. More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce（enhance） the vertical momentum transport over the cold（warm） eddy, resulting in the decrease（increase） of sea surface wind. Diagnoses of the model＇s momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.

Characteristics of subsurface mesoscale eddies in the northwestern tropical Pacifi c Ocean from an eddy-resolving model

Subsurface eddies(SSEs)are common features of the ocean interior.They are particularly abundant in oceanic basins and the vicinity of major intermediate water outfl ows.They are responsible for subsurface transport of mass,heat,and salt.Analysis of high-resolution general circulation model data has revealed the existence of subsurface anticyclonic eddies(SSAEs)and subsurface cyclonic eddies(SSCEs)in the northwestern tropical Pacifi c Ocean.SSEs are abundant east of the Philippines(0°–22°N,120°E–140°E)and in latitude bands between 9°N–17°N east of 140°E.The composite structure of SSEs was investigated.SSEs had a core at about 400-m water depth and their maximum meridional velocity exceeded 10 cm/s.They exhibited two cores with diff erent salinity polarities in the surface and subsurface.Additionally,spatial distributions of heat transport induced by SSEs in the northwestern tropical Pacifi c were presented for the fi rst time.A net equatorward heat fl ux toward a temperature up-gradient was observed.The analysis of eddy-mean fl ow interactions revealed that the circulation is baroclinically and barotropically unstable at diff erent depths and to diff ering degrees.The energy conversions suggest that both barotropic and baroclinic instabilities are responsible for SSE generation east of the Philippines,whereas baroclinic instability caused by a horizontal density gradient and vertical eddy heat fl ux are important between 9°N and 17°N east of 140°E.Meridional movement of the north equatorial current and the north equatorial undercurrent can contribute to SSE generation in our study region.

The numerical simulation of the Kuroshio frontal eddies in the East China Sea using a hybrid coordinate ocean mode

A hybrid coordinate ocean model （ltYCOM） is used to simulate the Kuroshio frontal eddies in the East China Sea （ECS）. The research area is located （20°-32°N, 120°-132°E）. Using tile simulating data, it is figured out that the Kuroshio frontal eddies occur in summer as well as in the other season in this area. The life cycle of the Kuroshio and its frontal eddies is different with the position. The life-cycle of the Kuroshio frontal eddies of the northwest Diaoyu Islands is about 14 d; and the life cycle of the Kuroshio frontal eddies of southwest Yakushima about 20 d. This result extends the in situ researching results greatly. In addition, the vertical impact depth of the Kuroshio frontal eddies is also changing with the position. On the whole, in the ECS, the maximum impact depth of the Kuroshio frontal eddies of the northwest Taiwan Islands is about 75 m; the maximum impact depth of the Kuroshio frontal eddies of the northwest Diaoyu Islands is more than 125 m, but no more than 200 m; and the maximum impact depth of the Kuroshio frontal eddies of southwest Yakushima is up to 100 m.

Multifractal filtering method for extraction of ocean eddies from remotely sensed imagery

Traditional methods of extracting the ocean wave eddy information from remotely sensed imagery mainly use the edge detection technology such as Canny and Hough operators. However, due to the complexities of ocean eddies and image itself, it is sometimes difficult to successfully detect ocean eddies using these methods. A mnltifractal filtering technology is proposed for extraction of ocean eddies and demonstrated using NASA MODIS, SeaWiFS and NOAA satellite data set in the typical area, such as ocean west boundary current. Results showed that the new method has a superior performance over the traditional methods.

Response of the Kuroshio Current to Eddies in the Luzon Strait

The impact of eddies on the Kuroshio Current in the Luzon Strait (LS) area is investigated by using the sea surface height anomaly (SSHA) satellite observation data and the sea surface height (SSH) assimilation data. The influence of the eddies on the mean current depends upon the type of eddies and their relative position. The mean current is enhanced (weakened) as the cyclonic (anticyclonic) eddy becomes slightly far from it, whereas it is weakened (enhanced) as the cyclonic (anticyclonic) eddy moves near or within the position of the mean current; this is explained as the eddy-induced meridional velocity and geostrophic flow relationship. The anticyclonic (cyclonic) eddy can increase (decrease) the mean meridional flow due to superimposition of the eddy-induced meridional flow when the eddy is within the region of the mean current. However, when the eddy is slightly far from the mean current region, the anticyclonic (cyclonic) eddy tends to decrease (increase) the zonal gradient of the SSH, which thus results in weakening (strengthening) of the mean current in the LS region.

Automatic detection of oceanic mesoscale eddies in the South China Sea

This study focuses on the spatial and temporal distribution characteristics of mesoscale eddies in the South China Sea(SCS). An automatic eddy detection method,based on the geometry of velocity vectors,was adopted to obtain an eddy dataset from 21 years of satellite altimeter data. Analysis revealed that the number of anticyclonic eddies was nearly equal to cyclonic eddies; in the SCS,cyclonic eddies are generally stronger than anticyclonic eddies and anticyclonic eddies are larger and longer-lived than cyclonic eddies. Anticyclonic eddies tend to survive longer in the spring and summer,while cyclonic eddies have longer lifetimes in the autumn and winter. The characteristics and seasonal variations of eddies in the SCS are strongly related to variations in general ocean circulation,in the homogeneity of surface wind stress,and in the unevenness of bottom topography in the SCS. The spatial and temporal variation of mesoscale eddies in the SCS could,therefore,be an important index for understanding local hydrodynamics and regional climate change.

Tracking the evolution processes and behaviors of mesoscale eddies in the South China Sea：a global nearest neighbor filter approach

The eddy tracking approach is developed using the global nearest neighbor filter（GNNF） to investigate the evolution processes and behaviors of mesoscale eddies in the South China Sea（SCS）. Combining the Kalman filter and optimal data association technologies, the GNNF algorithm is able to reduce pairing errors to 0.2% in tracking synthetic eddy tracks, outperforming other existing methods. A total of 4 913 eddy tracks that last more than a week are obtained by the GNNF during 1993–2012. The analysis of a growth and a decay based on 3 445 simple eddy tracks show that eddy radius, amplitude, and vorticity smoothly increase during the first half of lifetime and decline during the second half following a parabola opening downwards. The genesis of eddies mainly clusters northwest and southwest of Luzon Island whereas the dissipations concentrate the Xisha Islands where the underwater bay traps and terminates eddies. West of the Luzon Strait, northwest of Luzon Island, and southeast of Vietnam are regions where eddy splits and mergers are frequently observed. Short disappearances mainly distribute in the first two regions. Moreover, eddy splits generally result in a decrease of the radius and the amplitude whereas eddy mergers induce growing up. Eddy intensity and vorticity, on the contrary, are strengthened in the eddy splits and diminished in mergers.

Strengthening effect of super typhoon Rammasun(2014)on upwelling and cold eddies in the South China Sea

Typhoon is one of the frequent natural disasters in coastal regions of China.As shown in many studies,the impact of typhoons on the South China Sea(SCS)should not be overlooked.Super typhoon Rammasun(2014)was studied that formed in the northwestern Pacific,passed through the SCS,then landed in the Leizhou Peninsula.Remote sensing data and model products were used to analyze the spatiotemporal variations of the cold eddies,upwelling,sea surface temperature,mixed layer depth,rainfall,sea surface salinity,suspended sediment concentration,and surface-level anomaly.Results confirm the constant presence of upwelling and cold eddies in the southeast of Hainan(north of the Zhongsha Islands)and the southeast of Vietnam in July.In addition,we found the strengthening effect of super typhoon Rammasun on the upwelling and cold eddies in the SCS.The major reasons for the continuous decrease in sea surface temperature and the slow regaining of seawater temperature were the enhanced upwelling and vertical mixing caused by the typhoon.The increasing of the surface runoff in the Indochina Peninsula was mainly affected by the typhoon,with some contribution for the southeast of Vietnam’s cold eddy and upwelling.

Three-dimensional characteristics of mesoscale eddies simulated by a regional model in the northwestern Pacific Ocean during 2000–2008

Mesoscale eddies play vital roles in ocean processes.Although previous studies focused on eddy surface features and individual three-dimensional(3D)eddy cases in the northwestern Pacific Ocean,the analysis of unique eddy3D regional characteristics is still lacking.A 3D eddy detection scheme is applied to 9 years(2000-2008)of eddyresolving Regional Ocean Modeling System(ROMS)output to obtain a 3D eddy dataset from the surface to a depth of 1000 m in the northwestern Pacific Ocean(15°-35°N,120°-145°E).The 3D characteristics of mesoscale eddies are analyzed in two regions,namely,Box1(Subtropical Countercurrent,15°-25°N,120°-145°E)and Box2(Southern Kuroshio Extension,25°-35°N,120°-145°E).In Box1,the current is characterized by strong vertical shear and weak horizontal shear.In Box2,the current is characterized by the strong Kuroshio,topographic effect,and the westward propagation of Rossby waves.The results indicate the importance of baroclinic instability in Box1,whereas in Box2,both the barotropic and baroclinic instability are important.Moreover,the mesoscale eddies’properties in Box1 and Box2 are distinct.The eddies in Box1 have larger number and radius but a shorter lifetime.By contrast,Box2 has fewer eddies,which have smaller radius but longer lifetime.Vertically,more eddies are detected at the subsurface than at the surface in both regions;the depth of 650 m is the turning point in Box1.Above this depth,the number of cyclonic eddies(CEs)is larger than that of anticyclonic eddies(AEs).In Box2,the number of CEs is dominant vertically.Eddy kinetic energy(EKE)and mean normalized relative vorticity in Box2are significantly higher than those in Box1.With increasing depth,the attenuation trend of EKE and relative vorticity of Box1 become greater than those of Box2.Furthermore,the upper ocean(about 300 m in depth)contains 68.6%of the eddies(instantaneous eddy).Only 16.6%of the eddies extend to 1000 m.In addition,about87%of the eddies are bowl-shaped eddies in the two regions.Only about 3%are cone-shaped eddies.With increasing depth of the eddies,the proportion of bowl-shaped eddies gradually decreases.Conversely,the coneand lens-shaped eddies are equal in number at 700-1000 m,accounting for about 30%each.Studying the 3D characteristics of eddies in two different regions of the northwestern Pacific Ocean is an important stepping stone for discussing the different eddy generation mechanisms.

A method of automatic shape depiction and information extraction for oceanic eddies in SAR images

Synthetic aperture radar （SAR） provides a large amount of image data for the observation and research of oceanic eddies. The use of SAR images to automatically depict the shape of eddies and extract the eddy information is of great significance to the study of the oceanic eddies and the application of SAR eddy images. In this paper, a method of automatic shape depiction and information extraction for oceanic eddies in SAR images is proposed, which is for the research of spiral eddies. Firstly, the skeleton image is got by the skeletonization of SAR image. Secondly, the logarithmic spirals detected in the skeleton image are drawn on the SAR image to depict the shape of oceanic eddies. Finally, the eddy information is extracted based on the results of shape depiction. The sentinel 1 SAR eddy images in the Black Sea area were used for the experiment in this paper. The experimental results show that the proposed method can automatically depict the shape of eddies and extract the eddy information. The shape depiction results are consistent with the actual shape of the eddies, and the extracted eddy information is consistent with the reference information extracted by manual operation. As a result, the validity of the method is verified.