CHARACTERISTICS OF ANTICYCLONIC EDDY FORMED AT THE PACIFIC COAST OF THE SOUTHERN KURIL ISLANDS IN THE SPRING 1999

Based on the analysis of oceanographic survey materials and sea surface temperature satellite observations, the conditions of formation and characteristics of the anticyclonic eddy located in the region of the South Kuril Islands in the spring of 1999 were studied. It was shown that it separated from the subarctic front as result of its interaction with the Oyashio current stream in the second half of March and ceased to exist in mid-June, which is also confirmed by observations of the concentration of chlorophyll-a. In contrast the eddies formed as a result of meandering of the warm Kuroshio and Tsugaru currents, the eddy core, located at depths of 150–300 m, was characterized by lower values of temperature and salinity (4°–4.6°С and 33.55–33.65 е.п.с.) and low current velocities ( about 10 cm/s). The eddy was rather weakly distinguished in the density field both on horizontal and vertical distributions, as well as according to satellite altimetry data.

A Strong Kuroshio Intrusion into the South China Sea and Its Accompanying Cold-Core Anticyclonic Eddy in Winter 2020–2021

Multiple remote sensing datasets, combined with in-situ drifter observations, were used to analyze the Kuroshio intrusion through the Luzon Strait (LS). The results showed that a strong Kuroshio Current Loop (KCL) and accompanying anticyclonic eddy (ACE) existed in winter 2020–2021. As quantitatively demonstrated by the Double Index (DI), the Kuroshio Warm Eddy Index (KWI) had low values during a long sustained period compared to those in all other years in the available historical records. Remarkable kinematic properties (i.e., amplitude, diameter, propagated distance, lifespan and propagating speed) of the accompanying ACE were extracted by automatic eddy detection algorithms, showing that the ACE had a maximum diameter of 381 km and a peak amplitude of 50 cm, which significantly exceeded the previous statistics in winter. The orographic negative wind stress curl southwest of Taiwan Island and the westward Ekman transport through the LS during the winter half year of 2020–2021 both had large values beyond their historical maxima. Hence wind forcing is regarded as the primary forcing mechanism during this event. Alternating cyclonic eddies (CEs) and ACEs approaching on the east of the LS were identified, indicating that the interaction between the Kuroshio and the impinging CEs at proper locations made extra contributions to enhancing the KCL. The accompanying ACE had a distinctive feature of a cold-core structure at the surface layer, so as to be categorized as a cold core ACE (CC-ACE), and the temperature difference between the cold core and outer warm ring was maintained for three months. The generation and long duration of the CC-ACE may be due to the sustaining entrainment supported by the warm water from Kuroshio intrusion and the Northwest Luzon Coastal Current (NWLCC) successively.

Strain Evolution and Instability of an Anticyclonic Eddy From a Laboratory Experiment

Using the 13 m diameter LEGI-Coriolis rotating platform, the evolution processes of a generated anticyclonic eddy throughout its lifecycle are analyzed. Experimental results have shown that the eddy lasted for approximately 3T0, where T0 is the rotating period of 90 s. After T = 0.3T0, the eddy enters its mature phase, whereby following this event, eddy intensity slowly decreases from its maximum rotation speed. By T = 2.6T0, the eddy enters a stage of rapid weakening. In its decay period, two underlying mechanisms for this decay have been identified as inertial instability and eddy–eddy interactions.

Anomalous transport of heat and salt by a long-lived anticyclonic eddy in the northeast tropical Pacific Ocean

<p><span>During RV Sonne expedition SO268 to the northeast tropical Pacific Ocean between March and May 2019, the impact of a mesoscale eddy on the seawater properties was investigated by conducting a multiple of observations. A subsequent analysis of an altimeter data revealed the formation of an anticyclonic mesoscale eddy in the Tehuantepec gulf between 15 and 20 June 2018 with a mean radius of 185 km and an average speed of 13 cm/s. This extremely long-lived eddy carried sea-water characteristics from near coastal Mexican waters westward far into the open ocean. The water mass stayed largely isolated during the 11 months of travel time due to high rotational speed.</span></p><p><span>The eddy exhibited a conical-shape vertical structure with concurrent deepening of the main thermocline. The water in the eddy core showed an extreme positive temperature anomaly of 8</span><sup><span>◦</span></sup><span>C, a negative salinity anomaly of -0.5 psu and a positive dissolved oxygen concentration anomaly of +160 μmol/kg centered at 80 m depth. The sub-surface impact of the eddy is clearly evident in the temperature and salinity profiles at a depth of 1500 m. For dissolved oxygen the eddy-induced anomaly reached even deeper to the seafloor.</span></p><p><span>This study provides new insights to the offshore transport of heat and salt driven by the long-lived anticyclonic eddy in the northeast tropical Pacific Ocean. Considering the water column trapped within the eddy, a positive heat transport anomaly of 1-3 ×10</span><sup><span>11</span></sup><span> W and a negative salt transport anomaly of -8×10</span><sup><span>3</span></sup><span> kg/s were estimated. However, due to the rare occurrence of long-lived anticyclone eddies in this region, they likely do not play a significant role in affecting the heat and salt balance of the northeastern tropical Pacific Ocean. </span></p>

Characterizing an anticyclone in Western Mediterranean Sea using altimetric data and an eddy tracker

<p>In April 2019, a large anticyclonic Eddy has formed in Western Mediterranean Sea between Sardinia and Balearic Islands. This anticyclone was observable with Sentinel-3 SST satellite data for 7 months and its diameter was estimated to 150 km. Although mesoscale anticyclones are quite common in this part of the Mediterranean Sea, such large and long-live eddies remain exceptional and repercussions for ocean-atmospheric exchanges and for biodiversity might be consequent. However, due to the increase of temperatures during summer, the satellite SST track of the eddy has been lost during a few weeks in August and September. Indeed, the SST signature of the eddy was not distinguishable from surrounding waters anymore. In order to track the eddy during its entire life and have a better understanding of its characteristics, sea level anomaly derived from altimetric data will be analysed in this study with the Py Eddy Tracker toolbox to investigate the variation of its position, its altimetry and its size. The distribution of other remarkable eddies in this zone and period will also be considered. Moreover, a high-resolution SST field will be reconstructed with DINEOF method so the comparison between eddy’s SST and altimetric characteristics will be assured.</p>

Vertical Analysis of Mesoscale Eddies in the Northern Bay of Bengal

<p>Mesoscale eddies, coherent rotating structure with typical horizontal scale of ~100 km and temporal scales of a month, play a significant role in ocean energy and mass transports. Here both mesoscale cyclonic and anticyclonic eddies moving towards south in the northern Bay of Bengal during 20<sup>th </sup>March 2017 to 20<sup>th</sup> May 2017 are observed using a high resolution (~5 km) nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecological model embedded with Regional Ocean Modeling System (ROMS). Spatial maps of sea surface height anomaly (SSHA) from satellite-derived Archiving Validation, and Interpretation of Satellite Oceanographic (AVISO), and model are well matched. The centers and effective radii of both kind of eddies are identified using SSHA to proceed for their three-dimensional analysis. The extreme intensities of cyclonic and anticyclonic eddy centers are observed on 8<sup>th</sup> April 2017 at 86.40°E, 18.19°N and 84.80°E, 16.52°N respectively. Both kind of eddies are vertically extended upto 800 m and have radius ~100 km at surface. At these two locations, time-depth variations of zonal and meridional currents, and other physical (temperature and salinity) and bio-physical (chlorophyll-a, phytoplankton, zooplankton, detritus nutrient, dissolved oxygen and NO<sub>3</sub> nutrient) parameters are studied particularly from 8<sup>th</sup> March 2017 to 8<sup>th</sup> May 2017. Further vertical distribution of zonal and meridional currents, and other parameters are studied along the eddy diameters at their extreme intensity. In the vertical structure of both current components, an opposite sense between cyclonic and anticyclonic eddies are clearly captured, while other variables show strong upwelling and downwelling nature around the cyclonic and anticyclonic eddy centers respectively. Abundances (scarcities) of chlorophyll-a, phytoplankton, zooplankton and detritus nutrient are observed at 50 – 150 m depth of the cyclonic (anticyclonic) eddy center. The concentration of chlorophyll-a, phytoplankton, zooplankton and detritus nutrient reach to maximum of 1 mg/m<sup>3</sup>, 0.35 mMol/m<sup>3</sup>, 0.22 mMol/m<sup>3</sup> and 0.14 mMol/m<sup>3</sup> at ~80 m depth for the cyclonic eddy, while these are absent for the anticyclonic eddy.</p>

Generation of Vertical Fine Structure by the Internal Waves with the Regard for Turbulent Viscosity and Diffusion

Purpose. The study is aimed at evaluating effectiveness of the procedure of the observational data assimilation using the Kalman filter algorithm as compared to sequential analysis of the hydrophysical fields based on the optimal interpolation method, and at analyzing the mesoscale features of coastal circulation near the western Crimea coast and in the Sevastopol region. Methods and Results. Based on the hydrodynamic model adapted to the Black Sea coastal zone conditions including the open boundary and on the temperature and salinity data from the hydrological survey in 2007, the dynamic and energy characteristics of the Black Sea coastal circulation were calculated with high spatial resolution (horizontal grid is ~ 1.6 × 1.6 km and 30 vertical horizons). The hydrophysical fields were reconstructed using two algorithms of data assimilation: the sequential optimal interpolation and the modified Kalman filter. The kinetic energy changed mainly due to the wind action, vertical friction and the work of pressure forces; the potential energy – due to the potential energy advection and the horizontal turbulent diffusion. The following circulation features were reconstructed: the anticyclonic eddy with the radius about 15 km in the Kalamita Bay in the water upper layer, the anticyclonic eddy with the radius about 15 km between 32.2 and 32.6° E in the whole water layer, the intense current near Sevastopol and along the Crimea western coast directed to the north and northwest, and the submesoscale eddies of different signs of rotation in the upper layer. Conclusions. It is shown that having been taken into account, heterogeneity and non-isotropy of the error estimates of the temperature and salinity fields relative to the correlation function lead to qualitative and quantitative differences in the hydrodynamic fields (amplification of currents, change of the currents’ direction and eddy formations were better pronounced). At the same time, the mean square errors of the thermohaline fields’ estimates decreased. Formation of the anticyclonic eddy with the radius about 15 km in the Kalamita Bay could be related to the current shear instability. Submesoscale eddies with the diameters less than 5 km were formed when the current flowed around the coastline and the bottom topography inhomogeneities.

Analysis of the Dynamic and Energy Characteristics of Water Circulation near the Western Crimea Coast and in the Sevastopol Region Based on the Observational Data Assimilation in the Numerical Model of the Black Sea Dynamics

Purpose. The study is aimed at evaluating effectiveness of the procedure of the observational data assimilation using the Kalman filter algorithm as compared to sequential analysis of the hydrophysical fields based on the optimal interpolation method, and at analyzing the mesoscale features of coastal circulation near the western Crimea coast and in the Sevastopol region. Methods and Results. Based on the hydrodynamic model adapted to the Black Sea coastal zone conditions including the open boundary and on the temperature and salinity data from the hydrological survey in 2007, the dynamic and energy characteristics of the Black Sea coastal circulation were calculated with high spatial resolution (horizontal grid is ~1.6×1.6 km and 30 vertical horizons). The hydrophysical fields were reconstructed using two algorithms of data assimilation: the sequential optimal interpolation and the modified Kalman filter. The kinetic energy changed mainly due to the wind action, vertical friction and the work of pressure forces; the potential energy – due to the potential energy advection and the horizontal turbulent diffusion. The following circulation features were reconstructed: the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay in the water upper layer, the anticyclonic eddy with the radius about 15 km between 32.2 and 32.6° E in the whole water layer, the intense current near Sevastopol and along the Crimea western coast directed to the north and northwest, and the submesoscale eddies of different signs of rotation in the upper layer. Conclusions. It is shown that having been taken into account, heterogeneity and non-isotropy of the error estimates of the temperature and salinity fields relative to the correlation function lead to qualitative and quantitative differences in the hydrodynamic fields (amplification of currents, change of the currents’ direction and eddy formations were better pronounced). At the same time, the mean square errors of the thermohaline fields’ estimates decreased. Formation of the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay could be related to the current shear instability. Submesoscale eddies with the diameters less than 5 km were formed when the current flowed around the coastline and the bottom topography inhomogeneities