Warm Eddy Research Articles

Analysis on the Evolution and Seasonal Variation of the Ulleung Warm Eddy

The Ulleung Warm Eddy (UWE) is a semi-permanent eddy, which has a very important impact on material transport in the Japan Sea, so we use aviso(Archiving, Validation and Interpretation of Satellite Oceanographic data) data and ARM3D data to analyze the seasonal variation of UWE that appeared in a total of 28 years from 1993 to 2020. We also analyze the evolution of the UWE that appears in 2019, and found that the eddies merging (splitting) can cause sudden increases (drops) in the nature of UWE. Seasonally, UWE’s area and eddy amplitude are strong in winter and spring, and weak in summer and autumn. The Tsushima Current (TC) has an important influence on UWE. The stronger (weaker) flow of TC, the weaker (stronger) the area and eddy amplitude of UWE.

A Mesoscale Ocean–Atmosphere Coupled Pathway for Decadal Variability of the Kuroshio Extension System

Abstract The Kuroshio Extension (KE) system has been observed to experience a decadal cycle between dynamically stable and unstable states. However, divergent conclusions on its interaction with the atmosphere obfuscate the understanding of its oscillatory nature at the preferred decadal time scale. Here, using satellite observations and the ERA-Interim reanalysis in 2002–16, physical process-oriented diagnoses suggest that the wintertime finer-scale thermodynamic response to mesoscale oceanic surface conditions and slow oceanic Rossby wave adjustment frame a coupled ocean–atmosphere delayed oscillator for the decadal KE variability. During the stable state of the KE system, the downstream KE transition region is rich in mesoscale oceanic warming associated with warm eddies, which induces surface wind convergence and upward motion, probably via the enhanced turbulent mixing. Meanwhile, increased finer-scale diabatic heating in the lower troposphere with abundant moisture supply from warmer water likely facilitates the deep-reaching updraft that adiabatically cools the middle troposphere. The background northwesterly wind helps to spread out the cooling, leading to southward deflection of local atmospheric eddy available potential energy (EAPE) production by baroclinic conversion. Consequently, the synoptic eddy activity displaces southward across the basin with additional energy supply from the increased diabatic production of EAPE downstream. Anomalous synoptic eddy thermal and vorticity forcing eventually fosters the basin-scale equivalent-barotropic cyclonic circulation anomaly, which is further maintained by energy conversion from the background state. The resultant wind-driven negative sea surface height anomalies propagate westward into the upstream KE region with a delay of ∼4 years and can trigger the unstable state of the KE system.

Modulation Effects of Mesoscale Eddies on Sea Surface Wave Fields in the South China Sea Derived From a Wave Spectrometer Onboard the China‐France Ocean Satellite

AbstractThis study aims to examine the modulation effects of mesoscale eddies on sea surface wave fields in the South China Sea using data measured by a wave spectrometer named Surface Wave Investigation and Monitoring onboard the China‐France Ocean Satellite from July 2019 to August 2021. The statistical results show that the significant wave heights (SWHs) decrease (increase) from inside to outside of warm (cold) eddies, while the wavelengths have minimum values at the eddy edge in all cases. In cases of strong warm eddies, the maximum variations in SWH, wavelength, and wave direction caused by mesoscale eddy modulation reach up to 18%, 24%, and 28.4°, respectively. The eddy modulation on wave parameters is stronger at low sea states. The correlation analysis reveals that the wave parameter variations across eddies result from the eddy current shear and the direction difference between the eddy current and the waves. The dynamic analysis indicates that the deformation term of the eddy current is a dominant term affecting the SWH at the eddy edge, and the eddy current divergence and its product with wave direction determine the wavelength variation. The eddy vorticity deflects the wave direction by approximately 2°–3°, and the wave‐eddy angle and eddy‐modulated wind variation also contribute to the wave direction variation. The background current‐wave interaction is negligible in the eddy cases studied.

Summer Anticyclonic Eddies Carrying Kuroshio Waters Observed by a Large CPIES Array West of the Luzon Strait

Abstract The Kuroshio intrusion into the South China Sea (SCS) in summer is weak and has rarely been reported by in situ observations. Here, we describe a new form of Kuroshio water intrusion that is strongest during the summer, the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy originating north of Luzon Island. From early July to mid-September 2018, two NLWEs moving northwestward were captured by a mooring array consisting of 27 current- and pressure-recording inverted echo sounders (CPIESs). The three-dimensional CPIES estimates reveal that the NLWEs carried large amounts of saline Kuroshio waters (S > 34.7 psu) in the subsurface, which was also evidenced by Argo float profiles. The Kuroshio intrusion was confined to waters shallower than the 14.8°C isotherm. Historical data for NLWEs suggest that they occur mostly during the summer but are absent between November and March, which is attributed to seasonal wind stress curl (WSC). However, because the seasonal signal of WSC during summer is small, intraseasonal WSC is directly responsible for the generation of NLWEs. Significance Statement This paper describes a new type of Kuroshio water intrusion into the South China Sea (SCS)—the North Luzon Warm Eddy (NLWE), which is an anticyclonic eddy generated north of Luzon Island. The eddy mostly occurs during summer when the Kuroshio intrusion is commonly considered the weakest. From observations of a large CPIES array, we provide a cradle-to-grave picture of the NLWE. NLWEs are estimated to contribute almost half of the westward Luzon Strait transport during the summer and, as such, play an important role in the seasonal stratification and circulation in the northeastern SCS.

Observation of a mesoscale warm eddy impacts acoustic propagation in the slope of the South China Sea

Acoustic rays are modified while propagating through oceanic eddies. However, due to the lack of field synchronous observation, the impact of mesoscale eddy on the acoustic propagation is less clarified. To address the issue, an eddy-acoustic synchronous observation (EASO) field experiment for a mesoscale warm eddy was carried out in the slope of the South China Sea (SCS) in October, 2021. During the field experiment, a total of 105 conductivity-temperature-depth (CTD) stations, as well as a zonal acoustic survey line through the center of the warm eddy, were obtained. The vertical structures of temperature and salinity indicate that the warm eddy is surface-intensified with temperature and salinity cores confined within depths from 70 m to 200 m and 10 m to 70 m, respectively. The acoustic observation shows two obvious convergency zones (CZs) at about 39 km and 92 km in the eastern half acoustic line, and one convergency zones (CZ) at about 25 km in the western half acoustic line. By comparing with the none eddy circumstance, the respective impacts of the topography and warm eddy are quantitatively analyzed with a ray-tracing model. The results indicate that the topography shortens the horizontal span of the CZ by 11.4 km, while the warm eddy lengthens it by 1.7 km. Additionally, the warm eddy shallows the depth and broadens the width of the CZ by 32 m and 1.4 km, respectively. The anisotropy of 3D sound fields jointly influenced by the warm eddy and the local topography show that the distance differences of the first CZs in different horizontal directions can be as long as 31 km.

Physics-Guided Reduced-Order Representation of Three-Dimensional Sound Speed Fields with Ocean Mesoscale Eddies

Ocean mesoscale eddies have an important role in the ocean and affect the underwater sound speed field (SSF). Many physical models have been proposed for mesoscale eddy three-dimensional (3D) structure analysis and construction. Here, we propose a model for the reduced-order representation of 3D SSF with ocean mesoscale eddies. Particularly, the radial basis functions (RBFs), which are guided by the universal physics model of mesoscale eddy in horizontal dimensions, are employed. RBF and empirical orthogonal function (EOF) are used as basis functions for 3D representation. The proposed method is an approximation of the classical Gaussian eddy model in the first-order form. Simulation results confirm the reduced-order representation performance and effectiveness in reconstruction using 136 days of HYCOM data in the northwestward of the South China Sea with a warm eddy and a cold eddy. The proposed RBF + EOF method roughly halves the number of coefficients for mesoscale eddy representation, compared with classical methods. The reduced-order representation method can be utilized in ocean acoustic tomography and acoustic remote sensing in a mesoscale area.

Impacts of Mesoscale Eddies on Biogeochemical Variables in the Northwest Pacific

Mesoscale eddies play an important role in regulating biogeochemical cycles. However, the response of biogeochemical variables to cold and warm eddies has not been well elucidated, mainly due to most previous studies relying on remote sensing techniques and lacking in situ observations below the surface water. Here, we used hydrographic and biochemical data from one survey in the northwestern Pacific to document the vertical biogeochemical structure of one cold and two warm eddies. We first compared the changes of key variables in the eddy core relative to eddy outside, explained the role of key layers (the mixing depth, pycnocline, nutricline, euphotic) in causing these changes, and then analyzed the main environmental factors affecting chlorophyll a (Chla) and phytoplankton communities. Finally we focused on the response mechanisms of key biogeochemical variables to the cold and warm eddies. The results showed that biological variables (Chla, microphytoplankton, picophytoplankton), salinity, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphate (DIP), and dissolved inorganic silicate (DSi) in the cold eddy core increased by 0.2–134%, while in the warm eddy core, they decreased by 0.2–70% relative to the eddy outside. The cold and warm eddies were able to force the deep chlorophyll maximum (DCM), which rose or fell with the pycnocline, nutricline and euphotic depth (Zeu) as a whole. Cold eddies with a raised thermocline could lead to about 20 m elevated DCM and enhanced phytoplankton biomass when the nutricline and thermocline were coincident. In contrast, warm eddies drove isopycnals downward, resulting in a 10–25 m drop in DCM and a decrease in nutrient and Chla concentrations at the center of the eddies. The significant difference in the vertical structure of the phytoplankton community between the center and the outside of the eddy might be explained by the direct influence of both nutrient concentrations and stoichiometry changes. The contribution of microphytoplankton to total biomass was much smaller than that of picophytoplankton in oligotrophic waters where the DIN:DIP and DSi:DIN ratios are significantly low. Compared to nutrients, photosynthetically active radiation (PAR) might not be the main factor controlling phytoplankton biomass and abundance attributed to Zeu being consistently deeper than the mixed depth (Zm), whereas it was likely to be the key limiting factor affecting the vertical distribution of the phytoplankton community.

Evaluating the impacts of mesoscale eddies on abundance and distribution of neon flying squid in the Northwest Pacific Ocean

Mesoscale eddies are ubiquitous in global oceans yielding significant impacts on marine life. As a short-lived pelagic squid species, the population of neon flying squid (Ommastrephes bartramii) is extremely sensitive to changes in ambient oceanic variables. However, a comprehensive understanding of how mesoscale eddies affect the O. bartramii population in the Northwest Pacific Ocean is still lacking. In this study, a 10-year squid fisheries dataset with eddy tracking and high-resolution reanalysis ocean reanalysis data was used to evaluate the impact of mesoscale eddies and their induced changes in environmental conditions on the abundance and habitat distribution of O. bartramii in the Northwest Pacific Ocean. A weighted-based habitat suitability index (HSI) model was developed with three crucial environmental factors: sea surface temperature (SST), seawater temperature at 50-m depth (T50m), and chlorophyll-a concentration (Chl-a). During years with an unstable Kuroshio Extension (KE) state, the abundance of O. bartramii was significantly higher in anticyclonic eddies (AEs) than that in cyclonic eddies (CEs). This difference was well explained by the distribution pattern of suitable habitats in eddies derived from the HSI model. Enlarged ranges of the preferred SST, T50m, and Chl-a for O. bartramii within AEs were the main causes of more squids occurring inside the warm-core eddies, whereas highly productive CEs matching with unfavorable thermal conditions tended to form unsuitable habitats for O. bartramii. Our findings suggest that with an unstable KE background, suitable thermal conditions combined with favorable foraging conditions within AEs were the main drivers that yielded the high abundance of O. bartramii in the warm eddies.

Vertical patterns of chlorophyll a in the euphotic layer are related to mesoscale eddies in the South China Sea

Phytoplankton closely connects with the hydrodynamics and biogeochemical environment of the ocean. While research focusing on both the physiochemical factors and hydrodynamics regulating phytoplankton has already been conducted, the coupling mechanism between mesoscale eddies and the vertical distribution of phyto plankton in the South China Sea (SCS) is still not well understood. Here, phytoplankton was studied under one weak-cold and two warm eddies along the 18°N transect in the SCS. The results show that the vertical distribution of chlorophyll (chl-a) presented a similar pattern at all four sampling stations. The mixed layer is less than 50 m. It is the same as the meridional salinity gradients that may be distinguished above a depth of 60 m. The subsurface chlorophyll maximum at the edge of a warm (WI) and cold eddy (CI) at E413 and E411 was shallower than that at the edge of a warm eddy (WII) at E407 and E409, indicating that temperature and salinity may be the important driving factors. On the whole, mesoscale eddies had a significant influence on the vertical pattern of chl-a in the SCS during the study period.

Effects of mesoscale eddies on the spatial coherence of a middle range sound field in deep water

Mesoscale eddies have a remarkable influence on the underwater sound field. Many previous studies have investigated the effects of eddies on transmission loss, the convergence zone, time delay, etc. However, the effects of eddies on spatial coherence are less well studied and remain unclear. In this paper, the effects of eddies on spatial coherence at the subsurface in deep water are investigated. The eddy environments are simulated with Gaussian eddy equations, the complex pressure field is obtained using a range-dependent parabolic equation model and the associated mechanism is analyzed based on ray theory and models. The results show that cold/warm mesoscale eddies affect spatial coherence in a high-intensity zone by changing the locations and width of the convergence zone. In the shadow zone, the horizontal correlation radius and the vertical correlation radius increase with range and decrease with depth, and they are increased by warm eddies and decreased by cold eddies, mainly caused by variation of the multipath structure.

An Investigation of the Hydrography of Chabahar Bay Using FVCOM Model and EOF Analysis

Chabahar Bay is a semi-enclosed and shallow omega-shaped bay, located on the south-eastern coasts of Iran. The 3D unstructured-grid Finite-Volume Coastal Ocean Model was employed to study the physical characteristics of water in Chabahar Bay. The correlation coefficients between the model and available measurements were 0.99 and 0.84 for water level and current velocity, respectively. The numerical results show that the depth averaged temperature of the Bay generally decreases from north to south and varies between 20-26 °C during the year. The basin average of annual salinity is 36.1 psu. Hence, the density increases from north to south and is strongly affected by temperature changes. The vertically average of sigma-t varies between 21.5-24.0 kg/m3. In the northern half of the Bay a warm center eddy is formed from March; during July, it strengthened and peaked in August. With Applying EOFs analysis on model surface temperature four principal patterns were determined. EOF1 corresponds to the bathymetry of the region and shows the heat capacity of the Bay, it is in accordance with the monsoon conditions. EOF2, 3, and 4 are corresponding to the effect of the wind. Due to dominant winds a strong upwelling was determined along the western coast.

The Effects of Eddy Size on the Sea Surface Heat Flux

AbstractThe effects of eddy size on the surface turbulent heat flux anomaly (THFA) are revealed from the analysis of satellite observations. As eddy size increases from 80 to 140 km, the coupling coefficient between the sea surface temperature anomaly (SSTA) and THFA increases from 35.2 to 41.5 in the Kuroshio Extension (KE). Because eddy size decreases from the equator to the polar, the coupling coefficient () also weakens poleward. Analysis shows that for same SSTA, a large warm (cold) eddy deepens (shallows) the atmosphere boundary layer height (ABLH) more than a small one. This change in ABLH will lead to an increased (decreased) sea surface wind speed anomaly, which amplifies the THFA further. Theoretical results also show that the is proportional to a quarter of the eddy size, , where is depending upon SSTA and the transfer coefficient for momentum ().

Sinking Rate and Community Structures of Autumn Phytoplankton Responses to Mesoscale Physical Processes in the Western South China Sea.

To examine the influence of mesoscale eddy on the natural phytoplankton community and its sinking rate changes, a comprehensive investigation cruise was carried out in the western South China Sea in autumn 2016. A total of 108 phytoplankton species were found, which belong to 54 phytoplankton genera; most of them were dominated by Dinophyta (54 genera), followed by Bacillariophyta (50 genera), Cyanophyta (3 genera), and Chrysophyta (1 genus). Bacillariophyta and Dinophyta were the main phytoplankton communities in the investigated sea area. The sinking rate of phytoplankton ranged from 0.12 to 3.17 m day–1, determined by the SETCOL method. The highest phytoplankton sinking rate was found in the 200-m water layer, followed by the DCM layer. No significant correlation was found between phytoplankton sinking rates and most of the environmental parameters during this cruise. At a similar time, we have carried out the estimation of carbon flux in the investigated sea area by using the sinking rate of phytoplankton, which showed that the carbon flux ranged from 2.41 × 10–6 to 0.006 mg C m–2 day–1; in addition, the maximum was at the 200-m layer. Phytoplankton community and sinking rate were significantly affected by the mesoscale eddy processes. The cold eddy could affect the community distribution of diatom and dinoflagellate, and the upwelling mainly affects the community of dinoflagellate. Both of them could contribute to a higher sedimentation rate of phytoplankton in the surface and DCM layers. Warm eddy could reduce the abundance of phytoplankton in the surface layer; simultaneously, the sinking rate of phytoplankton in the shallow water layer above 100 m is also reduced. These results can fill in the knowledge gap of mesoscale eddy processes in the study of phytoplankton community change and sinking rate; furthermore, it can provide insights into phytoplankton carbon and its implementation in further carbon sink.

Impact of the warm eddy on planktonic ciliate, with an emphasis on tintinnids as bioindicator species

Mesoscale eddies are a common feature of world oceans. However, their influence on microzooplankton has not been well studied. This study investigated the impact of warm eddies (originating from the Kuroshio Current) on planktonic ciliates in the northern South China Sea and the western North Pacific. We found more tintinnid species in warm eddies than in reference areas in both seas. All species categorized as abundant (maximum abundance > 5 ind. L−1 and occurrence frequency > 10%) could be divided into subsurface-peak and surface-peak groups (highest abundance appeared in subsurface and surface layers, respectively). Subsurface-peak tintinnid species vertical distribution together with temperature-salinity-plankton diagrams at each sampling points revealed higher abundance in warm eddies than in reference areas, which suggested that they can act as warm eddy bioindicator species. Surface-peak tintinnid species were mainly distributed in both warm eddy and reference areas, depth and temperature were the main influence factors. The vertical distribution in warm eddies showed a bimodal pattern in both the South China Sea and North Pacific, with abundance peaks in surface and subsurface layers; whereas in reference areas, abundance peaks in surface layers. The average abundance of ciliate and tintinnids in subsurface layers were higher in warm eddies than in reference areas. Our study contributed to a better understanding of the impact of the warm eddy on microzooplankton and tintinnids function as usefully bioindicator species. Further research is needed to fully understand the ecological influence of eddies on the microzooplankton community.

Mesoscale Eddy-Induced Ocean Dynamic and Thermodynamic Anomalies in the North Pacific

Oceanic mesoscale eddies are associated with large thermodynamic anomalies, yet so far they are most commonly studied in terms of surface temperature and in the sense of composite mean. Here we employ an objective eddy identification and tracking algorithm together with a novel matching and filling procedure to more thoroughly examine eddy-induced thermodynamic anomalies in the North Pacific, their relationship with eddy amplitude (SSH), and the percentage of variability they explain on various timescales from submonthly to interannual. The thermodynamic anomalies are investigated in terms of sea surface temperature (SST), isothermal layer depth (ITD), and upper ocean heat content (HCT). Most eddies are weak in amplitude and are associated with small thermodynamic anomalies. In the sense of composite mean, anticyclonic eddies are generally warm eddies with deeper isothermal layer and larger heat content, and the reverse is true for cyclonic eddies. A small fraction of eddies, most probably subsurface eddies, exhibits the opposite polarities. Linear relationships with eddy amplitude are found for each of the thermodynamic parameters but with different level of scatter and seasonality. HCT-amplitude relation scatters the least and has the smallest seasonal difference, ITD-amplitude relation has the largest scatter and seasonality, while SST-amplitude relation is in between. For the Kuroshio and Oyashio Extension region, the most eddy-rich region in the North Pacific, eddies are responsible for over 50% of the total SSH variability up to the intra-seasonal scale, and ITD and HCT variability up to interannual. Eddy-induced SST variability is the highest along the Oyashio Extension Front on the order of 40–60% on submonthly scales. These results highlight the role of mesoscale eddies in ocean thermodynamic variability and in air-sea interaction.

Extreme Sea-Surface Cooling Induced by Eddy Heat Advection During Tropical Cyclone in the North Western Pacific Ocean

A tropical cyclone (TC) usually induces strong sea-surface cooling due to vertical mixing. In turn, surface cooling influences the intensities and tracks of TCs. Therefore, the relationship between sea-surface temperature (SST) and TC is one of the important components of air-sea interaction. Sea-surface cooling associated with three TCs (Bailu, Lingling, and Mitag) was investigated based on wave-glider observations, satellite altimetry, and Massachusetts Institute of Technology General Circulation Model (MITgcm) numerical experiments from August 3rd to October 10th, 2019. Surface cooling varied among the three TCs. TC Lingling had the nearest distance to the wave-glider position, the slowest translation speed, and the strongest intensity of three TCs, but extreme cooling (1.4) occurred during TC Bailu. Although MITgcm underestimated the extreme cooling, the SST trend driven by the net heat flux, advection, and vertical mixing within the mixed layer was greater during TC Bailu than during other TCs. Advection was the largest of the three heat balance terms during TC Bailu, while it was quite small during the other two TCs. Interestingly, the extreme cooling occurred at the position of preexisting warm eddy. Based on heat balance analysis, we found that the eddy-induced heat advection transport reached −0.4/day, contributing 60% of the heat balance; this was attributed to extreme coolingviaeddy disturbance. We suggest TC Bailu leads to the decrease in SST and increase in the area of the cold eddy, and then, the cooled-enlarged eddy is advected to the neighbored position of wave glider, which observes the extreme cooling. These findings provide the utilization of wave gliders and help improve air-sea coupled models during TCs.

The Increased Likelihood in the 21st Century for a Tropical Cyclone to Rapidly Intensify When Crossing a Warm Ocean Feature—A Simple Model’s Prediction

A warm ocean feature (WOF) is a blob of the ocean’s surface where the sea-surface temperature (SST) is anomalously warmer than its adjacent ambient SST. Examples are warm coastal seas in summer, western boundary currents, and warm eddies. Several studies have suggested that a WOF may cause a crossing tropical cyclone (TC) to undergo rapid intensification (RI). However, testing the “WOF-induced RI” hypothesis is difficult due to many other contributing factors that can cause RI. The author develops a simple analytical model with ocean feedback to estimate TC rapid intensity change across a WOF. It shows that WOF-induced RI is unlikely in the present climate when the ambient SST is ≲29.5 °C and the WOF anomaly is ≲+1 °C. This conclusion agrees well with the result of a recent numerical ensemble experiment. However, the simple model also indicates that RI is very sensitive to the WOF anomaly, much more so than the ambient SST. Thus, as coastal seas and western boundary currents are warming more rapidly than the adjacent open oceans, the model suggests a potentially increased likelihood in the 21st century of WOF-induced RIs across coastal seas and western boundary currents. Particularly vulnerable are China’s and Japan’s coasts, where WOF-induced RI events may become more common.

Observations of the variability of the New England shelf break by shipboard echosounders

The New England Shelf Break is a dynamic region where warm salty slope water meets with colder less saline shelf water to form a distinct front. The position and shape of the front is modified by a range of processes on differing temporal and spatial scales. While seasonal changes are well documented, the influence of shorter time scale processes throughout the water column are less well understood. Of interest is the influence of meso-scale physical oceanographic processes such as meanders in the Gulf Stream and warm core rings and eddies. From March to June 2021 five cruises to the New England Shelf Break were conducted as part of the ONR Task Force Oceans New England Shelf Break Acoustics Experiments. The shipboard broadband echo sounders on the R/V Neil Armstrong were used to ensonify the front during each of these five cruises. Significant changes were observed, especially during the transit of a warm core eddy through the study area. Groups of scatterers in the study area were targeted by Deep-See (a broadband tow sled) and a broadband echo sounder equipped REMUS 600 to enable ensonification of individual targets to aide in acoustic identification of biology at the shelf break front.

Giant ice rings in southern Baikal: multi-satellite data help to study ice cover dynamics and eddies under ice

Abstract. Ice cover on lakes is subject to atmospheric forcing from above and the influence of water dynamics and heat flux from below. One characteristic example of these influences in some large lakes, such as Lake Baikal in Russia, are the giant ice rings and the associated eddies under the ice cover. In April 2020 a giant ice ring appeared in southern Baikal, and a lens-like eddy was detected below the ice. We analysed the temporal changes of ice cover using satellite images from multiple satellite missions – MODIS on Terra and Aqua, Sentinel-1 SAR, Sentinel 2 MSI, Landsat 8, PlanetScope, satellite photography from the International Space Station, and radar altimetry data from Jason-3. Satellite imagery and meteorological data show unusual temporal changes of ice colour in April 2020, which were explained by water infiltration into the ice followed by the competing influences of cold air from above and the warm eddy below the ice. Tracking of ice floe displacement also makes it possible to estimate eddy currents and their influence on the upper water layer. Multi-satellite data contribute to a better understanding of the development of ice cover in the presence of eddies, the role of eddies in horizontal and vertical heat and mass exchange, and their impact on the chemistry and biology of the lakes and on human activity.

The influence of mesoscale eddies on sedimentary processes in the western South China Sea since 32 kyr BP

Mesoscale eddies play an important role in the transport of heat, salt and other chemical substances. However, the influence of these eddies on the sediment distribution has not been thoroughly explored. Here, we discuss the influence of eddies on sediment transport in the western South China Sea (WSCS), where the eddy kinetic energy is strong, using sedimentary records of the grain size, clay mineral assemblage and total organic carbon (TOC) since 32 kyr BP. Our results reveal that the mesoscale eddies that influence the sedimentation process are controlled by both sea level fluctuations and summer monsoon variations. During the glacial period, fluvial sediments from the Mekong River, Red River and central Vietnam mountainous rivers were mixed offshore of southeastern Vietnam, as indicated by the strong influence of these river-loaded sediments in gravity core 18XB47 (112.480°E, 15.442°N, water depth 1895 m). During that time, the particles could not effectively settle due to sea-water upwelling in the cold eddy region, while sea-water sinking in the warm eddy region promoted particle settling because the particles in the WSCS were confined by mesoscale eddies offshore of southeastern Vietnam. In contrast, during the last deglacial period when the activity of the Vietnam eddy dipole was reinforced by the rising sea level and strengthening of the summer monsoon, the particles from the WSCS could not be easily transported to the location of core 18XB47 because the eastern Vietnam upwelling moved closer to the coastal area.