Three-dimensional properties of mesoscale cyclonic warm-core and anticyclonic cold-core eddies in the South China Sea

In general,a mesoscale cyclonic(anticyclonic)eddy has a colder(warmer)core,and it is considered as a cold(warm)eddy.However,recently research found that there are a number of"abnormal"mesoscale cyclonic(anticyclonic)eddies associated with warm(cold)cores in the South China Sea(SCS).These"abnormal"eddies pose a challenge to previous works on eddy detection,characteristic analysis,eddy-induced heat and salt transports,and even on mesoscale eddy dynamics.Based on a 9-year(2000–2008)numerical modelling data,the cyclonic warm-core eddies(CWEs)and anticyclonic cold-core eddies(ACEs)in the SCS are analyzed.This study found that the highest incidence area of the"abnormal"eddies is the northwest of Luzon Strait.In terms of the eddy snapshot counting method,8620 CWEs and 9879 ACEs are detected,accounting for 14.6%and 15.8%of the total eddy number,respectively.The size of the"abnormal"eddies is usually smaller than that of the"normal"eddies,with the radius only around 50 km.In the generation time aspect,they usually appear within the 0.1–0.3 interval in the normalized eddy lifespan.The survival time of CWEs(ACEs)occupies 16.3%(17.1%)of the total eddy lifespan.Based on two case studies,the intrusion of Kuroshio warm water is considered as a key mechanism for the generation of these"abnormal"eddies near the northeastern SCS.

Nitrogen fixation driven by mesoscale eddies and the Kuroshio Current in the northern South China Sea and the East China Sea

N2 fixation rates(NFR,in terms of N)in the northern South China Sea(nSCS)and the East China Sea(ECS)were measured using the acetylene reduction assay in summer and winter,2009.NFR of the surface water ranged from 1.14 nmol/(L·d)to 10.40 nmol/(L·d)(average at(4.89±3.46)nmol/(L·d),n=11)in summer and 0.74 nmol/(L·d)to 29.45 nmol/(L·d)(average at(7.81±8.50)nmol/(L·d),n=15)in winter.Significant spatio-temporal heterogeneity emerged in our study:the anticyclonic eddies(AE)(P<0.01)and the Kuroshio Current(KC)(P<0.05)performed significantly higher NFR than that in the cyclonic eddies or no-eddy area(CEONE),indicating NFR was profoundly influenced by the physical process of the Kuroshio and mesoscale eddies.The depth-integrated N2 fixation rates(INF,in terms of N)ranged from 52.4μmol/(m2·d)to 905.2μmol/(m2·d)(average at(428.9±305.5)μmol/(m2·d),n=15)in the winter.The contribution of surface NFR to primary production(PP)ranged from 1.7%to 18.5%in the summer,and the mean contribution of INF to new primary production(NPP)in the nSCS and ECS were estimated to be 11.0%and 36.7%in the winter.The contribution of INF to NPP(3.0%–93.9%)also decreased from oligotrophic sea toward the eutrophic waters affected by runoffs or the CEONE.Furthermore,we observed higher contributions compared to previous studies,revealing the vital roles of nitrogen fixation in sustaining the carbon pump of the nSCS and ECS.

Interaction of an anticyclonic eddy with sea ice in the western Arctic Ocean:an eddy-resolving model study

The dramatic decline of summer sea ice extent and thickness has been witnessed in the western Arctic Ocean in recent decades, which has motivated scientists to search for possible factors driving the sea ice variability. An eddy-resolving, ice-ocean coupled model covering the entire Arctic Ocean is implemented, with focus on the western Arctic Ocean. Special attention is paid to the summer Maskan coastal current （ACC）, which has a high temperature （up to 5℃ or more） in the upper layer due to the solar radiation over the open water at the lower latitude. Downstream of the ACC after Barrow Point, a surface-intensified anticyclonic eddy is frequently generated and propagate towards the Canada Basin during the summer season when sea ice has retreated away from the coast. Such an eddy has a warm core, and its source is high-temperature ACC water. A typical warm-core eddy is traced. It is trapped just below summer sea ice melt water and has a thickness about 60 m. Temperature in the eddy core reaches 2-3℃, and most water inside the eddy has a temperature over 1℃. With a definition of the eddy boundary, an eddy heat is calculated, which can melt 1 600 km2 of 1 m thick sea ice under extreme conditions.

The characteristics of spontaneous near-inertial wave generation from an anticyclonic mesoscale eddy

The generation and propagation characteristics of near-inertial waves(NIWs)generated spontaneously from a quasi-geostrophic anticyclonic mesoscale eddy in a rotating and stratifi ed fl uid were investigated by three-dimensional numerical modeling.NIWs are generated over a long time interval as a forced response to balanced baroclinic mesoscale eddies.For such eddies,NIW generation from balanced flow is an inevitable result as the evolution of eddies.Moreover,the baroclinicity of mesoscale eddies is an essential condition for this NIW generation mechanism.The spontaneously generated NIWs radiate horizontally toward the eddy center and propagate upward in vertical direction.The forcing of the NIWs moves downward along the eddy axis from the location of maximum temperature anomaly of the mesoscale eddy.The moving speed of the forcing is independent on the balanced mesoscale eddies but is determined by the ratio of buoyancy to inertial frequency.When the forcing reaches the bottom of the mesoscale eddy,the spontaneous NIW generation process terminates.NIW intensity in this spontaneous generation process is strengthened with the increase of the Rossby and Froude numbers.Further research to gain a solid understanding of the role of the Rossby and Froude numbers is necessary for the parameterization of spontaneous NIW generation from quasi-geostrophic mesoscale eddies in general circulation model.

Short-term offshore extension of Brahmaputra-Ganges and Irrawaddy freshwater plumes to the central northern Bay of Bengal based on in situ and satellite observations

In this study, the short-term offshore extension of Brahmaputra-Ganges(BG) and Irrawaddy freshwater plumes to the central northern Bay of Bengal(BoB) was investigated based on in situ and satellite observations. In the summer and winter of 2015, two significant freshening events with periods of weeks were observed from a moored buoy at 15°N, 90°E in the BoB. Soil Moisture Active Passive(SMAP) satellite sea surface salinity compares well with the in situ data and shows that these freshening events are directly related to the short-term offshore extension of the BG and Irrawaddy freshwater, respectively. These data combined with the altimeter sea level anomaly data show that the offshore extending plumes result from freshwater modulated by eddies. During summer, the BG freshwater is modulated by a combination of three closely located eddies: a large anticyclonic eddy(ACE) off the northwestern BoB coast and two cyclonic eddies in the northern BoB. Consequently, the freshwater extends offshore from the river mouth and forms a long and narrow tongue-shaped plume extending southwestward to the central BoB. During winter, the Irrawaddy freshwater is modulated by two continuous ACEs evolved from Rossby wave propagating westward from the Irrawaddy Delta off Myanmar, forming a tongueshaped plume extending to the central BoB. Strong salinity fronts are formed along the boundaries of these tongue-shaped plumes. These findings confirm good capability of the SMAP data to investigate the short-term offshore extension of the BG and Irrawaddy freshwater. This study provides direct evidences of the pathways of the offshore extension of the BG and Irrawaddy freshwater and highlights the role of eddies in the northern BoB freshwater plume variability.

Evolution and propagation of a mesoscale eddy in the northern South China Sea during winter

In situ observations, satellite data, and the output from an eddy-resolving ocean circulation model were used to study the generations and propagations of an anticyclonic eddy in the northern South China Sea （NSCS） during the winter of 2009-2010. In the NSCS, the anticyclonic eddy firstly appeared to the west of the Luzon Strait, migrated generally along the continental slope and dissipated around the Xisha Archipela- go. The evolution of the warm eddy contains three phases： development, maturation, and decay. The eddy mainly stayed near 119.7°E in December and then gradually moved to 118.7°E until January 15, when its intensity, as indicated by the thermocline temperature and salinity anomalies, increased significantly, re- flecting the growth of the eddy. The eddy reached its peak on January 15 and persisted until February 23. During this period, the eddy propagated westward to 116.4°E. After, the warm eddy weakened significantly and dissipated finally near the Xisha Archipelago.

Direct measurements of surface and mid-depth circulationin the Shikoku Basin by Argo profiling floats

The circulation in the Shikoku Basin plays a very important role in the pathway of the Kuroshio and the water exchange in the subtropical gyre in the North Pacific Ocean. The Argo profiling floats deployed in the Shikoku Basin are used to study the circulations and water masses in the basin. The trajectories and parking depth velocity fields derived from all Argo floats show an anticyclonic circulation at 2 000 m in the Shikoku Basin. There are inhanced eddy activities in the Shikoku Basin, which have large influence on the Shikoku Basin circulation patterns. The characteristics of temperature - salinity curves indicate that there are North Pacific Ocean tropical water （NPTW）, North Pacific Ocean subtropical mode water （NPSTMW） and North Pacific Ocean intermediate water （NPIW） in the Shikoku Basin. The NPTW is only exists south of 32°N. In the middle part of the basin, which is 28°～31°N,133°～135°E, there is a confluence region. Water masses coming from the Kuroshio mix with the water in the Shikoku Basin.

Variations in Dissolved Oxygen Induced by a Tropical Storm Within an Anticyclone in the Northern South China Sea

Tropical storms(TSs)can induce sea surface cooling,freshening,and phytoplankton blooms.The dissolved oxygen(DO)concentration response to TSs within an anticyclone is still unclear due to the rarity of in situ observations.In this study,we investi-gate the variations in DO concentration attributed to TS‘Haitang’within an anticyclonic eddy in the northern South China Sea based on Chinese underwater glider data.DO concentrations have a higher value at the edge of eddy than at the core.Influenced by TS,DO concentrations decrease remarkably in the subsurface layer in all three regions(inside,edge,and outside of the anticyclonic eddy).The mean DO concentrations decrease more at the edge of the anticyclone than those inside the anticyclone.The recovery time of DO concentration after TS is around one week at the edge of the anticyclone and is>10 days within the eddy.Our observations show that the DO concentrations decrease above the subsurface chlorophyll a maxima layer.Quantitative analysis shows that variations in DO concentration are dominated by horizontal advection and vertical advection terms in the subsurface layer.