Mesoscale eddies in the South China Sea: Mean properties, spatiotemporal variability, and impact on thermohaline structure

Abstract

We investigated mean properties and the spatiotemporal variability of eddies in the South China Sea (SCS) by analyzing more than 7000 eddies corresponding to 827 eddy tracks, identified using the winding angle method and 17 years of satellite altimetry data. Eddies are mainly generated in a northeast-southwest direction and southwest of Luzon Strait. There is no significant difference between the numbers of two types of eddies (anticyclonic and cyclonic) in most regions. The mean radius and lifetime of eddies are 132 km and 8.8 weeks, respectively, both depending on where the eddies are formed. Anticyclonic and cyclonic eddies tend to deform during their lifetimes in different ways. Furthermore, eddy propagation and evolution characteristics are examined. In the northern SCS, eddies mainly propagate southwestward along the continental slope with velocities of 5.0-9.0 cm s(-1), while in the central SCS, eddies tend to move with slight divergence but still in a quasi-westward direction with velocities of 2.0-6.4 cm s(-1). Eddy propagation in the western basin to the east of Vietnam is quite random, with no uniform propagate direction. Investigation of 38 long-lived eddies shows that eddies have a swift growing phase during the first 12 weeks and then a slow decaying phase that affects the eddy radii and eddy energy densities. Nevertheless, vorticity has less variability. In addition, the effect of eddies on the thermocline and halocline is analyzed using 763 Argo temperature profile data. Cyclonic eddies drive the thermocline shallower and thinner and significantly strengthen the thermocline intensity, whereas anticyclonic eddies cause the thermocline to deepen and thicken and weaken the thermocline intensity to a certain degree. The halocline impacted by cyclonic eddies is also shallower and thinner than that impacted by anticyclonic eddies. Finally, eddy temporal variations are examined at seasonal and interannual scales. Eddy activity is sensitive to the wind stress curl and in the northern SCS it is also related with the strength of the background flows.