The 3-D evolutions of a deep-reaching anticyclonic eddy in the northeastern South China Sea

Surface-generated mesoscale eddies were observed to significantly contribute the lateral deep-sea sediment transport. However, a three dimensional structure of such eddies was not recognized and thus hindering our understanding of the dynamical capacity. However, the evolutionary features of the eddy along their track have not been investigated through the in-situ full-water-column observations. We deployed an integrated mooring system at 2100 m water depth in the northeastern South China Sea for one year, offering an opportunity to capture a deep-reaching anticyclonic eddy, which originated in the southwest of Taiwan and propagated southwestward along the isobaths from the end of November 2011 to March 2012. The observed first baroclinic mode feature of current velocity with enhanced magnitude and direction reversal exhibited a 7-day time lag between 600 m and 1700 m. Correspondingly, temperature increased in the upper layer when the eddy passed, and decreased pronouncedly in deeper layers 14 days later. Combined with Argo measurements, HYCOM model data, and the altimetry data, we found that the average radius of an anticyclonic eddy can reach 120 km with an amplitude as high as 29 cm. Such an eddy could propagate southwestward at a speed of 9.3 cm/s. Those parameters increased remarkably in the early stage with a slower translating speed, and decreased gradually as the eddy decayed. High nonlinearity over a value of 5 was kept during most of its lifetime, which may indicate sediment trapped and transported along its trajectory. It also found that the separation depth for this first mode baroclinic eddy was between 400 m and 700 m, depending on the strength of this eddy. Positive (negative) temperature anomalies occurred above (below) this depth. Horizontal velocity shear variance formed distinct circles above the transferring depth and maxima were in the peripheries, while below it clockwise flows were weaker with a northeastward incline. These findings are significant for a better understanding the mechanism for the vertical structure of mesoscale eddies and how the surface-generated eddies influence the deep circulation variations.