Statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island

Abstract

This study examined the statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island. In total, 315 eddies (138 anticyclonic and 177 cyclonic eddies) were detected from 19.5 years of satellite altimeter sea-level data, with more than 95% of these eddies being generated in the ocean west of the Mariana Islands. Eddy trajectory statistics indicated that eddies frequently intrude into the Kuroshio regime at two latitude bands, namely 18 degrees N-19 degrees N and 22 degrees N-23 degrees N, with periods of 146 +/- 62 and 165 +/- 46 days, respectively. The interaction time is longer within the two active bands (33 +/- 10 days at 18 degrees N-19 degrees N and 45 +/- 17 days at 22 degrees N-23 degrees N) than at other latitudes. These two eddy-intrusion bands are associated with the northern and southern Subtropical Countercurrents (STCCs). These STCCs have a vertically reversed sign of the meridional potential vorticity gradient, thus providing a key energy source for eddy generation. In addition, when westward-propagating eddies approach the Ryukyu Islands, the southwestward recirculation flow east of the island chain as well as topographic effects cause some eddies to head southwestward to the east of Taiwan and intrude into the Kuroshio at 22 degrees N-23 degrees N, rather than to dissipate directly. Therefore, we suggest that the STCCs play a key role in inducing the eddies to frequently intrude into the Kuroshio at 18 degrees N-19 degrees N and 22 degrees N-23 degrees N. In addition, the Ryukyu Islands are responsible for concentrating the eddies within 22 degrees N-23 degrees N.