Submesoscale Coherent Vortices Observed in the Northeastern South China Sea

Abstract

AbstractAlthough submesoscale coherent vortices (SCVs) have been observed in different parts of the world's oceans, most of them were captured by accident by a limited number of hydrographic profiles. Here, using concurrent velocity and temperature/salinity measurements from a submesoscale‐resolving mooring array (2 km), two oppositely rotating SCVs are for the first time reported in detail in the northeastern South China Sea (NESCS). For the anticyclonic (cyclonic) SCV, its core lays at 210 m (180 m) with a mean Rossby number of −0.76 (0.65); its maximum swirl velocity, radius, vertical scale, and Burger number are estimated to be 0.43 m s−1, 31 ± 5 km, 235 m, and 1.5 (0.23 m s−1, 16 ± 3 km, 100 m, and 2.5), respectively. Corresponding to the SCVs' submesoscale nature, their momentum is governed by gradient‐wind rather than geostrophic balance. Consequently, kinetic energy of the anticyclonic (cyclonic) SCV would be on average underestimated (overestimated) by 47% (68%) if the traditional geostrophic relation was used to diagnose the velocity. Further analysis shows that thermohaline properties within the anticyclonic and cyclonic SCVs are close to the Kuroshio water and the local NESCS water, respectively. By combing water mass tracing and high‐resolution simulations, we suggested that the anticyclonic (cyclonic) SCV was generated in the southern Luzon Strait (southwest of Taiwan) through current‐topography interaction. We further proposed that the observed SCVs here should be a common phenomenon in the NESCS, which may provide a novel route for the tracer exchange between the NESCS and the western Pacific.