[A-OS17] Ocean circulation and material cycle in coastal seas

[AOS17-09] Wind relaxation generates the counter-wind South China Sea Warm Current in winter

*Zhitao Yu¹, E. Joseph Metzger¹, Yalin Fan¹ (1.US Naval Research Laboratory)

Keywords:South China Sea warm current, wind relaxation, Ekman transport, Empirical Orthogonal Function, HYbrid Coordinate Ocean Model, reanalysis

We use the 22-year (1994-2015) HYbrid Coordinate Ocean Model (HYCOM) global reanalysis to study the generation mechanism of the counter-wind characteristic of the South China Sea Warm Current (SCSWC) in winter seasons. We perform an Extended Empirical Orthogonal Function (EEOF) analysis on the hourly Sea Surface Height (SSH) and wind stresses. The EEOF analysis reveals that the most dominant SSH mode over the continental shelf is an Ekman mode. When the wind relaxes, offshore Ekman transport anomaly generates a favorable cross-shore SSH slope for the northeastward surface currents over the continental shelf mainly by geostrophic balance. The alongshore pressure gradient force increases when the wind relaxes, and dominates the relaxed wind stresses to push the alongshore water northeastwards. This clearly demonstrates that the SCSWC is driven by wind relaxation. We also identify the region south of Taiwan Strait as the origin of the SCSWC, instead of the Hainan Island or Gulf of Tonkin, Vietnam due to the dominant Ekman dynamics.