Counter-rotating eddy pair in the Luzon Strait

Abstract

Based on satellite remote-sensing observation data and Hybrid Coordinate Ocean Model (HYCOM) re-analysis data, we studied the counter-rotating eddy pair in the Luzon Strait (LS). Statistical analysis reveals that when an anti-cyclonic mesoscale eddy (AE) (cyclonic mesoscale eddy [CE]) in the Northwest Pacific (NWP) gradually approaches the east side of the LS, a CE (an AE) gradually forms on the west side of the LS, and it is defined as the AE (CE) mode of the counter-rotating eddy pair in the LS. The counter-rotating eddy pair exhibits obvious seasonal variation: the AE mode mainly occurs in the summer half of the year, while the CE mode mainly occurs in the winter half of the year. The mean durations of the AE and CE modes are both approximately 70 d. Based on the vorticity budget equation and energy analysis, the dynamic mechanism of counter-rotating eddy-pair occurrence is determined to be as follows: the AE (CE) on the east side of the LS causes a positive (negative) vorticity anomaly through horizontal velocity shear on the west side of the LS, and the positive (negative) vorticity anomaly is transported westward by the zonal advection of the vorticity, finally leading to the formation of the CE (AE) on the west side of the LS.