The changing influences of ENSO and the Pacific meridional mode on mesoscale eddies in the South China Sea

Abstract

This study finds that the correlation between El Niño-Southern Oscillation (ENSO) and the activity of mesoscale oceanic eddies in the South China Sea (SCS) changed around 2004. The mesoscale eddy number determined from satellite altimetry observations using a geometry of the velocity vector method was significantly and negatively correlated with the Niño-3.4 index before 2004, but the correlation weakened and became insignificant afterward. Further analyses reveal that the ENSO-eddy relation is controlled by two major wind stress forcing mechanisms: one directly related to ENSO and the other indirectly related to ENSO through its subtropical precursor-the Pacific meridional modes (PMMs). Both mechanisms induce wind stress curl variations over the SCS that link ENSO to SCS eddy activities. While the direct ENSO mechanism always induces a negative ENSO-eddy correlation through the Walker circulation, the indirect mechanism is dominated by the northern PMM (nPMM), resulting in a negative ENSO-eddy correlation before 2004, and by the southern PMM (sPMM) after 2004, resulting in a positive ENSO-eddy correlation. As a result, the direct and indirect mechanisms enhance each other to produce a significant ENSO-eddy relation before 2004, but they cancel each other out, resulting in a weak ENSO-eddy relation afterward. The relative strengths of the northern and southernPMMs are the key to determining the ENSO-eddy relation and may be related to a phase change of the interdecadal Pacific oscillation.