A sub-seasonal oscillation of sea surface temperature in the Southern Indian Ocean during DJF and its excitation mechanism

Abstract Based on the ERA-Interim reanalysis data, a sub-seasonal oscillation of the sea surface temperature (SST) and its characteristics are revealed in the Southern Indian Ocean from December to February (DJF). The results of Empirical Orthogonal Function (EOF) analysis show that the first two dominant modes of SST in the Southern Indian Ocean are the spatial distribution of Southwest-Northeast direction dipole (SIOD) and triple (SIOT) respectively. The research shows that the intensity and zonal oscillation of the Mascarene High are conducive to the formation of SIOD (SIOT). The air-sea interaction during the formation process is composed of three stages. In the first stage the atmospheric forces the ocean to result in the Mascarene High westward (eastward) and enhances the abnormal anticyclone in the Southern Indian Ocean. There are abnormal northerly (Southerly) flows on the west (east) sides of the abnormal anticyclone respectively, which weakens (enhances) the Southeast trade wind in the climatology. The release of surface latent heat flux decreases (increases) and SST is warmed (cooled). During the following stage, the ocean feedback the atmosphere. The warm SST continues to increase, resulting in low-level convective is enhanced, which weakens the abnormal anticyclone. The third stage is again the atmosphere forcing the ocean. The abnormal anticyclone gradually turns into an abnormal cyclone and the meridional wind direction is reversed. The release of the latent heat flux increases (decreases) significantly which leads to the cooling (warming) of SST on the west (east) sides of an abnormal cyclone. In addition, the formation and extinction of SIOT easier are affected by the Southern Hemisphere annular mode (SAM) than SIOD. The abnormal zonal wave train with wavenumber 4 (3) propagates the Southern Indian Ocean by the westerly jet waveguide and results in an SST anomaly of SIOD (SIOT), accompanied by an obvious sub-seasonal meridional variation of the precipitation in Southern Africa.