El Nino and interannual variation of the sea surface temperature in the tropical Indian Ocean

By using observational data, we studied the relationship of the interannual variation of sea surface temperature anomalies (SSTA) between tropical Indian and Pacific Oceans. The empirical orthogonal function (EOF) analyses of SSTA showed that the dominant modes in tropical Indian Ocean are the unipole mode with same SSTA sign in basin-scale and the dipole mode with a reversal in sign across the basin, respectively. The dominant mode in tropical Pacific Ocean is the El Nino mode. All unipole, dipole and El Nino modes display noticeable interannual variations. The wavelet squared coherency analyses showed that in the interannual time-scale, the SSTA in NINO3 region (150degreesW-90degreesW, 5degreesS-5degreesN) in tropical eastern Pacific is closely related with the unipole and dipole modes in the tropical Indian Ocean. It is found that the El Nino lags the dipole mode and precedes the unipole mode. The El Nino from its developing phase to the decaying phase corresponds to SSTA in the tropical Indian Ocean varying from the dipole mode to the unipole mode. The physical connection of El Nino with dipole and unipole modes is also discussed. It is found that variations of the zonal wind anomalies over both tropical Indian and Pacific Oceans are affected greatly by the convection over the area in the eastern tropical Indian Ocean and western tropical Pacific. The variation of the convection accompanied with the evolution of El Nino alters the zonal winds and hence associated oceanic processes, which are responsible for SSTA in tropical Indian Ocean changing from the dipole to unipole mode.