A Diagnostic Study of the Indian Ocean Dipole Mode in El Nino and Non- El Nino Years

Abstract

The Indian Ocean Dipole Mode (IODM) is examined by comparing the characteristics of oceanic and atmospheric circulations, heat budgets, and possible mechanisms of IODM between El Nino and non-El Nino years. ERA-40 reanalysis data, Reynold SST, and ocean analysis from Modular Ocean Model with the assimilation of the temperature profile from World Ocean Dataset 1998 are used to form three-year composites of IODM during El Nino (72, 82, 97) and non-El Nino (61, 67, 94) years. In El Nino years, two off-equatorial, anti-cyclonic circulations develop as a Rossby-wave response to the increased pressure over the Indian Ocean. The resultant winds from easterlies to northeasterlies (from southerlies to southeasterlies) in the northwestern (southeastern) tropical Indian Ocean warms (cools) the mixed layer temperature by inducing an anomalous zonal (meridional and vertical) component in the ocean current that advects the basic-state mixed layer temperature. In non-El Nino years, a monsoon-like flow induces winds from westerlies to southwesterlies (from southerlies to southeasterlies) in the northwestern (southeastern) Indian Ocean. As a result, the cold advection by the anomalous eastward current (northward current) in the northwestern (southeastern) tropical Indian Ocean becomes dominant in non-El Nino years. In addition, the anomalous winds in these regions are the same sign as the climatological monthly mean winds. Hence the anomalous latent and sensible heat fluxes further contribute to the decrease of SST in the northwestern and the southeastern Indian Ocean. Consequently, the cooling of the eastern tropical Indian Ocean rather than the warming of western tropical Indian Ocean becomes the major feature of the IODM during non-El Nino years.