A diagnostic study of the Indian Ocean dipole mode in El Nino and non-El Nino years

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. Forty- year ECMWF Re- Analysis ( ERA- 40) data, Reynolds SST data, and ocean assimilation data from the Modular Ocean Model are used to form composites of the IODM that occur during El Nino ( 1972, 1982, and 1997) and non - El Nino ( 1961, 1967, and 1994) years. In El Nino years, two off- equatorial, anticyclonic circulations develop, associated with the increased pressure over the eastern Indian Ocean. The anticyclonic circulation over the Northern Hemisphere enhances the easterly component of the winds in the northwestern Indian Ocean. This enhanced easterly component increases the mixed layer temperature by inducing an anomalous westward ocean current that advects the warm mean mixed layer from the central to the western Indian Ocean. Meanwhile, the anticyclonic circulation over the southeastern Indian Ocean strengthens southeasterlies, thereby causing oceanic meridional and vertical advection of the cold mean temperature. Consequently, the IODM in El Nino years is characterized by the warming in the northwestern and the cooling in the southeastern Indian Ocean. In non - El Nino years, a monsoonlike wind flow increases the westerly and southeasterly components of the wind over the northwestern and southeastern Indian Ocean, respectively. Oceanic currents induced by these winds result in anomalous cold advection in both of these regions. In addition, the monsoonlike wind flow over the southeastern Indian Ocean enhances the anomalous latent and sensible heat fluxes in non - El Nino years. Hence, the cooling of the eastern tropical Indian Ocean, rather than the warming of the western Indian Ocean, becomes the major feature of the IODM during non - El Nino years.