Basin-wide warming of the Indian Ocean during El Nino and Indian Ocean dipole years

Basin-wide wintertime surface warming is observed in the Indian Ocean during El Ni (n) over tildeo years. The basin-wide warming is found to be stronger when El Ni (n) over tildeo and Indian Ocean Dipole (IOD) co-occur. The mechanisms responsible for the basin-wide warming are different for the years with El Ni (n) over tildeo only (El Ni (n) over tildeo without IOD) and for the co-occurrence (both El Ni (n) over tildeo and IOD) years. Strong westward propagation of downwelling Rossby waves is observed in the southern Indian Ocean during the IOD years. Such strong propagation is not seen in the case of the El Ni (n) over tildeo-only years. This indicates that the ocean dynamics play an important role in winter warming of the western Indian Ocean during the IOD years. The weak easterly wind anomalies in the El Ni (n) over tildeo-only years show no measurable impact on the Wyrtki Jets, but weakening or reversal of these jets is seen in the IOD years. This strongly suggests that the variability related to surface circulation is due to the local IOD forcing rather than El Ni (n) over tildeo induced wind anomaly. For the El Ni (n) over tildeo-only composites, surface heat fluxes (mainly latent heat flux and short wave radiation) play an important role in maintaining the basin-wide surface warming in the Indian Ocean. In the IOD-only composites (when there is no El Ni (n) over tildeo in the Pacific), such basin-wide warming is not seen because of the absence of ENSO (El Ni (n) over tildeo and Southern Oscillation) induced subsidence over the eastern Indian Ocean. For the years in which both El Ni (n) over tildeo in the Pacific and dipole in the Indian Ocean co-occur, warming in the western Indian Ocean is due to the ocean dynamics and that in the eastern Indian Ocean is due to the anomalous latent heat flux and solar radiation. Copyright (C) 2007 Royal Meteorological Society.