A mechanism for observed interannual variabilities over the equatorial Indian ocean

A mechanism is proposed that can give rise to a sustained interannual variability of sea surface temperature (SST) and other variables even over an ocean basin of small zonal extent such as the Indian Ocean, as observed. A simple one-dimensional ocean-atmosphere coupled climate model with ''minimal'' dynamics is considered to isolate and emphasize the basic physical mechanism. In particular, unlike in the conventional scenario of the El Nino-Southern Oscillation, the equatorial Rossby waves are not included in the dynamics of the model. However, although the dynamics is minimal, the model as a whole is nonlinear due to the nonlinear parameterization of the atmospheric heating. It is shown that even with this minimal dynamics the model exhibits interannual variabilities for appropriate air-sea interaction. Further, in the one-dimensional case a basic factor that suppresses variability over the smaller basins is the zonal extent of the ocean basin. For an ocean-covered globe and for two basins of equal size separated by a land strip, the model shows interannual variability of about 5-yr period over both the eastern and the western sector (or basins). For two unequal ocean basins, with zonal extents representative of the Indian Ocean and the Pacific Ocean, only the larger basin shows sustained interannual oscillations. However, the observed interannual viability over the smaller (Indian Ocean) basin can be recovered if evaporation-wind feedback is included in the modeling of air-sea interaction. The phase relationship of the two oscillations, on the other hand, is affected by both basin geometry and moist feedback. Thus, it is proposed that both basin geometry and ocean-atmosphere coupling in the presence of moist feedbacks are essential to account for the observed structure and distribution of interannual variabilities in the Tropics.