Intraseasonal Ocean-Atmosphere-Land Interaction during Northern Summer

Mechanism responsible for the northward propagation of intraseasonal oscillation in South Asia during northern summer is explored by examining the relationship between energy fluxes, circulation and outgoing longwave radiation. Singular value decomposition and regression analyses were applied to both ECMWF and NCEP reanalyses for the period 1979-1993. Major findings are as follows. When major convection region is located in the tropical Indian Ocean, convection over the land of South Asia are relatively inactive and wind is calm. At the same time, an anomalous anticyclonic circulation is located over the land area when an anomalous cyclonic circulation is located over the tropical Indian Ocean. The circulation pattern prevents the transport of moisture from the Arabian Sea and the Indian Ocean into the land area. The land is heated by the net downward radiation and, in turn, destabilizes the lower troposphere through the release of sensible heat flux. Destabilization triggers the northward movement of the major convection region and anomalous cyclonic circulation into Indian subcontinent and Bay of Bengal. Increase of near-surface speed encourages evaporation in the Arabian Sea and Bay of Bengal and leads to the decrease of surface temperature. The process increases moisture supply to deep convection. Latent heat release in deep convection further enhances the large-scale circulation that in turn draws more latent heat flux from the ocean. Analyses based on both ECMWF and NCEP reanalyses yield similar results. In conclusion, this study identifies a clear signal of ocean-atmosphere-land interaction that can be used to explain the northward propagation of intraseasonal oscillation in South Asia.