A Dipole-like Warming Trend Of Indian Ocean SST And Its Formation Mechanisms

Many studies have demonstrated the important roles of Indian Ocean in global climate system. Observational data reveal that the Indian ocean displays the most significant warming trend over the global oceans. In this paper, the spatial pattern of Indian Ocean warming trend and its formation mechanism are studied using observations, IPCC model output and a series of sensitivity experiments with the Fast Ocean-Atmosphere Model (FOAM). Observations reveal that the warming trend of Indian Ocean in twentieth century resembles the positive Dipole Zonal Mode (pDZM),characterized by stronger(weaker) warming trend in western(Southeastern) equatorial Indian Ocean. IPCC-AR4 models simulation show the positive dipole-like warming trend will persist in the twenty-first century. Previous studies suggested that the warming pattern is generated by the weakening of the Walker Circulation due to hydrology circulation variability in global warming.Base on in-house climate model FOAM, we conduct a series of sensitivity experiments in warm climate and find a new mechanism: the pDZM-like warming trend predominantly originates from an intensification of land-sea thermal contrast between Indian Ocean and adjacent Asian landmass in global warming, which induces an anticyclone wind anomaly in northern Indian Ocean and then triggers the equatorial easterly wind anomaly. The warming trend is further substantiated by local coupled ocean-atmosphere feedback, and regulates the response of the tropical atmospheric overturning circulation to global warming. The weakening of Walker Circulation is prominently generated by the tropical air-sea interaction, but not directly induced by the hydrological circulation in global warming.The response of Asian Summer Monsoon (ASM) to global warming is further studied by using both IPCC-AR4 models outputs and FOAM model simulation. There is a paradox of the ASM precipitation and circulation in global warming as described in previous modeling studies. Several sensitivity experiments with FOAM are conducted, in which SST warming anomalies are varied systematically. The results explicitly demonstrate that the ASM rainfall-circulation paradox in warm climate is primarily associated with the Indian Ocean warming, which enhances the middle to upper troposphere warming (thus reduce meridional thermal gradient) responsible for the weakness of the ASM circulation and intensify the moisture transport from warm tropical Indian Ocean to Asian land through the summer monsoon wind following the increase of the ASM rainfall. The further modeling studies suggest that the Pacific Ocean warming tends to suppress the paradox changes of ASM rainfall-circulation in warm climate.