Monsoon season local control on precipitation over warm tropical oceans

Abstract

Understanding local SST control on precipitation during monsoon is important for deducing climate change due to global warming, particularly for warm oceans. Studies of the relationship of the precipitation over tropical oceans with local sea surface temperature (SST), on the monthly scale, have shown that the propensity for precipitation is high for SST above a threshold of 27.5 $$^{\circ }$$ C/28 $$^{\circ }$$ C. However, for warm oceans with SST above the threshold such as the Bay of Bengal and South China Sea, for each SST, there is a large variation of precipitation and the SST–precipitation relationship is weak. On daily scale mean precipitation increases slightly with SST when precipitation lags SST by a few days, but the relationship between them is rather weak. But, when SST is above the threshold, for daily, pentad, 10-day and monthly time scales, and with or without time lag, the curve depicting the variation of mean precipitation with SST explains only a small fraction of precipitation variance and hence cannot be considered to be representative of the SST–precipitation relationship, or used to deduce the impact of SST on precipitation. On the other hand, the local control on precipitation is predominantly atmospheric dynamics with the relationship of variation of precipitation to low-level convergence on all timescales, being strong. This suggests that for warm oceans, the limiting resource for precipitation/convection is not SST but dynamics.