Role of the Arabian Sea Warm Pool on the Precipitation Characteristics during the Monsoon Onset Period

Regional distribution of precipitation during the onset phase of the Indian summer monsoon (15 May-15 June) shows distinct patterns in the years 2009 and 2010, with the latter having considerably more precipitation over the southeast Arabian Sea (AS) and the west coast of peninsular India. During these years, the location and regional extent of the warm pool in the AS are also distinct. In 2009, the warm pool core is located in the equatorial region, whereas in 2010 it spreads to a wide region of the AS. Sensitivity experiments with different SST forcings have been carried out using the Weather Research and Forecasting (WRF) model to understand the influence of the AS warm pool on the monsoon onset precipitation characteristics. Simulations with actual SSTs in the AS and climatological SSTs elsewhere are able to reproduce the distinct behavior of the monsoon onset precipitation observed during 2009 and 2010. These simulations show suppressed convection over the central and northern AS in 2009, while warmer SSTs in the AS favor enhanced convection during 2010 combined with a sharp contrast in the moisture transport. The strong intrusion of drier air from the north AS effectively confines the moist air mass from the south, causing a net transport of moisture toward the southwest coast of peninsular India and leads to positive anomalies in precipitation over the region in 2010. However, during 2009, the drier air from the north mixes rather easily over the AS, which suppress the convection.