Abstract/Summary

The west coast of India, dominated by the Western Ghats mountain range, is among the rainiest places in the tropics. The interaction between the land-sea contrast of the coast, the monsoonal westerlies, and the oblique mountains is subject to complex intraseasonal variability, which has not previously been explored in depth. This study investigates that variability from the perspective of the land-sea contrast, using empirical orthogonal function analysis to discern regimes of onshore and offshore rainfall over southwest India and the eastern Indian Ocean. Locally, it is found that the rainfall is most sensitive to mid-tropospheric humidity: when this is anomalously high, deep convection is encouraged; when this is anomalously low, it is suppressed. A moisture tracking algorithm is employed to determine the primary sources of the anomalously wet and dry mid-tropospheric air. There are important secondary contributions from low-level vorticity and cross-shore moisture flux. The dominant control on intraseasonal variability in coastal precipitation is found to be the BSISO: over 75% of the strongest offshore events occur during phases 3 and 4; and about 40% of the strongest onshore events occur during phases 5 and 6. The location of monsoon low-pressure systems is also shown to be important in determining the magnitude and location of coastal rainfall.