Abstract
AbstractThe 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 midtropospheric 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 midtropospheric 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 Boreal Summer Intraseasonal Oscillation (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.
Highlights
The west coast of India receives more monsoonal rainfall than any part of monsoonal South Asia except Meghalaya, averaging about three times as much as the rest of the region (Krishnamurthy and Shukla, 2000; Rajeevan et al, 2006)
The dominant control on intraseasonal variability in coastal precipitation is found to be the Boreal Summer Intraseasonal Oscillation (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
One could reasonably assume that the band of high rainfall along the coast of west India is caused by the forced ascent of moist monsoonal westerlies as they impinge upon the Western Ghats (WGs)
Summary
Modelling studies that have removed the WGs have found that rainfall along the west coast does not disappear, but is reduced in intensity by about half (Wu et al, 1999; Xie et al, 2006; Sijikumar et al, 2013), and is less constrained in location—though still parallel to the coastline These studies suggest that the lower level monsoonal jet would be stronger in the absence of the WGs, hinting that the blocking might have far-reaching effects across the Arabian Sea. Xie et al (2006) noted that the WGs were the only mountain range in South Asia to retain their associated precipitation band after the orography was flattened, suggesting that, while mountains in the Tropics act as as seeds for convection, the unique location of the WGs plays an important role.
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