Abstract
This study focuses on the interannual and intraseasonal variability of evaporative sources for rainfall events during the Indian monsoon. The monsoon is an important part of the economy and lifestyle in India, thus, any improvements in our understanding of its mechanisms would be directly beneficial to society. We first discuss the use of evaporative sources for rainfall events as an important tool to help increase our knowledge of the variations of the monsoon. We then outline the variability of the monsoon on an interannual (wet and dry years) and intraseasonal (active and break periods) time scale. We use three reanalyses (NCEP-R2, CFSR, and MERRA) and an IMD gridded rainfall dataset to trace the location and strength of evaporative sources via a quasi-isentropic back trajectory program. The program uses reanalysis winds and evaporation, among other parameters, to estimate these sources back in time. We discuss the differences in parameters between the datasets on a seasonal, interannual, and intraseasonal time scale. We then thoroughly investigate the strength and location of evaporative sources between datasets on interannual and intraseasonal time scales, and we attempt to explain the variations by analyzing the differences in the input parameters and circulation mechanisms themselves.
The study finds that the evaporative sources for given interannual or intraseasonal rainfall events do vary in strength and location. Interannually, the strongest change in evaporative source occurs over central India and the Arabian Sea, suggesting that the overall monsoon flow contributes moisture for Indian rainfall on this time scale. Intraseasonally, the strongest change in evaporative source occurs over the Bay of Bengal, suggesting that low pressure systems contribute moisture for Indian rainfall on this time scale. All three reanalyses yield similar fields of evaporative source. We conclude that accurate prediction of the Indian monsoon requires improved understanding of both interannual and intraseasonal oscillations since the sources of moisture for these events are unique.
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