Statistical evidence on distinct impacts of short- and long-time fluctuations of Indian Ocean surface wind fields on Indian summer monsoon rainfall during 1991-2014

This observational study mainly examines the impacts of short- and long-time fluctuations of surface wind fields over the Arabian Sea (AS), the Bay of Bengal (BoB), and the southern Indian Ocean (SIO) on Indian Summer Monsoon Rainfall (ISMR), with special reference to strong and weak Indian summer monsoons (ISM). Two datasets over 1991-2014 are used: (1) the daily gridded rainfall produced by India Meteorological Department (IMD), and (2) the Cross-Calibrated Multi-Platform (CCMP) wind product version 2.0 created by Remote Sensing Systems. Monthly mean surface wind speed, convergence, and curl in the AS, BoB, and SIO are overall not significantly different between strong and weak ISMRs except for wind speed in the AS in September. However, the probability density function (PDF) distribution of daily values over the AS, BoB, and SIO during strong ISMRs is different from during weak ISMs, suggesting that sub-monthly surface wind characteristics could be useful in diagnosing rainfall characteristics. Except for rainfall in the northeast part of India, Indian regional rainfalls are closely linked with surface wind speeds over the AS, and wind convergence and curl over the BoB on short timescales of up to 1 week. The daily area-averaged wind convergence over the BoB is better correlated with regional rainfall during strong ISMs than during weak ISMRs. Multiple linear regression analysis shows that the fluctuations of monthly wind fields in the AS and BoB can affect monthly rainfall in some regions but are not related to a significant change in rainfall over the whole India. It is the short-time fluctuations of wind speed over the AS as well as wind convergence and curl over the BoB rather than their long (monthly) timescale fluctuations that are related to the strength of ISMR. Surface winds over the SIO on weather timescales have little influence on ISMR.