Influence of convectively coupled MRG waves on the north-east monsoon rainfall characteristics over south India.

Abstract

The northeast (NE) monsoon contributes considerable annual rainfall over southern peninsular India. Here, an attempt has been made to evaluate the role of convectively coupled mixed Rossby-Gravity (MRG) waves on the variabilities of NE monsoon rainfall. The gridded rainfall data from 1980 to 2021 from the India Meteorological Department shows large interannual variability in NE monsoon rainfall over southern peninsular India. Wavelet analysis of outgoing longwave radiation (OLR) data shows the dominance of short-period waves over the equatorial Indian Ocean during the above-normal NE monsoon rainfall years compared to below-normal rainfall years. The space-time spectral analysis further confirms the dominance of the short-period westward propagating convectively coupled equatorial waves (CCEW) during above-normal excess NE monsoon rainfall years. In contrast, the eastward component of CCEWs dominates the deficit rainfall years. Further, the empirical orthogonal function (EOF) analysis of OLR filtered for MRG wave shows that the first two leading modes explain more than 14% of variability over the Indian Ocean domain. Moreover, the vertically integrated moisture flux convergence indicates the role of MRG wave in triggering the extreme rainfall over southern peninsular India and also offer favourable condition for the formation of cyclogenesis and, thereby, inducing heavy rainfall over the South Indian domain. Thus, the results suggest that the westward propagating MRG waves play a seminal role in modulating the NE monsoon rainfall over southern peninsular India. Keywords: mixed Rossby-Gravity wave, north-east monsoon, extreme precipitation