Contrasting trends in southwest monsoon rainfall over the Western Ghats region of India

Abstract

This study presents the contrasting trends of rainfall in the northern and southern Western Ghats (WG) and examines possible reasons for the phenomenon. The WG is one of the important mountain ranges that run parallel to the west coast of India. The mountain chain lies almost perpendicular to the low level jet stream (LLJ) and hence, receives about three times the average rainfall in India. The onset of southwest monsoon also occurs at this region, and thus WG plays a key role in regulating Indian climate through regional climate modulations. Therefore, detecting changes in the rainfall in WG is necessary to identify changes in regional climate. Here, we examine the features of rainfall received and reasons for the observed rainfall patterns in the northern and southern WG. In general, the rainfall peaks are observed in low elevated areas with high inter-annual variability. We find an increase (decrease) southwest monsoon rainfall of about 1.6 mm day−1 decade−1 in certain pockets of the northern (southern) WG. However, an average trend of + 0.3 (− 0.39) mm day−1 decade−1 is estimated in the northern (southern) WG for the 1931–2015 period. Our analyses reveal that this contrasting trend in rainfall (i.e. positive in the north and negative in the south WG) is due to the northward movement of LLJ; from 10°N to 15°N. This shift in LLJ is triggered by an abnormal increase in the surface temperature of the northern Arabian Sea and tropospheric temperature of the north India in the recent decades. The warming helped the LLJ core to move northwards and that weakened (strengthened) the westerly winds over the southern (northern) WG to significantly change the pattern of southwest monsoon rainfall. Henceforth, this study cautions the changes in the rainfall pattern over WG, which can have significant long-term implications for regional climate change.