The South Asian monsoon circulation sustains the lives of over one fifth of the world’s human population whose water supply is almost entirely dependent on the seasonal summer monsoon during June to September (Fig 1a). While on an average the seasonal monsoon rains contribute nearly 75–80 per cent of the annual precipitation in the region, their year-to-year variations have important implications. How the regional monsoon hydrological cycle might respond to global climate change thus becomes an issue of major concern.
Researchers point out that greenhouse warming is likely to intensify the monsoon precipitation over South Asia due to increased atmospheric moisture content and enhanced surface and tropospheric warming. However, assessments of future changes remain ambiguous due to wide variations among the climate model projections. Historical records over India during the twentieth century indicate absence of any significant long-term trend in the summer monsoon rainfall for the country as a whole, although specific areas of the subcontinent have been experiencing significant changes in the seasonal monsoon rainfall trends during the last century (S K Dash et. al., 2009 ‘Changes in the characteristics of rain events in India’, Journal of Geophysical Research; P Guhathakurha et. al., 2010, ‘Changes in extreme rainfall events and flood risk in India during last century’, National Climate Centre (NCC) Research Report, IMD).
The reasons for these regional differences in the rainfall trends are not yet fully clear. The fact is that the south Asian monsoon circulation is basically a large-scale convectively coupled phenomenon involving dynamical linkages between the monsoon circulation and precipitation (Fig 1b and c). Quite a few recent studies have alluded to a possible weakening of the large-scale monsoon circulation in the last few decades. Accordingly, it has been observed that the recent three decades were accompanied by significantly higher probability of occurrence of break-monsoon regime relative to the active-monsoon regime. The weakening of the large-scale summer monsoon circulation is also reflected in the activity of monsoon synoptic disturbances (i.e. monsoon depressions) which have significantly decreased in frequency during the last few decades (Fig 1d). The Western Ghats are one of the wettest regions in the world as the total summer monsoon seasonal precipitation over this region exceeds 2500 mm. This rainfall is largely associated with small and medium-sized convective systems which are present throughout the day, as a consequence of the orographic response to the monsoon southwesterly flow.
Drawn from a detailed analysis of observed datasets, (R Krishnan et. al. 2012, ‘Monsoon circulation interaction with western ghats orography under changing climate: Projection by a 20 km mesh AGCM’, Theoretical and Applied Climatology), a significant decrease in the frequency of moderate-to-heavy rainfall events over the Western Ghats during recent decades has been reported recently. The authors suggest that the decreasing orographic rains over the Western Ghats and weakening of the monsoonal winds during recent decades is related to increased stability of summer monsoon overturning circulations. By conducting numerical simulation experiments using very high resolution global climate model, the authors have suggested that the effects of global warming, due to increased greenhouse gas concentration, can induce a weakening of the summer monsoon circulation and reduction of orographic precipitation over the Western Ghats.