Recent drought events over the central Indian region: Pacific Ocean origin and insights from moisture budgets

Analysis of rainfall variation over the central Indian (CI) region, using a high resolution rainfall gridded data set over the Indian longitudes, shows that the recent decade (1998-2009) has large number of drought events, compared with the earlier two decades (1979-1997). Improved understanding of the underlying mechanism responsible for these summer monsoon droughts is important because of their profound socio-economic impact over this region. Our analysis here reveals that most of the recent droughts are associated with extended break events (breaks lasting more than 7 d) and that the process responsible for it may also be the mechanism responsible for these droughts. Sub-seasonal diagnostics, focusing on such drought-producing break monsoon events, show prominent structures resembling the central Pacific (CP) El Niño warming over central equatorial Pacific Ocean. We then performed sub-seasonal moisture budget diagnostics using newly available European Centre for Medium Range Weather Forecasts (ECMWF) interim reanalysis. The primary aim is to identify the various moist processes through which the break monsoon conditions are initiated and maintained over CI, thus leading to large-scale drought conditions. The budget diagnostic shows that dry advection is the principal moisture process to initiate drought-producing break conditions over CI. The primary processes leading to dry advection are as follows: the enhanced rainfall over western north Pacific (WNP) induces cyclonic circulation anomalies to its northwest as a Rossby wave response, and the northerlies at the poleward of this circulation advect dry air of low moisture content from continental subtropics to south Asia. The present result also indicates that the enhanced radiative cooling over CI plays an important role in maintaining such break conditions. A direct implication of our research is that observational efforts are essential to monitor the three-dimensional moisture distribution over the south Asian monsoon region, for a better understanding of these drought-producing break monsoon events.