资源环境科技发展态势分析平台

The present study examines the variability of Northeast Monsoon (NEM) rainfall over the southern Peninsular India (SPI) associated with multiyear-La Nina events using observations and Coupled Model Inter-comparison Project Phase 5 (CMIP5) models. The analysis is carried out for NEM season (October-December) of two successive La Nina years, referred to as the first and second year during the period 1900-2010. Observations show that despite noticeable weakening in the equatorial Pacific cooling from the first year to the second year, strong La Nina teleconnections and the rainfall deficiency over the SPI region remains the same in most of the multiyear-La Nina events (70%). In the first year, zonally protracted low level cyclonic circulation over the north Indian Ocean (NIO) region induces northwesterly wind anomalies over SPI region. Negative moisture anomalies corroborated by northwesterlies and subsidence cause deficient rainfall over this region. On the other hand, the western flank of NIO cyclonic circulation is pushed towards Indian longitudes in the second year, inducing dry northerlies over SPI region. Development of low level divergent flow over the western tropical Indian Ocean and eastward migration of convergence zone from Bay of Bengal are found to be responsible for the shift in circulation pattern. These anomalous circulations together with low moisture content are responsible for deficit NEM rainfall in the second year. Differences in circulation patterns noted in the first and second year are associated with changes in the La Nina related SST patterns over the equatorial Pacific and modulation of the upper/lower level convergence/divergence zones. Analysis reveals that most of the CMIP5 models have tendency to simulate excess rainfall over the SPI region (in 70-100% of multiyear-La Nina events) unlike in the observations (30% of events). The possible reasons for the simulation of such positive rainfall anomalies are explored in the present study.