Evolution of convection anomalies over the Indo-Pacific region in relation to Indian monsoon rainfall

Abstract

The present study is an attempt to explore a relationship between Indian Summer Monsoon Rainfall (ISMR) and the evolution of convective activity during winter to pre-monsoon seasons over the Indo-Pacific region. The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study from January 1975 to May 2004.  It is observed that the negative OLR (thus, convection) anomalies from January gradually strengthen in a west northwest direction from western Pacific region and got established over the southeast Asian region and adjoining eastern equatorial Indian Ocean by the month of May during excess ISMR years indicating a gradual reversal of anomalous rising motion over western Pacific in January to anomalous sinking motion in May over western Pacific region. However, during the deficient ISMR years the negative OLR anomalies established over the western Pacific region in January almost remain active over the same region till the month of May and consequently there is persistent presence of rising motion from northern winter to pre-monsoon seasons over the western Pacific.

Similar to the patterns during the deficient composite the evolution of OLR anomalies show stronger than normal convective activity over the western Pacific region east of 150° E compared to that of eastern Indian Ocean and adjoining southeast Asia from February to May during the deficient years of 1979, 1982, 1987, 2002, and 2004 though there exists strong interannual variability in its magnitude. The tendency of OLR anomalies over the western Pacific region from January to March and the difference of OLR anomalies between western equatorial Pacific and eastern Indian Ocean and adjoining southeast Asia regions in the month of May shows significant (95% level) correlation with ISMR and the predicted ISMR with these two predictors using simple regression model shows significant correlations (95% and 99% level respectively) with actual ISMR for the extreme years.