Northeast monsoon rainfall variability over south peninsular India vis-à-vis the Indian Ocean dipole mode

摘要

Whereas the June through to September southwest summer monsoon season over the Indian subcontinent has been the main focus of research, the October through to December northeast monsoon season over south peninsular India has received less attention. Hence, this study focuses on the northeast monsoon rainfall (NEMR) variability using historical data for a 131 year period (1871-2001) and its relationship with the neighbouring Indian Ocean sea-surface temperature (SST). Based on the observed available rainfall data, a time series of NEMR over the coherent south peninsular India has been developed. Similarly, an index to quantify the Indian Ocean dipole mode (IODM), exhibiting warm (cool) waters over the equatorial western (southeastern) Indian Ocean, has been developed using GISST data for the same period. The dynamics of the NEMR-IODM relationship are examined with the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data. Interannual and decadal variabilities in NEMR reveal alternate epochs of above- and below-normal rainfall. The epochs tend to last for about a decade or two. Rainfall epochs near the equatorial Indian Ocean tend to last for about a decade. No long-term trends in NEMR are noted. On the other hand, the decadal variability in the IODM reveals a dominance of the negative phase during the earlier decades (1880-1920) and a positive phase during recent decades (1960-2000), with suppressed activity in between. Long-term variability in the IODM clearly reveals an increasing trend, which could be related to greenhouse warming. Further, the NEMR variability is enhanced during the decades when the IODM exhibits its active phase, and is suppressed during the decades when the IODM is inactive. The NEMR and the IODM are directly related, suggesting that the positive (negative) phase enhances (suppresses) the northeast monsoon activity. During the positive phase, the anomalous flow pattern shows winds converging and suggesting moisture transport from the southeast Indian Ocean and the Bay of Bengal towards south peninsular India. In contrast, the negative phase reveals winds diverging and transporting moisture away from the south Indian region. The anomalous SST and circulation features associated with the positive IOD/excess rainfall and negative IOD/deficient rainfall are consistent and clearly bring out the coupled ocean-atmosphere dynamics over the Indian Ocean. These results show the direct influence of the IODM phenomenon on the interannual and decadal NEMR variability over south India.