The Abnormal Indian Summer Monsoon of 2000

Abstract

Diagnostic analysis of observations and a series of ensemble simulations using an atmospheric general circulation model (GCM) have been carried out with a view to understanding the processes responsible for the widespread suppression of the seasonal summer monsoon rainfall over the Indian subcontinent in 2000. During this period, the equatorial and southern tropical Indian Ocean (EQSIO) was characterized by persistent warmer than normal sea surface temperature (SST), increased atmospheric moisture convergence, and enhanced precipitation. These abnormal conditions not only offered an ideal prototype of the regional convective anomalies over the subcontinent and Indian Ocean, but also provided a basis for investigating the causes for the intensification and maintenance of the seasonal anomaly patterns. The findings of this study reveal that the strengthening of the convective activity over the region of the southern equatorial trough played a key role in inducing anomalous subsidence over the subcontinent and thereby weakened the monsoon Hadley cell. The leading empirical orthogonal function (EOF) of the intraseasonal variability of observed rainfall was characterized by a north?south asymmetric pattern of negative anomaly over India and positive anomaly over the region of the EQSIO and accounted for about 21% of the total rainfall variance during 2000. The GCM-simulated response to forcing by SST anomalies during 2000 is found to be consistent with observations in reasonably capturing the seasonal monsoon anomalies and the intraseasonal variability. Further, it is shown from the GCM experiments that the warm Indian Ocean (IO) SST anomalies influenced the regional intraseasonal variability in a significant manner by favoring higher probability of occurrence of enhanced rainfall activity over the EQSIO region and, in turn, led to higher probability of occurrence of dry spells and prolonged break-monsoon conditions over the subcontinent. In particular, the simulated break-monsoon anomaly pattern of decreased rainfall over the subcontinent and increased rainfall over the EQSIO is shown to intensify and persist in response to the IO SST anomalies during 2000. These results clearly bring out the significance of the IO SST anomalies in altering the regional intraseasonal variability and thereby affecting the seasonal mean monsoon. Further studies will be required in order to investigate the detailed physical mechanisms that couple the variability of convection over the IO region with the local SST boundary forcing and the large-scale monsoon dynamics.