The Delayed Effect of Major El Nino Events on Indian Monsoon Rainfall

Abstract

Previous studies have shown that boreal summer Indian monsoon rainfall is, on average, significantly above normal after major El Nino events. In this study, the underlying causes of this rainfall response are examined using both observational analysis and atmospheric general circulation model (AGCM) simulations. Moist static energy budgets for two strong El Nino events (1982/83 and 1997/98), estimated from monthly 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), suggest that stronger low-level moisture transport and reduced moist stability associated with a warmer north Indian Ocean (NIO) can increase monsoon rainfall, despite a weakened monsoon circulation. The trade-off between a dynamically weaker monsoon and moist processes favoring enhanced monsoonal rainfall is broken during the late monsoon season (August-September) as the warm NIO enhances surface latent heat flux and the monsoon circulation relaxes back to the climatological mean. The monsoon circulation strength and the moist processes work together in the late season, which explains the observed tendency for monsoonal rainfall increases during the late monsoon season after strong winter El Nino conditions. Idealized AGCM experiments with a fixed-depth ocean mixed layer demonstrate that the remnant but weaker-than-peak warm SSTs in the eastern equatorial Pacific during spring and the early summer following winter El Ninos substantially contribute to the NIO warming. The results suggest that local air-sea interactions in the tropical Indian Ocean after winter El Nino are strongly dependent on the details of El Nino's decaying trend.