Anatomy of the Indian Summer Monsoon and ENSO relationships in state-of-the-art CGCMs: role of the tropical Indian Ocean

Indian Summer Monsoon (ISM) rainfall and El Niño-Southern Oscillation (ENSO) exhibit an inverse relationship during boreal summer, which is one of the roots of ISM interannual variability and its seasonal predictability. Here we document how current climate and seasonal prediction models simulate the timing and amplitude of this ISM-ENSO teleconnection. Many Coupled General Circulation Models (CGCMs) do simulate a simultaneous inverse relationship between ENSO and ISM, though with a large spread. However, most of them show significant negative correlations before ISM, which are at odd with observations. Consistent with this systematic error, simulated Niño-3.4 Sea Surface Temperature (SST) variability has erroneous high amplitude during boreal spring and ISM rainfall variability is also too strong during the first part of ISM. The role of the Indian Ocean (IO) in modulating the ISM-ENSO relationships is further investigated using dedicated experiments with the SINTEX-F2 CGCM. Decoupled tropical Pacific and IO experiments are conducted to assess the direct relationship between ISM and IO SSTs on one hand, and the specific role of IO feedback on ENSO on the other hand. The direct effect of IO SSTs on ISM is weak and insignificant at the interannual time scale in the Pacific decoupled experiment. On the other hand, IO decoupled experiments demonstrate that El Niño shifts rapidly to La Niña when ocean–atmosphere coupling is active in the whole IO or only in its western part. This IO negative feedback is mostly active during the decaying phase of El Niño, which is accompanied by a basin-wide warming in the IO, and significantly modulates the length of ENSO events in our simulations. This IO feedback operates through a modulation of the Walker circulation over the IO, which strengthens and shifts eastward an anomalous anticyclone centered on the Philippine Sea and associated easterly wind anomalies in the equatorial western Pacific during boreal winter. In turn, these atmospheric anomalies lead to a fast ENSO turnabout via oceanic adjustement processes mediated by eastward propagating upwelling Kelvin waves. An experiment in which only the SouthEast Indian Ocean (SEIO) is decoupled, demonstrates that the equatorial SST gradient in the IO during boreal winter plays a fundamental role in the efficiency of IO feedback. In this experiment, simulated ISM-ENSO lead-lag correlations match closely the observations. This success is associated with removal of erroneous SE