Role of the Indian Ocean in the biennial transition of the Indian summer monsoon

The biennial variability is a large component of year-to-year variations in the Indian summer monsoon (ISM). Previous studies have shown that El Nino-Southern Oscillation (ENSO) plays an important role in the biennial variability of the ISM. The present study investigates the role of the Indian Ocean in the biennial transition of the ISM when the Pacific ENSO is absent. The influence of the Indian and Pacific Oceans on the biennial transition between the ISM and the Australian summer monsoon (ASM) is also examined. Controlled numerical experiments with a coupled general circulation model (CGCM) are used to address the above two issues. The CGCM captures the in-phase ISM to ASM transition (i.e., a wet ISM followed by a wet ASM or a dry ISM followed by a dry ASM) and the out-of-phase ASM to ISM transition (i.e., a wet ASM followed by a dry ISM or a dry ASM followed by a wet ISM). These transitions are more frequent than the out-of-phase ISM to ASM transition and the in-phase ASM to ISM transition in the coupled model, consistent with observations. The results of controlled coupled model experiments indicate that both the Indian and Pacific Ocean air-sea coupling are important for properly simulating the biennial transition between the ISM and ASM in the CGCM. The biennial transition of the ISM can occur through local air-sea interactions in the north Indian Ocean when the Pacific ENSO is suppressed. The local sea surface temperature (SST) anomalies induce the Indian monsoon transition through low-level moisture convergence. Surface evaporation anomalies, which are largely controlled by surface wind speed changes, play an important role for SST changes. Different from local air-sea interaction mechanisms proposed in previous studies, the atmospheric feedback is not strong enough to reverse the SST anomalies immediately at the end of the monsoon season. Instead, the reversal of the SST anomalies is accomplished in the spring of the following year, which in turn leads to the Indian monsoon transition.