Influences of the Indian Ocean SST on the Indian Summer Monsoon and its Seasonal Prediction in El Niño Developing Years

This study examines how the summertime Indian Ocean (IO) SST anomalies (SSTAs) affect the Indian Summer Monsoon (ISM) and its predictability in the El Niño developing years from the perspective of seasonal predictions. The CFSv2-COLA ensemble seasonal reforecasts successfully predicted the ISM in 1972 but failed in 1997, as those years exhibited drastically different ISM states. Our sensitivity experiments, in which the ocean and atmosphere are decoupled in the tropical IO with the prescribed SST, reveal that the erroneous prediction of cold IO SSTAs in 1997 exacerbates an El Niño-induced ISM drought and “correcting” these SST errors improves the ISM prediction substantially, whereas a good prediction of the summertime IO SSTAs contributes positively to the skillful ISM reforecast in 1972. It is also demonstrated that the warm IO SSTAs centered in the Arabian Sea in 1997 reduce sea-level pressures locally and steer the low-level anomalous winds to transport water vapor into the India. This regional process counters the El Niño-induced drought tendency and results in a nearly normal ISM that defies the historical El Niño-ISM relation. However, the warm SSTAs centered at the western equatorial IO in 1972 strengthen the anomalous Walker circulations originally set up by the developing El Niño in the Indo-Pacific domain, which further enhance the El Niño evolution and its teleconnection to the ISM. This inter-basin feedback process intensifies the typical El Niño-ISM relation. The spatial structure of the summer IO SSTAs may determine whether the IO regional process or the inter-basin process prevails. The year 2015 is also discussed as another relevant case.