Interdecadal variability of the Asian summer monsoons

The Asian monsoon represents one of the major modes of the seasonal cycle on the planet and undergoes variations on intraseasonal through interannual time scales that affect the water supply for more than half the global population. But the monsoon also undergoes appreciable variability on decadal time scales, noted both in the Asian monsoon and also as part of more coherent variations in the global monsoon. In the observed data, large variations on multi-decadal time scales are present but are confounded by external drivers including anthropogenic aerosols and greenhouse gas emissions. This review examines some of the mechanisms linking internal multi-decadal ocean modes with Asian monsoon decadal variability, including from the Pacific and Atlantic. Since much focus is still being given to seasonal monsoon prediction, we also examine perturbations caused by long time-scale oceanic modes to the monsoon-ENSO teleconnection, affecting seasonal monsoon predictability. Finally, we examine new prospects for decadal monsoon prediction in research considering the CMIP5 decadal hindcasts and forecasts, which are particularly encouraging for Atlantic multi-decadal variation modes. If future work can establish links between synoptic or intraseasonal monsoon variability and longer-term variations in the mean state, then there is potential for a cascade of predictability, improving our forecasts of the monsoon.