Influences of subseasonal to interannual oscillations on the SCS summer monsoon onset in 2018

Abstract

The seasonal transition from December 2017 to May 2018 occurred during the final decaying stage of the La Niña phase following the 2015/16 El Niño. In this report we documented the anomalous cyclonic flow that persisted over the South China Sea (SCS) in winter and over the western North Pacific in spring was maintained by the anomalous heating in the equatorial Pacific and the extratropical influences that consist of weakened Aleutian low and subtropical wind-SST coupled air-sea fluxes over the NW Pacific. Persistent anomalous north-easterlies along with negative OLR and warm SST anomalies over the SCS in spring was found to be sustained by the cold and dry air advected from East Asia into the lower latitudes through the anomalous easterlies associated with the cyclonic flow to the south and anticyclonic flow to the north at 20 - 30°N. Besides the above interannual scale influences, heating over tropical Indian Ocean (IO) associated with two intra-seasonal oscillation (ISO) episodes in April and May caused strong easterly flow over eastern IO and Maritime Continent. The interannual and intra-seasonal influences together maintained an anticyclonic flow from SCS to Bay of Bengal in April and May, and delayed the SCS monsoon onset. A dry Equatorial Rossby wave that arrived at the SCS in late May further enhanced the delay and resulted in a late SCS monsoon onset in early June.