Stratospheric control of the Indian Summer Monsoon Onset

This paper reports a significant linkage between the seasonal timing of boreal spring stratospheric final warming (SFW) events and the onset of the Indian summer monsoon (ISM). The leading singular vector decomposition pattern reveals a significant, coupled interannual variation between the SFW-related circulation in the lower stratosphere and the ISM-related circulation in the lower troposphere, objectively confirming the intimate relationship between SFWs and the ISM. Regression and composite maps show that, associated with a late SFW, the stratospheric polar vortex and the polar jet are anomalously stronger in April to early May, which is coupled with positive anomalies of the Northern Annular Mode (NAM) and the Arctic Oscillation (AO) in the troposphere. These tropospheric NAM/AO anomalies act to pass the extratropical anomaly signals to western central Asia via a NAM-/AO-related Rossby wave train in the upper troposphere, which is initiated over the North Atlantic jet exit region and extends across Eurasia. This results in an anomalous upper tropospheric anticyclone accompanied by anomalous descent over western central Asia that in turn warms the in situ air column and results in an enhanced meridional gradient of tropospheric temperature over the land to the north of the Arabian Sea and the Indian Ocean and therefore an early onset of the ISM. In contrast, an early SFW corresponds to the negative NAM/AO phase, an anomalous upper-level low over western central Asia in the following April and May, and thus a late onset of the ISM.