An Emerging Precursor Signal in Stratosphere for the Onset of South Asian Summer Monsoon

This study reports a significant temporal relationship that has emerged in recent decades between the Indian summer monsoon (ISM) onset and the seasonal timing of the stratospheric final warming (SFW). Specifically, early SFWs in spring tend to be followed by late ISM onset, and vice versa. During the ISM onset season after 1979, there exist significant SFW-related zonal wind anomalies over South Asia, and they are highly consistent with that of the ISM-related in both the upper and lower troposphere. These SFW-related zonal wind anomalies are not observed before 1979, corresponding to the absence of the SFW–ISM relationship. Further analysis indicates that the tropospheric Arctic Oscillation anomalies that follow the stratospheric SFWs to appear during April–early May after 1979 are crucial for the emerging SFW–ISM linkage. This implies that the seasonal timing of SFWs may act as an emerging precursor for the ISM onset since 1970s.