Observed impacts of the North Pacific Victoria Mode on Indian summer monsoon onset

The onset of the Indian summer monsoon (ISM) marks the arrival of the rainy season and affects billions of people. Therefore, exploring the factors influencing ISM onset is essential, especially sea surface temperature (SST) anomalies. In addition to the well-known El Ni & ntilde;o-Southern Oscillation in the tropical Pacific, in this study, we find that the SST anomalies in the extratropical Pacific (Victoria mode, VM) can significantly impact ISM onset. The positive phase of the May VM causes monsoon onset delay, and two distinct mechanisms linking these phenomena are identified. The tropical pathway involves divergent circulation and equatorial Rossby waves. The VM-associated SST gradient causes low-level divergence and suppressed rainfall around the Indo-China Peninsula. Such suppressed rainfall induces anomalous anticyclone circulation by exciting the equatorial Rossby wave response, which prevents the establishment of monsoonal southwesterly winds and moisture transport, therefore delaying ISM onset. The extratropical pathway involves the Rossby wave train and land-sea thermal contrast. The VM triggers an eastward Rossby wave train via upper-level divergence around the Gulf of Alaska. This wave train propagates downstream and generates anomalous negative geopotential heights and low air temperatures to the northwest of India, which reduces the land-sea thermal contrast and delays ISM onset. Our result suggest that the ISM onset predictions should consider the role of May VM.