Distinct response of Asian summer monsoon rainfall during the first and third years of triple-dip La Niña events

This study investigates triple-dip La Niña events and their influence on Asian summer monsoon rainfall during the first and third years. In the first year, a significant positive rainfall anomaly extends from the Bay of Bengal to the Yangtze River Basin, concurrent with reduced rainfall over the northern Indian Peninsula and the Indochina Peninsula. This pattern largely reverses during the third year. Convergent empirical and modeling evidence reveal that the distinct rainfall responses are linked to different evolutions of the El Niño–Southern Oscillation (ENSO). In the first year, following a preceding El Niño, La Niña onset is rapid. This rapid onset strengthens the Western Pacific Subtropical High (WPSH) and a negative-phase Scandinavian pattern, which jointly shape the initial rainfall distribution. Conversely, by the third year, sustained La Niña amplifies zonal sea surface temperature (SST) gradients in the equatorial western Pacific. This shifts the WPSH northward and triggers a positive Circumglobal Teleconnection (CGT) pattern, collectively driving the reversed rainfall anomalies. Additionally, the second year exhibits relatively weak rainfall anomalies, as both the La Niña onset rate and the zonal SST gradients are near climatological normals. These results are useful for predicting Asian summer monsoon rainfall.