Distinct Influence of Multiyear La Niña Events on Northwestern Pacific Tropical Cyclone Activity during Their Developing and Persisting Periods

Abstract Tropical cyclone (TC) activity over the western North Pacific (WNP) during the developing and persisting periods of multiyear La Niña events exhibits distinct spatial patterns, particularly during boreal summer. In multiyear La Niña’s developing summers, TC activity is suppressed basinwide due to an anomalous anticyclonic circulation over the WNP. This circulation arises from the synergistic effects of the Indo-Pacific Ocean Capacitor effect following the preceding El Niño and strengthened tropical easterlies driven by concurrently developing La Niña. In contrast, during multiyear La Niña’s persisting summers, TC activity markedly increases over the northwestern WNP and adjacent coastal regions of the northwestern WNP. This arises from enhanced convection over the western WNP induced and maintained by long-lasting La Niña, which, in summer, generates Rossby wave trains that induce an anomalous cyclone over the northwestern WNP and an anticyclone east of Japan. These systems provide favorable environments for TC activity over the northwestern WNP and amplify TC risks in coastal regions of the northwestern WNP. The differences in summer TC activity arise from the synergistic effects of different preceding El Niño–Southern Oscillation (ENSO) conditions and the concurrent La Niña. In addition, TC activity is relatively similar in subsequent autumns, which may be due to the dominant modulation of the current La Niña. Our findings provide a more detailed picture beyond canonical views of ENSO–TC relationships from the perspective of multiyear La Niña events, which could advance our physical understanding of TC activity and help TC seasonal prediction. Significance Statement El Niño–Southern Oscillation (ENSO) dominantly modulates the interannual variability of tropical cyclones (TCs) over the western North Pacific (WNP), but multiyear La Niña events—often analyzed as isolated episodes—may have different impacts in their different periods. This study reveals distinct patterns of WNP TC activity between the developing and persisting periods of multiyear La Niña events, as preceding ENSO conditions generate different synergistic impacts with the concurrent La Niña. The findings advance our understanding of the ENSO–TC relationship from the perspective of multiyear La Niña events and may be helpful for TC seasonal prediction and risk adaptation under global warming.