Large-Scale Anomalous Cyclone in the Western North Pacific

The large-scale anomalous anticyclone in the western North Pacific (WNP) has been extensively studied, but the large-scale anomalous cyclone has not received much attention in the past years. In this study, we use observational data to find that the occurrence numbers of the anomalous cyclone and anticyclone in the WNP have been roughly the same from 1979 to 2020. Our analyses indicate that the WNP anomalous cyclone is an interannual circulation anomaly in the WNP, which can persist from boreal autumn to the subsequent spring during a La Niila year and from spring to sum-mer during a developing El Niilo year. To confirm the roles of the central equatorial Pacific, tropical Indian Ocean, and central WNP sea surface temperatures, we perform a suite of model experiments using an atmospheric general circulation model. The model experiments demonstrate that central equatorial Pacific warming contributes to the WNP anomalous cyclone during a developing El Niilo year. Cooling in the central equatorial Pacific or the tropical Indian Ocean alone can-not induce the WNP anomalous cyclone, but the combination of central equatorial Pacific cooling, tropical Indian Ocean cooling, and central WNP warming can jointly induce the WNP anomalous cyclone during a La Niila year. Similar to the WNP anomalous anticyclone, the WNP anomalous cyclone and its climatic impacts deserve attention.

Automated summary

Based on observational data, this study finds that there are large-scale anomalous cyclones and anticyclones in the western North Pacific (WNP), with similar occurrence numbers. The WNP anomalous cyclone is identified as an interannual circulatory anomaly that can persist from autumn to spring during La Niña years and from spring to summer during developing El Niño years. The central equatorial Pacific warming contributes to the WNP anomalous cyclone during developing El Niño years, while the combination of central equatorial Pacific cooling, tropical Indian Ocean cooling, and central WNP warming induces the WNP anomalous cyclone during La Niña years.