Record-Low WNP Tropical Cyclone Activity in Early Summer 2020 due to Indian Ocean Warming and Madden-Julian Oscillation Activity

摘要

During early boreal summer 2020, the western North Pacific (WNP) was characterized by record-low tropical cyclone (TC) activity, reaching the lowest historical level since 1979. Only one short-lived TC was observed in June and the whole of July underwent an unprecedented quiescent TC period. Concurrent Indian Ocean warming provided unfavorable conditions for cy-clogenesis over the WNP, which partly (～40%) explains the extremely low TC number from a statistical standpoint. Accompanied with the Indian Ocean warming, an unprecedentedly long-lasting Madden-Julian Oscillation (MJO) event stalled over the Indian Ocean sector, persisting almost through the entire June and July. This simultaneous quasi-stationary MJO activity further caused unfavorable background conditions for cyclogen-esis in the WNP. The Indian Ocean warming combined with the anomalous MJO conditions together largely (＞80%) explain the record-low TC number over the WNP during early summer 2020.