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 cyclogenesis 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 cyclogenesis 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.

中文翻译：

---

由于印度洋变暖和马登-朱利安振荡活动，2020 年初夏的 WNP 热带气旋活动创下历史新低

2020 年北半球初夏，西北太平洋 (WNP) 的热带气旋 (TC) 活动创下历史新低，达到 1979 年以来的历史最低水平。 6 月仅观测到 1 次短命 TC，整个 7 月都经历了前所未有的静止 TC 时期。同时发生的印度洋变暖为 WNP 上的气旋形成提供了不利条件，这部分（~40%）从统计的角度解释了极低的 TC 数量。伴随着印度洋变暖，一场空前持久的马登-朱利安振荡 (MJO) 事件在印度洋海域停滞不前，几乎持续了整个 6 月和 7 月。这种同时发生的准平稳 MJO 活动进一步导致了 WNP 中气旋发生的不利背景条件。