Why rare tropical cyclone formation after maturity of super El Niño events in the western North Pacific?

  • Rare TC formation over the WNP in the first-halfyears of 1983, 1998 and 2016
  • A Walker circulation anomaly forced by CT El Niño is found to suppress TC formation
  • The SGP is a nice metric to measure the environmental factors’ control on TC activity

Using Gray’s “seasonal genesis parameter” (SGP) as a metric, we investigate the large-scale environmental factors and the associated circulation changes attributing to rare tropical cyclone (TC) formation in the western North Pacific (WNP) in the first-half years of 1983, 1998, and 2016, which are known to be the decaying phase of the 1982/83, 1997/98, and 2015/16 super El Niño events. A budget analysis of SGP shows that decreases in relative vorticity and specific humidity, due to the existence of a Philippine Sea anticyclonic anomaly, are the major factors attributing to strong suppression of TC formation mentioned above. From a broader basin-scale view, we demonstrate that this Philippine Sea anticyclone could also be a response resulting from the descending branch of a northwest-and-southeast tilting anomalous overturning (Walker) circulation forced by asymmetric SST anomalies typical of the Cold Tongue El Niño.

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