Remote forcing effect of sea surface temperatures in the northern tropical Atlantic on tropical cyclone genesis over the Western North Pacific in July

In July 2020, neither genesis nor landfall of typhoons occurred in the western North Pacific (WNP). This was the first time that the "no-typhoon" phenomenon had occurred since 1949. This study analysed the variation in the tropical cyclone genesis frequency (TCGF) in the WNP in July over the past 40 years, including the corresponding atmospheric background circulation and the possible influence of tropical sea surface temperature (SST). The results revealed that the eastward and northward location of the western Pacific subtropical high (WPSH), the deepened and eastward expanded tropical monsoon trough (TMT) in the WNP, the active convection in the main body of TMT, the weakened southwest water vapour transport of the East Asian summer monsoon, as well as the strengthened and eastward expanded equatorial westerly wind were conducive to the tropical cyclone genesis in the WNP in July. Furthermore, we used regression analysis to explore the possible impact of tropical SSTs. We found that the relatively high SSTs in the northern tropical Atlantic (NTA) in June and July triggered a cyclonic circulation over the tropical East Pacific and NTA, resulting in strong upward motion. The airflow subsidence in the tropical central Pacific then caused the WPSH to extend westward and the easterly airflow on the south side of WPSH to strengthen, which weakened the WNP monsoon trough. Besides, the influence of warming NTA SSTs could be conveyed to the Indian Ocean and indirectly induce the enhancement of WPSH. Such a series of the large-scale circulation variations were not favourable for tropical cyclone genesis in the WNP. As a result, the TCGF was relatively low in July. Thus, one of the causes of the July 2020 "no-typhoon" phenomenon in the WNP could be the remote forcing effect of SST in NTA.