The record-breaking northward shift of the western Pacific subtropical high in summer 2018 and the possible role of cross-equatorial flow over the Bay of Bengal

In July-August (JA) 2018, the western Pacific subtropical high (WPSH) was the most northward since 1961, and this led to the record-breaking highest temperature in China for the last 58 years. In this study, the possible effects of the cross-equatorial flows (CEFs) on the extremely northward shift of the WPSH are analyzed. The results show that significant relationships exist between the WPSH ridge line with the westerly wind over the eastern Indian Ocean and western Pacific (EIOWP), and with the CEF over the Bay of Bengal (BOBCEF). In JA 2018, the westerly wind over the EIOWP and the BOBCEF were the strongest during 1981-2018. The strengthening of the BOBCEF can enhance the westerly anomalies from the east of the Arabian Sea to the northwestern Pacific and favor the northward shift of the WPSH. Based on the composite analysis of 29 other northward shift cases of WPSH in 1981-2017, it is shown that the strengthening of the BOBCEF and that of the westerly wind over EIOWP occur 2 weeks and 1 week before the beginning of the WPSH northward shift, respectively. The similarity between the 29 cases and the northward extremity of the WPSH in 2018 suggests that the anomalous BOBCEF was one of the factors that contributed to the record-breaking northward shift of the WPSH in the summer.