Understanding the 2018 Heatwave in Japan from the Pacific–Japan Teleconnection Pattern and its Relationship with the Seasonal March of the Asian Summer Monsoon

Abstract:

East Asia experienced extremely hot summer in 2018, and East Japan had the hottest July on record since 1946. This study investigated the heatwave that occurred over Japan from mid- to late July 2018, focusing on the Pacific–Japan (PJ) teleconnection pattern in sub-seasonal timescale. During the 2018 heatwave, an enhanced Pacific High was recognized over Japan, and an enhanced convection activity was recognized over the vicinity of the Philippines, showing the enhancement of the PJ teleconnection pattern. To understand this enhancement, we focused on the sub-seasonal transition of the PJ teleconnection pattern. The spatial pattern of the PJ teleconnection pattern showed that the area where the mid-latitude response to the convection activity over and around the Philippines as an anomalous high shift in sub-monthly timescale. The anomalous high appeared in the western North Pacific in the second half of June and moved over Japan in the second half of July. This shows that it was a preferable period for the PJ teleconnection pattern response to appear over Japan when the 2018 heatwave occurred. Because the PJ teleconnection pattern appears in the Asian summer monsoon (ASM) region, the shift of the mid-latitude response area of the PJ teleconnection pattern can be related to the seasonal march of the climatological background of the ASM. Moreover, in the ASM region, the monsoon trough deepened with tropical cyclone activity over the western North Pacific and the Pacific High extend westward in July. Thus, the climatological background and interannual variation acted in phase to strengthen anticyclonic conditions over Japan in 2018, resulting in the extremely hot weather event.