An increasing trend in the early-winter precipitation around Japan and its linkage with the enhanced heating over the tropical eastern Indian Ocean

Our previous paper (Yasunaga and Tomochika 2017) examined long-term changes in the monthly precipitation along the coastal areas of the Sea of Japan and found that the December precipitation significantly increases by approximately 50% of the climatological precipitation at most of the observational stations from the mid-1980s to 2015, while there are no remarkable changes in the precipitation one month before and after December (in November and January). Although sea surface temperature shows remarkably positive trends over the central and southern parts of the Sea of Japan in December of the corresponding period, it is suggested that the intensified north-westerly (NW) monsoonal flow is mainly responsible for the precipitation increase. On the other hand, a question is still left open why the NW wind gets stronger in December of recent years (and not in November and January). Following the previous investigation, therefore, the present paper explores long-term trends of the large-scale atmospheric pressure pattern leading to the intensified NW monsoonal flow.

Results indicate that precipitation around tropical Eastern Indian Ocean and maritime-continent also significantly increases in recent years. It is suggested that a wave-packet of the stationary Rossby wave associated with the enhanced tropical heating lead to the negative height anomalies in the lower level around Japan, through deflecting the subtropical jet to the south (Fig. 1) and giving the favorable conditions for the development of a low-pressure trough on the eastern side. Anomalies of geopotential height also show significant positive trends in the pole region. Based on the multiple regression analysis, however, tropical convection is found to be of primary importance to the formation of the negative height anomalies in the lower level around Japan. Although tropical precipitation increases also in November and January, the location of the heating is not favorable to the development of an upper-level trough on the western side of Japan.