Characteristic large-scale circulation pattern of the persistent droughts of Northeast Asia from premonsoon season to monsoon season

Abstract

Characteristic patterns of atmospheric circulation anomalies over the northeast Asian region in the dry and wet premonsoon seasons have been analyzed. The relationship between these patterns in the premonsoon season and the precipitation anomalies in the monsoon season has been also investigated. Since this study focuses on dry premonsoon season, features of anomalous large-scale atmospheric circulations during springtime droughts that occurred over the northeast Asia are revealed. The Palmer drought severity index is used in order to define drought years. In drought years, the anomalous circulation over the northeast Asia exhibits the weakened northward flow from the western North Pacific, and the position of the East Asian westerly Jet (EAWJ) maximum core at 300hPa is shifted southward. Thus, the rainfall band is moved southward, and the associated precipitation is suppressed over Korea, Japan, and southeastern China during spring droughts. The Sea Surface Temperature (SST) anomalies and outgoing longwave radiation anomalies, which show a north–south dipole pattern between the Philippines and the northeast Asian region, support these dry conditions through the Hadley circulation. The anomalously warm SSTs in the western North Pacific seem to play an important role in the atmospheric circulation associated with persistent northeast Asian droughts The persistent features of extreme precipitation over the northeast Asia from

premonsoon season to monsoon season are found. In order to understand some characteristics of the circulation anomalies associated with persistent features of extreme precipitation, six dry cases and six wet cases in the premonsoon season are selected. Both of the extreme conditions are nearly persistent in each case. As mentioned in spring drought analysis, the weakened western North Pacific subtropical high (WNPSH) and the southward-shifted EAWJ are predominant for dry cases. For wet cases, the strengthening of WNPSH, the northward-shifted EAWJ in spring, and southward-shifted EAWJ in summer are apparent. However, dry cases are associated with the warm SST anomalies of the western North pacific, whereas wet cases are related to the warm SST anomalies of the Indian Ocean and Bay of Bengal. The dependency of extreme precipitation on this SST anomaly pattern is supported through numerical experiments. Based on the observational analysis and model experiments, the main connection system between extreme precipitation such as drought and SST is the anomalous cyclonic circulation over the western North Pacific.