Zonally Asymmetric Variation of Summer Asian Upper-tropospheric Jet and Its Possible Association with North Atlantic SST Anomaly and Eurasian Land Surface Thermal Anomaly

Based on the ERA-Interim reanalysis data during 1979-2015, the zonally-asymmetric variation of summer Asian upper-tropospheric jet and its possible external forcing factors are analyzed. The second empirical orthogonal function mode of summer Asian 200-hPa zonal wind anomalies, which accounts for 16.4% of total variance, is characterized by zonally asymmetric spatial distribution. This mode reflects the out-of-phase changes of meridional displacement of jet over West Asia and East Asia. The North Atlantic sea surface temperature anomalies (SSTA) and the Eurasian surface thermal anomalies may have a linkage with the zonally asymmetric variation. The tripole pattern of North Atlantic SSTA in the spring can trigger anomalous wave trains that propagate downstream with anomalous circulation centers over Eurasian in the summer corresponding to circulation anomaly centers associated with the zonally-asymmetric variation of the Asian jet. The anomalous anticyclonic circulation around the East European Plain and anomalous cyclonic circulation around Lake Balkhash associated with the wave trains can affect the West Asian jet. In addition, the East Asian jet is well correlated with the anomalous cyclonic circulation to the north of Lake Baikal and the anomalous anticyclonic circulation to the south. Land surface thermal anomaly may also lead to changes in local circulation and affect the jet variation by modulating vertical motion and land-atmosphere interaction over soil moisture key areas and the impacts are regionally different.