Decadal changes of East Asian jet streams and their relationship with the Mid-high Latitude Circulations

This study investigates the concurrent location and intensity changes of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on decadal time scale using NCEP/NCAR, ERA5 and JRA-55 reanalysis data, along with the relationship with the mid-high latitude circulation changes. Results show that EAPJ was intensified accompanied by the weakening of EASJ after 1985, and shifted equatorward in conjunction with the poleward migration of EASJ after 1999. The three-dimensional structures of the atmospheric circulation over East Asia exhibited distinct changes corresponding to the two regime shifts. Dipole anomalous sea level pressure (SLP) between the Arctic and the North Pacific, a NAM-like geopotential height anomalous pattern, and a deepened polar vortex occurred after 1985, accompanied with the out-of-phase intensity changes of the two jets. In comparison, dipole SLP anomalies between the mid-high latitudes and Tibetan Plateau, a zonal wave train over Eurasia accompanied with a meridional wave train over East Asia, and a polar vortex shifting toward the Siberia were observed after 1999, when the two jets approached each other. These circulation changes exerted pronounced effects on the East Asian climate anomalies. After the mid-1980s, the positive SLP anomaly over the northern North Pacific led to decreased land-sea SLP contrast, and the positive mid-tropospheric geopotential height center near Lake Baikal implied weakened East Asian trough (EAT). Such changes favored the warming of southern East Asia and the weakening of the southern mode of East Asian winter monsoon (EAWM). In contrast, the high latitude positive SLP anomalies denoted enhanced Siberian high (SH) and the East Asian meridional wave train resulted in tilted EAT after the late 1990s, benefitting the cold anomalies over northern Eurasia with the northern mode of EAWM intensified. Possible mechanisms for the decadal changes of the jets are discussed from the perspective of the dynamic forcing of the synoptic-scale transient eddy activities (STEA), and the thermal forcings from the East Asian landmass, the North Pacific sea surface and the Arctic sea ice cover.