Intraseasonal Variation of the Strength of the East Asian Trough and its Climatic Impacts in Boreal Winter

Abstract:

The East Asian trough (EAT) is a distinct component of the boreal winter circulation whose strength corresponds to the amplitude of the Northern Hemispheric stationary waves. In this study, the mechanism and climatic impacts of the intraseasonal variation of the EAT's strength are investigated through composite analysis and dynamical diagnostics. It reveals that both the low-frequency Rossby wave (RW) and synoptic transient eddy (TE) play significant but different roles in the EAT's variability. Before the peak of strong EAT events, an upper-tropospheric RW train propagates across the northern Eurasia and interacts with pre-existing surface cold anomalies over central Siberia. It intensifies the Siberian high and causes RW convergence towards the EAT. However, this RW does not amplify the EAT directly. Instead, it weakens the background baroclinicity, which acts to reduce TE activities near the entrance region of the North Pacific storm track. It is the TE-induced feedback forcing that leads to the amplification of the EAT directly. Although the evolution of the weak EAT events resembles that of the strong events with opposite sign, the dynamical processes are distinctly different in that both the RW and TE weaken the EAT directly with comparable contributions. These physical pictures differ from the conventional knowledge and provide new insights on the mechanism of the strong/weak EAT events. Variations of the EAT’s strength exert significant climatic impacts on East Asia and its downstream region. Near-surface air temperature is below (above) normal over East Asia during the development and peak stages of the strong (weak) EAT events and above (below) normal over North America afterwards.