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 variations of the EAT’s strength are investigated through composite analysis and dynamical diagnostics. The significant roles played by the low-frequency Rossby wave (RW) and synoptic transient eddy (TE) are revealed. Before the peaks of strong EAT events, an upper-tropospheric RW train propagates across northern Eurasia and interacts with preexisting surface cold anomalies over central Siberia. This pattern intensifies the Siberian high and causes RW convergence toward the EAT, leading to 30% of the EAT’s amplification directly via the RW-induced feedback forcing. Meanwhile, RW weakens the background baroclinicity and reduces TE activities near the entrance region of the North Pacific storm track. The TE-induced feedback forcing leads to another 30% of the EAT’s amplification. The evolution and dynamical processes of the weak EAT events generally resemble those of the strong events with opposite signs. These results are consistent with the knowledge on the mechanism of the strong and weak EAT events regarding the role of RWs with additional quantitative description and provide new insights regarding the role of TEs. 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 growth and peak stages of the strong (weak) EAT events and above (below) normal over North America afterward.

Comparison of Intraseasonal East Asian Winter Cold Temperature Anomalies in Positive and Negative Phases of the Arctic Oscillation

“…A connection of positive height anomalies with those in the Arctic region is obvious in phases 6–7 (Figures f and g). Negative height anomalies over East Asia denote a deepened East Asian trough, which, together with a strengthened surface Siberia high (Figure h) and anomalous lower level northerly wind anomalies (Figure h), provide favorable condition for occurrence of cold anomalies in eastern China (Song et al, ; Song & Wu, ). The weakening/deepening of the East Asian trough and the weakening/strengthening of the Siberian high are related to the Rossby wave activity.…”

Impact of the December North Atlantic Oscillation on the following February East Asian trough

“…According to the research of Song et al . [], an accumulation of the wave energy from upstream incoming waves can affect the EAT directly. On the other hand, upstream waves interact with surface air temperature anomalies and lead to upward wave propagation converging toward the EAT, contributing to the EAT indirectly.…”

Individual and Combined Impacts of Two Eurasian Wave Trains on Intraseasonal East Asian Winter Monsoon Variability

“…This cyclone appears as a merge of the two wave trains. Previous studies have identified the effects on the cold events over East Asia of either the north wave train (Bueh et al, ; Hsu, ; Hsu & Wallace, ; Joung & Hitchman, ; Lau & Lau, ; Park et al, ; Song et al, ; Suda, ; Takaya & Nakamura, , , ; Wu & Chan, ; Yang & Li, ; Yao et al, ) or the south wave train (Song & Wu, ; Wen et al, ). Here, we detect that the two wave trains may coexist in association with intraseasonal variability of the East Asian winter monsoon.…”

“…Some studies detected a wave train over the subtropics in the upper troposphere (Branstator, ; Li & Sun, ; Li & Zhou, ; Song et al, ; Song & Wu, ; Watanabe, ; Wen et al, ). The subtropical wave train begins from the North Atlantic and the Mediterranean regions and propagates to East Asia along the subtropical jet stream (Hu et al, ; Song et al, ; Watanabe, ).…”