Characteristics of cold surge and temperature variation over East Asia in connection with stratospheric circulation during the boreal winter season

Abstract

In this study, I investigated the tropospheric circulation and temperature variation related with the weakening of stratospheric polar vortex. In particular, I examined how the stratospheric circulation dynamically influences the temperature variation over East Asia during the stratospheric weak vortex (SWV) period. During the SWV period, the cold condition is dominant over East Asia. A notable feature is the fluctuation of temperature with longer timescale than 10 days during the SWV event while stratospheric circulation varies slowly. The temperature fluctuation in the lower-troposphere over East Asia is mainly forced by the variation of climatological trough located in the upper-troposphere over the East Asian coast during the SWV event. The trough varies in a close connection with the evolution of the SWV. Particularly, in the beginning period of the SWV event (Peak period, -5 - +5 days) when East Asia shows extreme coldness, the trough is deepened exceptionally by wave forcing from the upstream side over the area of Ural Mountain as well as the direct impact related to the decrease of stratospheric geopotential height (Z) over East Asia. In the subsequent period (Decay 1 period, +6 - +16 days), I can notice the weakening of the upper-tropospheric trough over East Asia, which mitigates the coldness over the region. The weakening of the trough is caused by the westward migration of strong positive Z anomaly from the North Pacific. The westward positive anomaly comes from the clockwise rotation of stratospheric positive Z anomaly. After about 20 days since the SWV occurrence (Decay 2 period, +17 – +27), the coldness over East Asia gains its strength again. This coldness is induced by the re-developing of the trough over East Asia, which is related to the negative AO-like type pattern associated with the SWV. During the Peak period, the anomalous positive Z is stronger over the North Pacific than over other regions influenced by the zonally symmetric positive Z anomaly over the stratospheric polar cap area. This feature is relevant to the contribution of feedback between the synoptic eddy and tropospheric flow in low-frequency variation of the SWV. The North Pacific positive Z related to the SWV enhances its strength through the divergence of potential vorticity flux by the synoptic eddies. In particular, the eddy vorticity (heat) flux contributes to the strengthening of positive Z in the upper (lower) troposphere. The above results indicate that the synoptic eddies play an important role in the intensification of stratospheric impact on the tropospheric circulation. The circulation change and temperature variation over East Asia related with the SWV influence the cold surge characteristics such as its frequency, duration and strength. In comparison of the characteristics of cold surge for the SWV and the stratospheric strong vortex (SSV), the cold surges occurred more frequently in the SWV than in the SSV. Moreover, they are stronger and longer-lasting in SWV than in SSV. The differences between these cold surge characteristics are originated by the impact of anomalous tropospheric circulation (long-lasting and deepening of East Asian trough in the upper-troposphere), which has a close relationship with the SWV. Furthermore, the cold surge characteristic changed in the decadal time scale during the past three decades (1979-2011). They are stronger and longer-lasting during the two decades of 1980s and 2000s than during the 1990s. The features of cold surges arise from the large-scale atmospheric conditions associated with the different phases of the AO, which showed a prominent decadal fluctuation. In particular, more (less) frequent strong and long-lasting cold surge in the 1980s and 2000s (1990s) are preceded by more dominant negative (positive) phase of the AO. These features indicate that the decadal change of AO, which preconditioned the cold surge, plays an important role in the decadal change of cold surge characteristics. In addition, the decadal change in the AO preconditioning of cold surge during the recent decades has a strong relationship with the negative phase of tropospheric AO during the periods of SWV events.