Effects of a Secondary Circulation over the Eastern Tibetan Plateau on Long-Lived Cold Surges over the South China Sea in Boreal Winter

Abstract A persistent cold surge over the South China Sea (SCS) can sometimes cause severe disasters and economic losses in China and Southeast Asia during boreal winter. In this study, cold surge events were classified into long-lived (≥6 days) and short-lived (<6 days) events based on the lower-tropospheric circulation anomalies. A secondary circulation was observed between the eastern Tibetan Plateau (TP) and downstream East Asia during the period 2 days before and 3 days after the outbreak of long-lived cold surge events, and its cause was revealed by employing the temperature tendency equation. It was established that long-lived cold surges are preceded by a Rossby wave train propagating from the North Atlantic to East Asia, which triggers a cyclonic anomaly over the northern TP. Together with strengthening of the southwesterly wind to the south and west of the TP, the warm advection of anomalous temperature by mean westerly wind causes anomalous warming and ascent motion over the eastern TP. Meanwhile, it is accompanied by westerly wind anomalies in the upper troposphere and anomalous subsidence over East Asia; hence, the secondary circulation emerges and strengthens. Cold advection by anomalous northerly wind causes temperature decrease and anomalous subsidence over East Asia after the outbreak. The existence and enhancement of the secondary circulation significantly strengthen this subsidence, contributing to the vertical circulation between the midlatitude area of East Asia and the SCS. Finally, anomalous lower-tropospheric northerly wind invading from East Asia toward the SCS is strengthened, thereby enhancing the intensity and duration of long-lived cold surges over the SCS. Significance Statement The purpose of this study is to understand the physical mechanisms of the Tibetan Plateau (TP) impacting long-lived cold surges over the South China Sea (SCS) during boreal winter. Although previous studies have demonstrated that the TP exerts influence on the weather in East Asia, few studies explore its effect on long-lived cold surges over the SCS. Our study indicates that a secondary circulation is observed between the eastern TP and downstream East Asia and influences long-lived cold surges. Utilizing temperature tendency equation, we reveal that this secondary circulation is triggered by upstream Rossby wave trains for warm advection over the eastern TP. These results have important implications for improving forecasting of cold surges over the SCS.