Circulation Anomaly and Sea Surface Temperature Forcing in Rare Continuous Rainy Weather in Zhejiang during the Winter of 2018/2019

Based on the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) global reanalysis data, ground observation data, and automatic station precipitation data, this study analyzed circulation anomalies such as winter monsoon circulation and the South Branch trough in rare continuous rain and oligoscale weather in Zhejiang in the winter of 2018/2019. Moreover, the study investigated the causes of local circulation anomalies from aspects such as westerly wind fluctuation and sea temperature forcing. The results showed that in the winter of 2018/2019, the rainy days and sunshine hours surpassed the historical record, and the rainfall was significantly above normal. The main circulation anomalies were the abnormal northerly western North Pacific anomalous anticyclone (WNPAC). Meanwhile, the Aleutian low-pressure and the Siberian high-pressure systems were also northerly. There was a strong southerly wind anomaly south of 40°N in East Asia, and the winter monsoon was weak. The southern branch trough was stronger than the perennial, ensuring that there was a continuous water vapor and disturbance transport over Zhejiang. In the middle layer of the troposphere, a wave energy propagated along Europe to East Asia and the western Pacific. The wave energy spread southward to the south of 20°N in East Asia, which might lead to a significant north lift of the WNPAC and the strengthening of the southern branch trough. The El Niño–Southern Oscillation (ENSO) warm phase caused abnormal convective cooling in the maritime continent, while the convection over Zhejiang strengthened, and ENSO also had a significant effect on the activity intensity of the southern branch trough. The high sea surface temperature in the offshore waters of China was an important factor for the WNPAC and the Aleutian Low to significantly jump north. The abnormal circulation in the northern hemisphere in the winter of 2018/2019 might have been caused by ENSO and China’s offshore sea temperature collaborative forcing.