Abstract:
Based on daily temperature and precipitation observations collected at weather stations in China during 1981-2010, persistent anomalous events in southern China are identified and classified into four types, i.e. cold-wet-event, cold-dry-event, warm-wet-event, and warm-dry-event. Features of large-scale atmospheric circulations associated with the four types of events are examined using the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalysis data. The anomalous "north high-south low" pattern in geopotential height over Eurasia in the middle and high latitudes results in a weaker than normal temperate jet and a stronger than normal subtropical jet, which are favorable for the southward invasion of cold air. On the other hand, the anomalous "west low-east high" pattern over the subtropics indicates that the subtropical western Pacific High is stronger and shifts farther north than normal, while the southern branch trough deepens. The southerlies in front of the trough transport abundant water vapor into southern China and increase precipitation there. Jointly affected by anomalous circulations both in middle and higher latitudes and in the subtropics, southern China often experiences various persistent abnormal events during winter. Among them, the wet-cold event occurs when the above two anomalous patterns appear simultaneously. Therefore, our results suggest that the prediction skill of severe freezing disaster weather over southern China would be improved if taking into account the variations of key circulations associated with cold-wet events.