Cross-Seasonal Precursors of Strong Persistent Freezing Rain and Snow Events in South and Southwest China

Abstract We used daily temperature and precipitation data from 753 stations provided by the National Meteorological Information Center of China for the time period 1950–2018 and NCEP/NCAR reanalysis data to explore winter weather events in southwest China (100–110° E, 22.5–32.5° N). We found a cross-seasonal precursor signal for the occurrence of freezing rain and snow events. Key zones of the tropical Pacific (160° E–90° W, 5° S–5° N) sea surface temperature (SST), the north Pacific decadal oscillation (PDO)-like SST mode and the Indian Ocean (40–100° E, 10° S–20° N) SST in the previous autumn were significantly correlated with the key anomaly combination index in winter. Forcing of the atmosphere by these three key areas in terms of SST led to significant atmospheric circulation anomalies in the north Pacific and its lower reaches. The coupling effect of the cold SST at (160° E–90° W, 5° S–5° N) and the north Pacific PDO-like cold SST mode in the previous autumn caused an abnormal longwave disturbance over the north Atlantic Ocean in winter, which was dispersed downstream. This caused mid- to high-latitude circulation anomalies in East Asia that favored the diffusion of cold air. The cold SST at (40–100° E, 10° S–20° N) deepened the southern branch trough, favoring the occurrence of freezing rain and snow events. The CESM air–sea coupling model simulated the forcing effect outside the three key areas of SST in November, consistent with the observed statistical results. An inter-seasonal prediction index for strong persistent freezing rain and snow events in southwestern China was established based on the average SST anomalies in key areas in the previous November. This index provides a reference for the inter-seasonal prediction of winter freezing events in southern China.