Winter monsoon over South China in ENSO and non-ENSO years

The interannual variability of the winter monsoon (WM) over South China (SC) is identified based on the unified monsoon index (UMI) defined by Lu and Chan (1999). Using the same classification of ENSO events by Xu and Chan (2000), it is found that the WM over SC is likely to be strong (weak) following the onset of an La Niña (El Niño) episode. Those years with a strong (weak) WM after an La Niña (El Niño) are regarded as non-ENSO WM. In the present study, differences between strong (weak) WM over SC in the two categories are investigated. Results suggest that the strong WM following La Niña onset (non-ENSO onset) is associated with an anomalous high (low) at 1000-hPa over the Eurasian continental (Siberia-Mongolia). Consistently, the 500-hPa ridge-trough patterns are also opposite in the two categories. Similarly, different features are observed between the weak WM that followed the onset of El Niño event and that from non-ENSO onset. Thus, the synoptic patterns are quite different between the winters of strong/weak WM over SC that followed and not followed from the onset of ENSO events. In other words, the physical processes leading to a strong/weak WM over SC accompanying with ENSO event will be different from that accompanying with non-ENSO phenomena.