Analysis on Anomalous Precipitation in Southern China During Winter Monsoons

The winters of 1997/1998 and 1998/1999, corresponding to El Nieno and La Niena episodes,respectively, were two typical rain-abundant and -scarce seasons for the southern China. In order to understand the cause of the anomalous precipitation during the two winters, a comparative analysis technique has been employed to investigate the differences in general circulation and moisture transportation between the two seasons.The results show that the abundant rainfall during the winter of 1997/1998 was associated with the ENSO warm episode event, eastward shifted weak westerly trough/ridge,weakened East Asian winter monsoon (EAWM), strengthened subtropical high, and presented two anti-cyclonic circulations over Hokkaido and the Philippine Sea, respectively, as well as one cyclonic circulation over the Yangtze River Basin in the anomalous wind fields of the lower troposphere. During the rain-scarce winter, however , the patterns of equatorial sea surface temperature anomalies and the circulation systems both in upper and lower levels were nearly the opposite of those during the rain-abundant winter. It has also been discovered that the water vapor over southern China during the winters came mainly from the southwesterly flow ahead of troughs in the southern branch of westerlies and the turning flow over the South China Sea??Indo-China Peninsula area ; and the moisture transportation channels varied significantly with regard to height. The intensified flow in the southern branch of westerlies and the anti-cyclonic circulation anomaly over the Philippine Sea during the winter of 1997/1998 were favorable for moisture transportation to mainland China, however the two moisture transportation streams were dramatically weakened during the winter of 1998/1999 due to weak westerly flow and the dominance of a cold high system in the lower level over the southeast coast of China. Such a significant inter-annual change of moisture transportation is a key factor resulting in the obvious difference in precipitation between the two winters.