CLIMATOLOGY AND INTERANNUAL VARIATION OF THE EAST-ASIAN WINTER MONSOON - RESULTS FROM THE 1979-95 NCEP NCAR REANALYSIS/

This paper presents the climatology and interannual variation of the E ast Asian winter monsoon based on the 1979-95 National Centers for Env ironmental Prediction/National Center for Atmospheric Research reanaly sis. In addition to documenting the frequency, intensity, and preferre d propagation tracks of cold surges and the evolution patterns of rela ted fields, the authors discuss the temporal distribution of the Siber ian high and cold surges. Further, the interannual variation of the co ld surges and winter monsoon circulation and its relationship with ENS O were examined. There are on average 13 cold surges in each winter se ason (October-April), of which two are strong cases. The average inten sity of cold surges, measured by maximum sea level pressure, is 1053 h Pa. The cold surges originate from two source regions: 1) northwest of Lake Baikal, and 2) north of Lake Balkhash. The typical evolution of a cold surge occurs over a period of 5-14 days. Trajectory and correla tion analyses indicate that, during this time, high pressure centers p ropagate southeastward around the edge of the Tibetan Plateau from the mentioned source regions. Some of these high pressure centers then mo ve eastward and diminish over the oceans, while others proceed southwa rd. The signatures of the associated temperature, wind, and pressure f ields propagate farther southward and eastward. The affected area enco mpasses the bulk of the maritime continent. Although the intensity of the Siberian high peaks during December and January, the frequency of cold surges reaches a maximum in November and in March. This result su ggests that November through March should be considered as the East As ian winter monsoon season. Two stratifications of cold surges are used to examine the relationship between ENSO and the interannual variatio n of the winter monsoon. The first one, described as conventional cold surges, indicates that the surge frequency reaches a minimum a year a fter El Nino events. The second one, defined as a maximum wind event n ear the South China Sea, shares the same origin as the first. The surg e frequency is in good agreement with the Southern Oscillation index ( SOI). A low (high) SOI event coincides with a low (high) frequency of cold surges. The interannual variation of winter northerlies near the South China Sea is dominated by the South China Sea cold surges and is well correlated (R = 0.82) with the SOI. Strong wind seasons are asso ciated with La Nina and high SOI years; on the other hand, weak wind s easons are associated with El Nino and low SOI years. This pattern is restricted to an area north of the equator within the region of (0 deg rees-20 degrees N,100 degrees-130 degrees E) and is confined to the ne ar-surface layer. The SST variation in the same region is similar to t he wind pattern but lags the wind for approximately 1-5 months, which suggests that the SST variation is forced by the wind. The surface Sib erian high, 500-hPa trough, and 200-hPa jet stream, all representing t he large-scale monsoon flow, are weaker than normal during El Nino yea rs. In particular, the interannual variation of the Siberian high is i n general agreement with the SOI.