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There are on average thirteen cold surges in each winter season (October to April), of which two are strong cases. The averaged intensity of cold surges, measured by maximum surface pressure, is 1053 hPa. The cold surges originate from two primary source regions: 1) northwest of Lake Baikal, and 2) north of Lake Balkhash. The typical evolution of a cold surge occurs over the period of 5-14 days. Trajectory and correlation analyses indicate that, during this time high pressure centers propagate southeastward around the edge of the Tibetan Plateau from the mentioned source regions. Some of these high pressure centers then move eastward and diminish over the ocean, while others proceed southward. The signature of the associated temperature, wind, and pressure fields propagate farther southward and eastward. The affected area encompasses the bulk of the maritime continent. Although the intensity of the Siberian high is found to peak during December and January, the frequency of cold surges has a maximum in November and in March. This result suggests that November through March should be considered as the East Asian winter monsoon season.
Two stratifications of cold surges are used to examine the relationship between ENSO and the interannual variation of the winter monsoon. The first one, described as the conventional cold surges, indicates that the cold surge frequency reaches a minimum a year after El Nino events. The second one, defined as the maximum wind events near the South China Sea, shares the same source regions as the first. This stratification of surges is found to be in good agreement with the Southern Oscillation Index (SOI). Low SOI (high SOI) events coincide with high (low) frequencies of cold surges.
The interannual variation of averaged meridional wind near the South China Sea and the maritime continent is dominated by the South China Sea cold surges, and is also well correlated (R=0.82) with the SOI. Strong wind seasons are associated with La Nina and high SOI events; likewise, weak wind years are linked with El Nino and low SOI cases. This pattern is restricted north of the equator within the region of (0°N-20°N, 110°E-130°E), and is confined to the near surface layer. The surface Siberian high, 500 hPa trough and 200 hPa jetstream, all representing the large-scale monsoon flow, are found to be weaker than normal during El Nino years. In particular, the interannual variation of the Siberian high is in general agreement with the SOI.(pdf)
UCRL-MI-123395
