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Climatology and Interannual Variation of the East Asian Winter Monsoon: Results from the 1979–95 NCEP/NCAR Reanalysis

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  • 1 Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, California
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Abstract

This paper presents the climatology and interannual variation of the East Asian winter monsoon based on the 1979–95 National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. In addition to documenting the frequency, intensity, and preferred propagation tracks of cold surges and the evolution patterns of related fields, the authors discuss the temporal distribution of the Siberian high and cold surges. Further, the interannual variation of the cold surges and winter monsoon circulation and its relationship with ENSO were examined.

There are on average 13 cold surges in each winter season (October–April), of which two are strong cases. The average intensity of cold surges, measured by maximum sea level pressure, is 1053 hPa. 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 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 oceans, while others proceed southward. The signatures 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 peaks during December and January, the frequency of cold surges reaches 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 conventional cold surges, indicates that the surge frequency reaches a minimum a year after El Niño events. The second one, defined as a maximum wind event near the South China Sea, shares the same origin as the first. The surge 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 associated with La Niña and high SOI years; on the other hand, weak wind seasons are associated with El Niño and low SOI years. This pattern is restricted to an area north of the equator within the region of (0°–20°N, 100°–130°E) and is confined to the near-surface layer. The SST variation in the same region is similar to the wind pattern but lags the wind for approximately 1–5 months, which suggests that the SST variation is forced by the wind. The surface Siberian high, 500-hPa trough, and200-hPa jet stream, all representing the large-scale monsoon flow, are weaker than normal during El Niño years. In particular, the interannual variation of the Siberian high is in general agreement with the SOI.

Corresponding author address: Dr. Yi Zhang, Program for ClimateModel Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Mail Stop L-264, P.O. Box 808, Livermore, CA 94550.

Email: zhang@pcmdi.LLNL.gov

Abstract

This paper presents the climatology and interannual variation of the East Asian winter monsoon based on the 1979–95 National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. In addition to documenting the frequency, intensity, and preferred propagation tracks of cold surges and the evolution patterns of related fields, the authors discuss the temporal distribution of the Siberian high and cold surges. Further, the interannual variation of the cold surges and winter monsoon circulation and its relationship with ENSO were examined.

There are on average 13 cold surges in each winter season (October–April), of which two are strong cases. The average intensity of cold surges, measured by maximum sea level pressure, is 1053 hPa. 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 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 oceans, while others proceed southward. The signatures 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 peaks during December and January, the frequency of cold surges reaches 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 conventional cold surges, indicates that the surge frequency reaches a minimum a year after El Niño events. The second one, defined as a maximum wind event near the South China Sea, shares the same origin as the first. The surge 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 associated with La Niña and high SOI years; on the other hand, weak wind seasons are associated with El Niño and low SOI years. This pattern is restricted to an area north of the equator within the region of (0°–20°N, 100°–130°E) and is confined to the near-surface layer. The SST variation in the same region is similar to the wind pattern but lags the wind for approximately 1–5 months, which suggests that the SST variation is forced by the wind. The surface Siberian high, 500-hPa trough, and200-hPa jet stream, all representing the large-scale monsoon flow, are weaker than normal during El Niño years. In particular, the interannual variation of the Siberian high is in general agreement with the SOI.

Corresponding author address: Dr. Yi Zhang, Program for ClimateModel Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Mail Stop L-264, P.O. Box 808, Livermore, CA 94550.

Email: zhang@pcmdi.LLNL.gov

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