COMPARISON OF FOUR EAST ASIAN WINTER MONSOON INDICES
Gao Hui
Laboratory for Climate Studies，National Climate Center, CMA，Beijing 100081
Abstract
Monsoon circulation is an important component of the global circulation systems.
Based on the monthly NCEP/NCAR 50-year reanalysis dataset in 1958-2003, the global
gridded CPC Merged Analysis of Precipitation (CMAP) dataset in 1979-2003 and the
observed precipitation, temperature, and cold air activity frequencies at 160
stations in China in 1958-2003, the main members of the East Asian winter monsoon
(EAWM) circulation systems are summarized in this paper, and four typical EAWM
indices (EAWMI) reflecting the basic characteristics of the main member are
calculated and compared, respectively. Results show that all four indices display
similar interannual and interdecadal variation characteristics in the research
periods, and are positively, significantly correlated each other at a confidence
level of 99%, indicating that each index is able to describe the anomaly pattern
of the EAWM circulations. The power spectra of four EAWM indices reveal that the
indices all pose the most predominant interannual period of 3-4 years, the secondary
period of about 6.5 years, and the main interdecadal period of 9-15 years. Besides,
most of four indices have shown a significant decreasing trend since the
1980s.Results also indicate that in strong (weak) EAWM years, both the Siberian high
and the upper
level subtropical westerly jet are stronger (weaker), and the
Aleutian low and the East Asia trough are deeper (shallower) than normal, thus
favorable (unfavorable) for the dominance of more powerful northwesterly wind and
lower temperature over the subtropical region of East Asia. However, none of four
indices has good relationship with the frequency of cold air activities in winter
in China. Besides, after strong (weak) EAWM, less (more) summer precipitation will
be seen over the regions from the Yangtze River valley to southern Japan and more
(less) summer precipitation from the South China Sea to the tropical western Pacific.
Therefore, the EAWM also has a remarkable influence on the next summer monsoon.
Key words: East Asian winter monsoon index, General circulation, Westerly jet.