Based on NCEP/NCAR reanalysis data and by using harmonic analysis method, extra-long waves have been separated out from the parallel of latitude band of 30S-30N. Then employing the circulation diagnostic method, we obtained the stream function field and the potential function field, from which, the regional circulation was found in the SCS monsoon region. The flow pattern distributes a alternation of cyclone and anticyclone alone parallel of latitude, and the pattern of heat source, also, distributes alone parallel of latitude.
In winter, anticyclonic circulation is taken on in the low and middle level of troposphere, on both sides of which are cyclonic circulations, however, a opposite pattern appears at 200hPa level. A divergence field presents in the low and middle level, and a convergence field is on either side. And also, the pattern reverses at 200hPa level. From the above pattern, we can see that the SCS is a cold source, and both sides are heat source, which is entirely different to the pattern of the general circulation of atmosphere.
In summer, cyclonic circulation appears in low and middle level, on both sides of which are anticyclonic circulations. Noticeably, the pattern of the potential function field is basicly the same to that in winter, so the pattern of the cold and heat sources in this region is relatively stable in a whole year.
3-dimension structure of the regional circulation was given in this article. The pattern of potential function field has no obviously variety in both winter and summer. While, the pattern of stream function field of low and middle troposphere in winter is opposite to that in summer, however, they are basic similar each other in high level. So the regional circulation of the SCS in a whole year is more like that in winter. The structure of the regional circulation supplies a background, which is why the spatial pattern of air-sea coupled oscillation in a whole year is mostly similar to that in winter in the SCS monsoon region.
The summer monsoon in SCS breaks out earlier in the north than that in the south. Maybe some important gist could be found, if the problem is considered on the aspect of the structure of regional circulation. The component of ageostrophic wind is S-SW at low and middle level in the north of the SCS, while N-NE in the south. The stability of the component of ageostrophic wind is benefit to the earlier formation of S-SW stream field in the north.
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