THE SEASONAL VARIATION OF THE EAST ASIAN SUBTRPICAL WESTERLY
Kuang Xueyuan
1
1，2
Zhang Yaocun
JET AND ITS THERMAL MECHANISM
1
Department of Atmospheric Sciences, Nanjing University,
2
Nanjing 210093
Guangxi Climate Center，Nanning 530022
Abstract
The seasonal variations of the intensity and location of the East Asian subtropical Westerly
Jet (EAWJ) and its thermal mechanism are analyzed by using the NCEP/NCAR monthly reanalysis data
from 1961 to 2000. It is found that the seasonal variation of the EAWJ core not only has
significantly meridional migration, but also shows the rapid latitudinal displacement during
June-July. Moreover, there exists the zonal inconsistency in the northward shift process of the
EAWJ axis, and the change in the jet intensity usually occurs before the meridional shift of the
EAWJ. The thermal mechanism of the seasonal variation of EAWJ is also examined by tracking the
core of the EAWJ. The results indicate that the annual cycle of the EAWJ matches very well with
the structure of the meridional temperature differences in the upper and middle troposphere,
suggesting that the seasonal variation is the response of EAWJ to the seasonal variation of solar
radiation, and the thermal contrast of land/sea and the megarelief of the Tibetan Plateau. Through
investigating the relation between the EAWJ and heat transports, it is found that the EAWJ weakens
and shifts northwards during the warming period from wintertime to summertime, whereas the EAWJ
intensifies and shifts southward during the cooling period from summertime to wintertime. The
meridional difference of heat advection transport is the domonant factor determining the
meridional temperature difference around the EAWJ center, and the south-north shift of the EAWJ
follows the maximum meridional gradient of the heat advection transport. During the period from
April to October the diabatic heating plays a leading role in the longitudinal displacement of
the EAWJ center, and the strong diabatic heating of the Tibetan Plateau in the mid-upper
troposphere leads to the rapid longitudinal displacement of the EAWJ center in early summer during
June to July.
Key words: East Asian subtropical，Westerly jet, Seasonal variation, Thermal mechanism.