Berichte zur Erdsystemforschung - Max-Planck-Institut für ...

Intertropical Convergence Zone (ITCZ). In contrast, the East Asian summer monsoon is
controlled by the monsoon trough on the Asian continent and the western Pacific
subtropical high. It is, therefore, characterised by a zonal pressure-gradient and has to be
seen in association with the Walker Circulation (Webster et al., 1998). As a result of the
east-west thermal contrast, a planetary subtropical front is formed (Meiyu-Baiu frontal
system) whose seasonal march determines the propagation of the East Asian summer
monsoon rainbelt (e.g. Ding, 2007). The subtropical front is separated from the ITCZ,
which forms a second rainband in East Asia (tropical rainbelt) affecting mainly the western
North Pacific monsoon.
According to Ding and Chang (2005), Ding (2007) and He et al. (2007), the summer
monsoon season first starts in the equatorial East Indian Ocean and Sumatra in late April.
In a second stage beginning early to mid of May, the monsoon expands to the Bay of
Bengal, southern Indochina and the South China Sea. From these regions, two branches
move north-west- and north-eastward to South China (mid and late May) and the Indian
subcontinent (early to mid June) establishing the onset of the East Asian and the Indian
summer monsoon, respectively. The Indian monsoon then penetrates further inland
arriving in Northwest India in the mid of July. The East Asian monsoon moves northward
in a stepwise manner that is characterised by abrupt jumps from one region to another. In
early June, it arrives in the Yangtze River valley and Japan bringing the so called Meiyu
and Baiu rain, respectively. It then jumps to Northeast China and the Korean Peninsula
(Changma rainy season) in early and mid-July where it stays until the end of August or
early September before it starts to continuously move back to the South China Sea. The
Indian monsoon terminates not until late September.
The East Asian monsoon system differs from the Indian monsoon also in other respects.
The interplay of the mid-latitude westerlies, the subtropical circulation around the western
Pacific subtropical high and the tropical circulation including the Indian monsoon flow
makes the East Asian monsoon domain unique and enables the occurrence of many
significant weather and climate phenomena (Ding, 2007). In contrast to other monsoon
regions, the precipitation in the East Asian monsoon domain is not temporally confined to
the summer monsoon season. The wintertime pressure-gradient between the Siberian high
and the Aleutian low induces cold northerly winds embedding regularly cold-air outbreaks
(Wang B., 2006) in the entire East Asian monsoon region. Therefore, the East Asian
monsoon domain is not characterised by the alternation of wet and dry seasons but rather
by the alternation of particularly wet and cold seasons.
Recent studies have shown that climate models reveal large deficits in representing the
mean precipitation distribution, the season cycle and the interannual variability of the
Asian monsoon system (Kang et al. 2002; Zhou et al., 2009; Boo et al., 2011). In contrast,
the temperature distribution and monsoon circulation can be captured by the models.
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