Geoscience Reference
In-Depth Information
mA
mA
BAI-U SEASON
MIDSUMMER
cP
cP
L
H
mP
H
40°N
40°N
L
30°
30°
H
mT
20°
20°
mT
H
110°
120°
130°
140°
150°E
110°
120°
130°
140°
150°E
Figure 11.35
Schematic surface circulation pattern and frontal locations (Siberian-Canadian Arctic Front S-C AF,
Eurasian Polar Front EPF, Pacific Polar Front PPF and Monsoon Trough MT/ Inter-tropical Convergence Zone
MT/ITCZ) over East Asia during the Bai-u (i.e., July-August) season.
Source: Matsumoto (1985). Reproduced by permission, University of Tokyo.
originating in a southwesterly stream that is the
extension of the monsoon circulation of Southeast
Asia (
Figure 11.23
). The southeast circulation
is displaced westward from Japan by a zonal
expansion of the subtropical anticyclone during
late July and August, giving a period of more
settled, sunny weather. The secondary precipi-
tation maximum of the Shurin season during
September and early October coincides with an
eastward contraction of the Pacific subtropical
anticyclone, allowing low pressure systems and
typhoons from the Pacific to swing north towards
Japan. Although much of the Shurin rainfall is of
typhoon origin (see
Figure 11.36
), some is
undoubtedly associated with the southern sides
of depressions moving along the southward-
migrating Pacific Polar Front to the north (see
Figures 11.22
and
11.35
), because there is a
marked tendency for the autumn rains to begin
first in the north of Japan and to spread south-
ward. The manner in which the location of the
western margin of the North Pacific subtropical
high pressure cell affects the climates of China
and Japan is well illustrated by the changing
seasonal trajectories of typhoon paths over East
Asia (
Figure 11.37
). The northward and south-
ward migrations of the cell zonal axis through 15
°
of latitude, the northwestern high pressure cell
extensions over eastern China and the Sea of Japan
in August, and its southeastern contraction in
October are especially marked.
Northern Australia experiences a monsoon
regime during the austral summer. Low-level
westerlies develop in late December associated
with a thermal low over northern Australia.
Analogous to the vertical wind structure over
Asia in July, there are easterlies in the upper
troposphere. Various wind and rainfall criteria
have been used to define monsoon onset. Based on
the occurrence of (weighted) surface to 500mb
westerly winds, overlain by 300-100mb easterlies
at Darwin (12.5°S, 131°E), the main onset date
is 28 December and retreat date 13 March.
Despite an average duration of 75 days, monsoon
