Geoscience Reference
In-Depth Information
Indian Ocean in winter (December to February),
spring (April) and autumn (September), together
with those associated with active and break periods
during the June to August summer monsoon.
June-Aug (Active)
J
W
15
J
E
J
W
10
5
0
40°S
20°S
Eq
20°N
40°N
1 Winter
Near the surface, this is the season of the out-
blowing 'winter monsoon', but aloft westerly
airflow dominates. This reflects the hemispheric
pressure distribution. A shallow layer of cold, high
pressure air is centered over the continental
interior, but this has disappeared even at 700mb
(see
Figure 7.4
) where there is a trough over East
Asia and zonal circulation over the continent. The
upper westerlies split into two currents to the
north and south of the high Tibetan (Qinghai-
Xizang) Plateau (
Figure 11.17
), to reunite again
off the east coast of China (
Figure 11.18
). The
plateau, which exceeds 4000m over a vast area, is
a tropospheric cold source in winter, particularly
over its western part, although the strength of this
source depends on the extent and duration of
snow cover (snow-free ground acts as a heat
source for the atmosphere in all months). Below
600mb, the tropospheric heat sink gives rise to a
shallow, cold plateau anticyclone, which is best
developed in December and January. The two jet-
stream branches have been attributed to the
disruptive effect of the topographic barrier on the
airflow, but this is limited to altitudes below about
4km. In fact, the northern jet is highly mobile and
may be located far from the Tibetan Plateau. Two
currents are also observed farther west, where
there is no obstacle to the flow. The branch over
northern India corresponds to a strong latitudinal
thermal gradient (from November to April) and
it is probable that this factor, combined with the
thermal effect of the barrier to the north, is
responsible for the anchoring of the southern jet.
This branch is the stronger, with an average speed
of more than 40m s
-1
at 200mb, compared with
about 20-25m s
-1
in the northern one. Where the
two unite over north China and south Japan the
average speed exceeds 66m s
-1
(
Figure 11.19
).
June-Aug (Break)
15
J
E
J
W
J
W
10
5
0
40°S
20°S
Eq
20°N
40°N
Sept
15
J
W
J
E
10
5
0
40°S
20°S
Eq
20°N
40°N
Dec-Feb
15
J
W
J
E
J
W
10
5
0
40°S
20°S
Eq
20°N
40°N
April
15
10
5
0
40°S
20°S
Eq
20°N
40°N
Figure 11.16
Schematic representation of the meridional
circulation over India at 90°E at five characteristic periods of
the year: winter monsoon (December to February); approach
of the monsoon season (April); the active summer monsoon
(June to August); a break in the summer monsoon (June
to August); and the retreat of the summer monsoon
(September). Easterly (JE) and westerly (JW) jet streams are
shown at sizes depending on their strength; the arrows mark
the positions of the overhead sun; and zones of maximum
precipitation are indicated.
Source: After Webster (1987a). Copyright © 1987. Reproduced by kind
permission of John Wiley and Sons, Inc.
Air subsiding beneath this upper westerly
current gives dry out-blowing northerly winds
from the subtropical anticyclone over northwest
India and Pakistan. The surface wind direction
is northwesterly over most of northern India,
