Geoscience Reference
In-Depth Information
Australia. In summer, the southward movement
of the Intertropical Convergence Zone and its
transformation into a Monsoon Trough brings
on the wetter season in northern Australia (see
Chapter 11D), and the onshore southeast trades
bring rain along the eastern seaboard.
New Zealand is subject to climatic controls
similar to those of southern Australia ( Figure
10.33 ). Anticyclones, separated by troughs associ-
ated with cold fronts often deformed into wave
depressions, cross the region on average once a
week. Their most southerly track (38.5°S) is taken
in February. The eastward rate of anticyclonic
movement averages about 570km/day in May to
July and 780km/day in October to December.
Anticyclones occur some 7 percent of the time and
are associated with settled weather, light winds, sea
breezes and some fog. On the eastern (leading)
edge of the high pressure cell the airflow is usually
cool, maritime and southwesterly, interspersed
with south or southeasterly flow producing
drizzle. On the western side of the cell, the airflow
is commonly north or northwesterly, bringing
mild and humid conditions. In autumn, high
pressure conditions increase in frequency up to 22
percent, giving a drier season.
Simple troughs with undeformed cold fronts
and relatively simple interactions between the
trailing and leading edge conditions of the anti-
cyclones persist in about 44 percent of the time
during winter, spring and summer, compared
with only 34 percent in autumn. Wave depres-
sions occur with about the same frequency. If a
wave depression forms on the cold front to the
west of New Zealand, it usually moves south-
eastward along the front, passing to the south of
the country. In contrast, a depression forming
over New Zealand may take 36-48 hours to clear
the country, bringing prolonged rainy conditions
( Figure 10.34 ). Relief, especially the Southern
Alps, predominantly controls rainfall amounts.
West- or northwest-facing mountains receive an
average annual precipitation in excess of 2500mm,
with some parts of South Island exceeding
10,000mm (see Figure 5.16 ). The eastern lee areas
120°
130°
140°
150°E
10°
m T i
miles
0
0
400
600
km
m T p
20°
c T
30°
m P
(A) January
120°
40°S
130°
140°
150°E
10°
0
0
miles
400
600
km
20°
c T
30°
(B) July
40°S
Figure 10.32 Air-mass frequencies, source
areas, wind directions and dominance of the cT
high pressure cell over Australia in summer (above)
and winter (below).
Source: After Gentilli (1971). By permission of Elsevier
Science, NL.
anticyclones and the passage of inter-anticyclonic
fronts cause periodic inflows of surrounding
maritime tropical air masses from the Pacific
(mTp) and the Indian (mTi) Oceans. There are
also incursions of maritime polar air (mP) from
the south, and variations in strength of the local
source of continental tropical (cT) air masses (see
Figure 10.32 ).
The high pressure conditions over Australia
promote especially high temperatures over central
and western parts of the continent, towards which
there is a major heat transport in summer. These
pressures keep average rainfall amounts low; these
normally total less than 250mm annually over
37 percent of Australia. In winter, upper-air
depressions along the inter-anticyclonic fronts
bring rain to southeastern regions and also, in
conjunction with mTi incursions, to southwest
 
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