Geoscience Reference
In-Depth Information
New Zealand is subject to climatic controls similar to
those of southern Australia (Figure 10.33). Anticyclones,
separated by troughs associated 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 570 km/day in May to July
and 780 km/day in October to December. Anticyclones
occur some 7 per cent 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 per cent, giving a drier
season.
Simple troughs with undeformed cold fronts and
relatively simple interactions between the trailing
and leading edge conditions of the anticyclones persist
in about 44 per cent of the time during winter, spring and
summer, compared with only 34 per cent in autumn.
Wave depressions occur with about the same frequency.
If a wave depression forms on the cold front to the west
of New Zealand, it usually moves southeastward along
the front, passing to the south of the country. In contrast,
a depression forming over New Zealand may take thirty-
six to forty-eight hours to clear the country, bringing
prolonged rainy conditions (e.g. 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
2500 mm, with some parts of South Island exceeding
10 000 mm (see Figure 5.15). The eastern lee areas have
much lower amounts, with less than 500 mm in some
parts. North Island has a winter precipitation maximum,
but South Island, under the influence of depressions in
the southern westerlies, has a more variable seasonal
maximum.
A
B
Figure 10.32
Airmass frequencies, source areas, wind directions
and dominance of the cT high-pressure cell over Australia in
summer (A) and winter (B).
Source
: After Gentilli (1971).
addition, there are incursions of maritime polar air (mP)
from the south, and variations in strength of the local
source of continental tropical (cT) airmasses (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 250
mm annually over 37 per cent of Australia. In winter,
upper-air depressions along the inter-anticyclonic fronts
bring rain to southeastern regions and also, in conjunc-
tion with mTi incursions, to southwest 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.
D HIGH LATITUDES
1 The southern westerlies
The strong zonal airflow in the belt of the southern west-
erlies, which is apparent only on mean monthly maps,
is associated with a major frontal zone characterized
