Geoscience Reference
In-Depth Information
such as the tendency for anticyclonic weather in mid-
September, have been recognized in Britain, and major
seasonal trends in occurrence of airflow regimes can be
used to define five natural seasons. Abnormal weather
conditions (synoptic anomalies) are associated particularly
with blocking anticyclones, which are especially prevalent
over Scandinavia and may give rise to cold, dry winters and
warm, dry summers.
The climate of North America is similarly affected by
pressure systems that generate airmasses of varying
seasonal frequency. In winter, the subtropical high-
pressure cell extends north over the Great Basin with
anticyclonic cP air to the north over Hudson Bay. Major
depression belts occur at about 45 to 50°N, from the
central USA to the St Lawrence, and along the east coast
of Newfoundland. The Arctic front is located over north-
west Canada, the polar front lies along the northeast coast
of the United States, and between the two a maritime
(Arctic) front may occur over Canada. In summer, the
frontal zones move north, the Arctic front lying along the
north coast of Alaska, Hudson Bay and the St Lawrence
being the main locations of depression tracks. Three major
North American singularities concern the advent of spring
in early March, the midsummer northward displacement
of the subtropical high-pressure cell, and the Indian
summer of September to October. In western North
America, the coast ranges inhibit the eastward spread of
precipitation, which may vary greatly locally (e.g. in British
Columbia), especially as regards snowfall. The strongly
continental interior and east of the continent experiences
some moderating effects of Hudson Bay and the Great
Lakes in early winter, but with locally significant snow belts.
The climate of the east coast is dominated by continental
pressure influences. Cold spells are produced by winter
outbreaks of high-latitude cA/cP air in the rear of cold
fronts. Westerly airflow gives rise to chinook winds in the
lee of the Rockies. The major moisture sources of the Gulf
of Mexico and the North Pacific produce regions of differ-
ing seasonal regime: the winter maximum of the west coast
is separated by a transitional intermontane region from the
interior, with a general warm season maximum; the north-
east has a relatively even seasonal distribution. Moisture
gradients, which strongly influence vegetation and soil
types, are predominantly east-west in central North
America, in contrast to the north-south isotherm pattern.
The semi-arid southwestern United States comes
under the complex influence of the Pacific and Bermudas
high-pressure cells, having extreme rainfall variations, with
winter and summer maxima due mainly to depression and
local thunderstorms, respectively. The interior and east
coast of the United States is dominated by westerlies in
winter and southerly thundery airflows in summer.
The subtropical margin of Europe consists of the
Mediterranean region, lying between the belts dominated
by the westerlies and the Saharan-Azores high-pressure
cells. The collapse of the Azores high-pressure cell in
October allows depressions to move and form over the
relatively warm Mediterranean Sea, giving well-marked
orographic winds (e.g. mistral) and stormy, rainy winters.
Spring is an unpredictable season marked by the collapse
of the Eurasian high-pressure cell to the north and the
strengthening of the Saharan-Azores anticyclone. In
summer, the latter gives dry, hot conditions with strong
local southerly airstreams (e.g. scirocco). The simple
winter rainfall maximum is most characteristic of the
eastern and southern Mediterranean, whereas in the north
and west, autumn and spring rains become more
important. North Africa is dominated by high-pressure
conditions. Infrequent rainfall may occur in the north with
extratropical systems and to the south with Saharan
depressions.
Australian weather is determined largely by travelling
anticyclone cells from the southern Indian Ocean and
intervening low-pressure troughs and fronts. In the winter
months, such frontal troughs give rains in the southeast.
The climatic controls in New Zealand are similar to those
in southern Australia, but South Island is greatly influenced
by depressions in the southern westerlies. Rainfall amounts
vary strongly with the relief.
The southern westerlies (30 to 40° to 60 to 70°S)
dominate the weather of the Southern Ocean. The strong,
mean zonal flow conceals great day-to-day synoptic
variability and frequent frontal passages. The persistent
low-pressure systems in the Antarctic trough produce the
highest year-round zonally averaged global cloudiness.
The Arctic margins have six to nine months of snow
cover and extensive areas of permanently frozen ground
(permafrost) in the continental interiors, whereas the
maritime regions of northern Europe and northern
Canada-Alaska have cold, stormy winters and cloudy,
milder summers influenced by the passage of depressions.
Northeast Siberia has an extreme continental climate.
The Arctic and Antarctic differ markedly because of the
types of surface - a perennially ice-covered Arctic Ocean
