Geoscience Reference
In-Depth Information
conditions of this continent were barriers to
progress. Furthermore, while the Arctic was a key
strategic region during the Cold War, leading to
extensive research and the rapid establishment of
observing networks, the Antarctic did not benefit
from Cold War activity to the same degree. Some
features had long been recognized, such as the
existence of a trough of low pressure surrounding
the continent, and persistent strong katabatic
(downslope) winds. Much progress was made
following extensive observations made during
the International Geophysical Year (IGY) of
1957-1958, which was modeled on the Inter-
national Polar Years of 1882-1883 and 1932-1933
(
Box 1.1
). A preliminary survey of the Southern
Hemisphere westerlies, based in part on upper-air
observations during the IGY, was published by
H.H. Lamb in 1959. Even today, direct observa-
tions are much more sparse over the Antarctic
than over the Arctic. Weather forecasts in this
region rely especially strongly on data collected by
earth-orbiting satellites.
Southern Hemisphere. The huge instability of
tropical airmasses means that even mild conver-
gence in the Trade Winds gives rise to atmospheric
waves travelling westward with characteristic
weather patterns.
Above the Pacific and Atlantic Oceans the ITCZ
is quasi-stationary with a latitudinal displacement
annually of 5° or less, but elsewhere it varies
between latitudes 17
N in January and
between 2°N and 27°N in July - i.e., during the
southern and northern summer monsoon seasons,
respectively. The seasonal movement of the ITCZ
and the existence of other convective influences
make the south and east Asian monsoon the most
significant seasonal global weather phenomenon.
Investigations of weather conditions over
the broad expanses of the tropical oceans were
assisted by satellite observations after about 1960.
Observations of waves in the tropical easterlies
began in the Caribbean during the mid-1940s, but
the structure of meso-scale cloud clusters and
associated storms was only recognized in the
1970s. Satellite observations also proved very
valuable in detecting the generation of hurricanes
over the great expanses of the tropical oceans.
In the late 1940s and subsequently, important
work was conducted on the relations between the
south Asian monsoon mechanism concerning the
westerly subtropical jet stream, the Himalayan
mountain barrier and the displacement of the
ITCZ. The very significant failure of the Indian
summer monsoon in 1877 had led Blanford
(1860) in India, Todd (1888) in Australia, and
others, to seek correlations between Indian
monsoon rainfall and other climatic phenomena
such as the amount of Himalayan snowfall (which
influences large-scale differential heating between
the land and ocean) and the strength of the
southern Indian Ocean high pressure center. Such
correlations were studied intensively by Sir Gilbert
Walker and his co-workers in India between about
1909 and the late 1930s. In 1924 a major advance
was made when Walker identified the 'Southern
Oscillation' - an east-west seesaw of atmospheric
pressure and resulting rainfall (i.e., negative
°
S and 8
°
G TROPICAL WEATHER
The success in modeling the life cycle of the mid-
latitude frontal depression, and its value as a
forecasting tool, naturally led to attempts in the
immediate pre-World War II period to apply it to
the atmospheric conditions which dominate the
tropics (i.e., 30°N-30°S), comprising half the
surface area of the globe. This attempt was largely
doomed to failure, as observations made during
the air war in the Pacific soon demonstrated. This
failure was due to the lack of frontal temperature
discontinuities between air masses and the
absence of a strong Coriolis effect and thus of
Rossby-like waves. Tropical airmass discontinu-
ities are based on moisture differences. Tropical
weather results mainly from strong convectional
features such as heat lows, tropical cyclones
(hurricanes and typhoons) and the Inter-Tropical
Convergence Zone (ITCZ), the axis of which
represents the dividing line between the southeast
and northeast Trade Winds of the Northern and
