Geoscience Reference
In-Depth Information
the Rossby waves. The most important jet stream,
located at 10 km, clearly affects surface weather by
guiding the low pressure systems which tend to form
beneath it. In addition, air subsiding beneath the jet
streams strengthens the subtropical high pressure cells.
Asian monsoon mechanism in relation to 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 and the strength of
the southern Indian Ocean high pressure centre. 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 correlation) between Indonesia
and the eastern Pacific. Other north-south climatic
oscillations were identified in the North Atlantic
(Azores vs. Iceland) and the North Pacific (Alaska vs.
Hawaii). In the phase of the Southern Oscillation when
there is high pressure over the eastern Pacific, westward-
flowing central Pacific surface water, with a consequent
upwelling of cold water, plankton-rich, off the coast
of South America, are associated with ascending air,
gives heavy summer rains over Indonesia. Periodically,
weakening and breakup of the eastern Pacific high
pressure cell leads to important consequences. The chief
among these are subsiding air and drought over India
and Indonesia and the removal of the mechanism of the
cold coastal upwelling off the South American coast
with the consequent failure of the fisheries there. The
presence of warm coastal water is termed 'El Niño'.
Although the central role played by lower latitude high
pressure systems over the global circulations of atmos-
phere and oceans is well recognized, the cause of the
east Pacific pressure change which gives rise to El Niño
is not yet fully understood. There was a waning of
interest in the Southern Oscillation and associated
phenomena during the 1940s to mid-1960s, but the work
of Berlage (1957), the increase in the number of Indian
droughts during the period 1965 to 1990, and especially
the strong El Niño which caused immense economic
hardship in 1972, led to a revival of interest and
research. One feature of this research has been the
thorough study of the 'teleconnections' (correlations
between climatic conditions in widely separated regions
of the earth) pointed out by Walker.
F TROPICAL WEATHER
The success in modelling the life cycle of the mid-
latitude frontal depression, and its value as a forecasting
tool, naturally led to attempts in the immediate pre-
Second World War 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 doomed largely to failure, as obser-
vations 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 discontinuities are
based on moisture differences, and tropical weather
results mainly from strong convectional features such
as heat lows, tropical cyclones and the intertropical
convergence zone (ITCZ). The huge instability of trop-
ical airmasses means that even mild convergence in the
trade winds gives rise to atmospheric waves travelling
westward with characteristic weather patterns.
Above the Pacific and Atlantic Oceans the inter-
tropical convergence zone is quasi-stationary with
a latitudinal displacement annually of 5° or less, but
elsewhere it varies between latitudes 17°S and 8°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 mesoscale cloud clusters
and associated storms was recognized only 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, most important
work was conducted on the relations between the south
