Geoscience Reference
In-Depth Information
the conservation of potential vorticity by the air. About 10
per cent of wave disturbances later intensify to become
tropical storms or cyclones. This development requires a
warm sea surface and low-level convergence to maintain
the sensible and latent heat supply and upper-level
divergence to maintain ascent. Cumulonimbus 'hot
towers' nevertheless account for a small fraction of the
spiral cloud bands. Tropical cyclones are most numerous
in the western oceans of the northern hemisphere in the
summer to autumn seasons.
The monsoon seasonal wind reversal of South Asia
is the product of global and regional influences. The
orographic barrier of the Himalayas and Tibetan Plateau
plays an important role. In winter, the subtropical westerly
jet stream is anchored south of the mountains. Subsidence
occurs over northern India, giving northeasterly
surface (trade) winds. Occasional depressions from the
Mediterranean penetrate to northwestern India-Pakistan.
The circulation reversal in summer is triggered by the
development of an upper-level anticyclone over the
elevated Tibetan Plateau with upper-level easterly flow
over India. This change is accompanied by the northward
extension of low-level southwesterlies in the Indian Ocean,
which appear first in southern India and along the Burma
coast and then extend northwestward. The summer
'monsoon' over East Asia also progresses from southeast
to northwest, but the Mai-yu rains are mainly a result of
depressions moving northeastward and thunderstorms.
Rainfall is concentrated in spells associated with 'monsoon
depressions', which travel westward steered by the upper
easterlies. Monsoon rains fluctuate in intensity, giving rise
to 'active' and 'break' periods in response to southward
and northward displacements of the monsoon trough,
respectively. There is also considerable year-to-year
variability.
The West African monsoon has many similarities to
that of India, but its northward advance is unhindered by
a mountain barrier to the north. Four zonal climatic belts,
related to the location of overlying easterly jet streams and
east-west-moving disturbances, are identified. The Sahel
zone is reached by the monsoon trough, but overlaying
subsiding air greatly limits rainfall.
The climate of equatorial Africa is influenced strongly by
low-level westerlies from the South Atlantic high (year-
round) and easterlies in winter from the South Indian
Ocean anticyclone. These flows converge along the Zaire
air boundary (ZAB) with easterlies aloft. In summer, the
ZAB is displaced southwards and northeasterlies over
the eastern Pacific meet the westerlies along the ITCZ,
oriented north-south from 0° to 12°S. The characteristics
of African disturbances are complex and barely known.
Deep easterly flow affects most of Africa south of 10°S
(winter) or 15 to 18°S (summer), although the southern
westerlies affect South Africa in winter.
In Amazonia, where there are broad tropical easterlies
but no well-defined ITCZ, the subtropical highs of the
North and South Atlantic both influence the region.
Precipitation is associated with convective activity triggering
low-level convergence, with meso- to synoptic-scale
disturbances forming in situ, and with instability lines
generated by coastal winds that move inland.
The equatorial Pacific Ocean sector plays a major role
in climate anomalies throughout much of the tropics. At
irregular, three- to five-year intervals, the tropical easterly
winds over the eastern-central Pacific weaken, upwelling
ceases off South America and the usual convection over
Indonesia shifts eastward towards the central Pacific. Such
warm ENSO events, which replace the normal La Niña
mode, have global repercussions since teleconnection links
extend to some extratropical areas, particularly East Asia
and North America.
Variability in tropical climates also occurs through
diurnal effects, such as land-sea breezes, local topographic
and coastal effects on airflow, and the penetration of extra-
tropical weather systems and airflow into lower latitudes.
Short-range tropical weather prediction is commonly
limited by sparse observations and the poorly understood
disturbances involved. Seasonal predictions show some
success for the evolution of the ENSO regime, Atlantic
hurricane activity and West African rainfall.
