Geoscience Reference
In-Depth Information
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 south-westerlies in the Indian Ocean, which appear first in southern India and along the
Burma coast and then extend northwestwards. 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 strongly influenced 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 southward and northeasterlies over eastern Africa meet the westerlies along the ITCZ,
oriented north-south from 0
°
to 12
°
S. The characteristics of African disturbances are complex and
poorly known. Deep easterly flow affects most of Africa south of 10
°
S (winter) or 15-18
°
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.