Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 66

these winds. The trade winds are a highly significant part of global atmospheric
circulation, controlling a large portion of the Earth's climate regions. The trade
winds form the equatorward limb of the Hadley circulation in each hemisphere.
They flow out of the subtropical high pressure cells as northeasterlies in the
Northern Hemisphere, and southeasterlies in the Southern Hemisphere.
As they travel across the oceans, the trade winds accumulate latent and
sensible heat. This is transported to the zone of the ITCZ where it contributes
toward massive convection which, in turn, fuels the Hadley circulation. Over
continents, the annual cycle of heating and cooling disrupts the patterns of the
trades.
Throughout much of the trade wind regions, a persistent temperature inver-
sion occurs. Known as the trade wind inversion, it is a highly stable layer that
effectively caps vertical motion, and hence deep convection seldom occurs in the
trade wind zone. The trade wind inversion is not the upper limit of the trades, but
separates the well-mixed, humid marine air from arid upper air. In fact, four
layers in the trade zone may be identified: the subcloud layer, cloud layer,
inversion layer, and free atmosphere.
The climate of the trade wind zone is remarkably consistent, in large part due
to the persistence of the trade winds and the trade wind inversion as illustrated
by weather in Hawaii and other similarly positioned oceanic island groups
(Giambelluca and Schroeder 1998 ). The inversion limits rainfall in the region
near Hawaii to around 700 mm per year. However, because of orographic lifting
of trade winds on the windward slopes, some spots in Hawaii receive spectacular
amounts. Mount Waialeale on Kauai, one of the wettest places on Earth,
averages around 11 m of rainfall annually.
3.2.4 The monsoons
In a later chapter the monsoons are used as an illustration of climatic modeling.
Accordingly, at this juncture only basic characteristics of the monsoon climates
will be discussed.
Monsoon, a term derived from an Arabic word meaning season, refers to the
conditions in which the mean surface wind reverses its direction from summer to
winter. Most regions lying between 358 Nand258 S and between 308 W and 1708 E
experience a monsoon effect. They are best developed in south and east Asia.
Generally, the summer monsoons of both the hemispheres are wet and winter
monsoons are dry. The Asian summer monsoon consists of the Indian monsoon
and the east Asian monsoon, both of which are responsible for abundant summer
rainfall. West Africa also experiences a wind reversal from southwesterly in
summer to northeasterly in winter. As a result, on the west coast of Africa the
heaviest rainfall occurs from June to August.
The east Asian winter monsoon and the north Australian summer monsoon are
intermingled because the dry winter air of the Northern Hemisphere flows across