Geoscience Reference
In-Depth Information
In monsoonal and seasonal forests, there is a true dry season, usually for four to
seven months, but that is compensated for by abundant precipitation throughout
the rest of the year.
In Tropical Forests, day length (photoperiod) and the angle at which sunlight
strikes the Earth's surface varies little throughout the year. Slight shifts can create
the seasonal weather patterns that typically occur in the Tropical Seasonal Forest
Biome. In all parts of these biomes, gradual changes in climate patterns occur
across large distances.
Global Circulation
Two major global circulation systems affect the Tropical Forests of the world. They
are the Intertropical Convergence Zone (ITCZ) and the Trade Winds. These two
systems regulate the climate and weather patterns in the tropics. With abundant
and direct sunlight, the equatorial zone is an intensive source of heat and moisture.
As airmasses are intensely heated at the Equator, they rise into the atmosphere.
This rising air creates a pressure gradient, with low pressure at the Equator and
higher pressure at the mid-latitudes. The pressure gradient causes a shifting of the
warmer airmasses toward the pole in the upper atmosphere, and a shifting of colder
airmasses from mid-latitudes toward the Equator in the lower atmosphere. This
movement of air toward the Equator occurs in the Northern and Southern hemi-
spheres and initiates air movement and the general circulation patterns around the
globe (see Figure 1.2). As these airmasses flow together at the Equator, a zone of
unstable air is created. This is called the ITCZ. The ITCZ occurs between about
5
S of the Equator, but can extend farther in certain regions. The ITCZ
moves north or south depending on the season and amount of solar energy
received. There is often a difference in the ITCZ boundary on land and over the
oceans. Above the oceans, unstable air (generating heavy rains) and unpredictable
winds, or in some cases no wind at all, are typical within the ITCZ. This zone at
sea was known as the ''doldrums'' by earlier mariners.
The movement of air from the mid-latitudes toward the Equator creates a gen-
eral wind circulation pattern north and south of the ITCZ. The typically steady
flows of air known as the Trade Winds facilitated the movement of trade from
Europe to the Americas and back again. This trade wind circulation is most promi-
nent over the Atlantic and Pacific oceans and least in the monsoon-controlled
regions surrounding the Indian Ocean. Trade wind circulation is an easterly flow
in both hemispheres—that is, the winds travel from the east toward the west. The
winds tend to converge toward the Equator into the ITCZ. The Trade Winds are
capable of bringing large amounts of rainfall, especially on the east coast of tropical
countries, such as in the Caribbean and in Central America.
On the continents, the ITCZ shifts poleward from the Equator into the sub-
tropics with the solstices. These seasonal shifts can bring strong winds and heavy
rains into certain areas north and south of the tropics. Differences in temperature
between land and sea during changing seasons causes shifts in wind patterns. These
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