Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 60

Time of year

Figure 3.1 The
relationship between annual
and diurnal energy cycles in
the tropics. (After Oliver and
Hidore 2002 )

January July January July January

14 8

diurnal
cycle

annual
cycle

0 Mean 0

-14 -8

PM AM PM AM PM AM PM AM PM

Time of day

AM PM AM PM AM PM AM PM

night to which plants must adjust, varies between 11 and l3 hours from winter to
summer. Though solar radiation is relatively high all year, it is sometimes not as
high as it is in mid-latitudes, particularly in summer. Cloud cover in wet
equatorial lands usually reflects more than half of the incident total solar
radiation.
In relation to the energy inputs, annual temperatures average about the same
throughout the moist tropical regions and the annual range in temperature
depends on the length of the dry season. Where there is no dry season, the annual
range in mean monthly temperatures may be as little as one or two degrees.
Where there is dry weather in the winter months the mean temperatures decrease
to increase the annual temperature range.
In equatorial climates the primary energy flux is diurnal (Figure 3.1 ) and the
variation in temperature from day to night is greater than the variation from
season to season. Tropical regions are not places of continuous high tempera-
tures. Nights can be rather cool so that the diurnal radiation and temperature
cycles are more important than the annual temperature cycle as a regulator of life
cycles.
Given these characteristics, it is not surprising that rainfall rather than tem-
perature determines the seasons and it is the amount and timing of rainfall that
form the chief criterion for distinguishing the various climates. The largest
portion of tropical environments has a marked seasonal regime of rainfall that
governs the biological productivity of the system. The remaining areas are
deserts, where rainfall is incidental throughout the year. It is, in fact, the seasonal
moisture pattern that distinguishes the major tropical environments - the rain
forest, the savannas, and the desert - from each other.
Essentially, the dominant dynamic controls of tropical climates are the inter-
tropical convergence zone (ITC or ITCZ) and the subtropical high pressure
systems. A further aspect over large areas of the tropical world concerns the
seasonal change of winds to give the monsoon climates.