Geoscience Reference
In-Depth Information
11
Tropical weather and climate
Learning objectives
When you have read this chapter you will:
Understand the characteristics and significance of the intertropical convergence zone,
Be familiar with the principal weather systems that occur in low latitudes and their distribution,
Know some of the diurnal and local effects that influence tropical weather,
Know where and how tropical cyclones tend to occur,
Understand the basic mechanisms and characteristics of El Niño and La Niña events.
Tropical climates are of especial geographical interest
because 50 per cent of the surface of the globe lies
between latitudes 30°N and 30°S, and over 75 per cent
of the world's population inhabit climatically tropical
lands. This chapter first describes the trade wind
systems, the intertropical convergence zone and tropical
weather systems. The major monsoon regimes are then
examined and the climate of Amazonia. The effects
of the alternating phases of the El Niño - Southern
Oscillation in the equatorial Pacific Ocean are discussed
as well as other causes of climatic variation in the
tropics. Finally, the problems of forecasting tropical
weather are briefly considered.
The latitudinal limits of tropical climates vary greatly
with longitude and season, and tropical weather condi-
tions may reach well beyond the Tropics of Cancer and
Capricorn. For example, the summer monsoon extends
to 30°N in South Asia, but to only 20°N in West Africa,
while in late summer and autumn tropical hurricanes
may affect 'extra-tropical' areas of East Asia and eastern
North America. Not only do the tropical margins extend
seasonally poleward, but also in the zone between the
major subtropical high-pressure cells there is frequent
interaction between temperate and tropical disturbances.
Elsewhere and on other occasions, as illustrated in Plate
23 over the western North Pacific, distinct tropical and
mid-latitude storms are observed. In general, however,
the tropical atmosphere is far from being a discrete
entity and any meteorological or climatological bound-
aries must be arbitrary. There are nevertheless a number
of distinctive features of tropical weather, as discussed
below.
Several basic factors help to shape tropical weather
processes and also affect their analysis and interpreta-
tion. First, the Coriolis parameter approaches zero at
the equator, so that winds may depart considerably
from geostrophic balance. Pressure gradients are also
generally weak, except for tropical storm systems. For
these reasons, tropical weather maps usually depict
streamlines, not isobars or geopotential heights. Second,
temperature gradients are characteristically weak.
Spatial and temporal variations in moisture content are
