Environmental Engineering Reference
In-Depth Information
THE EQUATORIAL TROUGH
The equatorial trough, or Intertropical Convergence Zone (ITCZ), is a shallow trough of
low pressure generally situated near the equator. Over the oceans it is fairly static,
because seasonal temperature changes are small. In the Pacific, for example, its average
position varies by no more than 5° of latitude within the course of a single year (Figure
6.1). The situation is very different over the continents. During summer in continental
areas the trough sweeps polewards, reaching 30° or even 40° latitude over eastern China.
Behind the trough the winds are predominantly westerly and are the main rain-bearing
winds to most of those areas. Where they reach into higher latitudes they are called
monsoons (an Arabic word meaning 'season') and they show an almost complete reversal
of direction from summer to winter, a change that tends to occur with uncanny regularity
about the same dates each year.
With the exception of the monsoon, the winds in the equatorial trough tend to be light
and variable and, because sailors often found themselves becalmed there, the area became
known as the Doldrums.
THE TRADE WINDS
The trade-wind belts lie between the equatorial trough and the subtropical highs (Figure
6.14). This zone occupies nearly half the globe, much of it ocean, and within that area the
steady trade winds provide a stable and relatively constant climate. At the surface the
winds have a component towards the equator, this being from the north-east in the
northern and from the south-east in the southern hemisphere. Above the surface friction
layer the winds become more easterly.
Viewed from the air, the oceanic trade winds contain innumerable uniform small
clouds, all with a similar base and depth (see Plate 7.4). These are the visible expression
of the transfer of latent heat from the sea surface, through evaporation, before
condensation at higher levels.
As we have noted, the seasonal movement of the equatorial trough is slight over the
oceans, so the oceanic tropical areas are dominated by the trades. On the continents the
trades are far more restricted in extent, and the equatorial westerlies and monsoons are
more important. The two belts interact closely; it is the convergence of moisture in the
trade winds that feeds the equatorial trough. The shift in the position of the trough thus
determines the relative extent of the easterlies and westerlies. When the trough is farther
north, with the overhead sun in July, the trades are restricted in the northern
GENERAL CIRCULATION MODELS
applications
The rapid development of computing power has made it possible to model the physical
processes which operate in the atmosphere and the oceans and to simulate Earth's
atmospheric circulation realistically. Such dy namic models are called
general circulation
models
(GCMs).
A GCM uses mathematics and the laws of physics to describe the operation of the
