Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 65

The classical explanation for causation and location of the STHPs suggests
that air rising at the equator flows poleward aloft and converges at about 208 to
308 N and S. At those latitudes, convergence of air aloft creates a high pressure at
the surface. However, in order for the STHPs to be as persistent as they are, the
winds aloft, the antitrades, would need to be constant. It just so happens that the
antitrades are frequently absent, so that this process alone could not result in
the STHPs.
Another potential cause relies upon the differential heating and cooling of air
in the cells. In the poleward flows of the Hadley Cells, continuous longwave
radiation to space causes upper air cooling. The more distant from the equator,
the more the air will become cooler and denser. Following this, a higher pressure
will occur at all levels of the system resulting in the creation of the surface high
pressure systems. The cold water of cold currents on the eastern sides of the cells
(and the cold air over continents, when the cells expand in winter) would
enhance this effect.
The STHPs have also been attributed to middle-latitude dynamics. At the
boundary between polar and tropical air (polar front), changes in Coriolis force
would cause anticyclonic cells to move equatorwards while low pressure sys-
tems would move polewards. These cold equatorward-moving anticyclones
would reinforce and regenerate the STHPs. At the very least, the infusion of
the cold anticyclonic air would provide an explanation of short-term changes in
intensity of the STHPs.
Clearly each of these explanations is insufficient in itself. McIlveen ( 1992 ,
p. 417) expertly summarizes the interaction of these processes and his wording
is used here. ''During the development of a particular subtropical high-pressure
cell, convergence aloft obviously exceeds divergence below. However, during
long quiet periods in which the intensity of an anticyclone changes little,
divergence and convergence must balance closely in a dynamic equilibrium.
No simple argument explains the continuing presence of the high surface
pressure, despite the unwise claims of some descriptive texts. For example it is
not the direct consequence of the downward motion of air in the main mass of
air, since this would imply a gross failure of the hydrostatic approximation,
which actually must be especially accurate in the quiet conditions prevailing
there. It seems that dynamic constraints such as the Coriolis effect maintain the
excess of atmospheric mass in the central parts of the anticyclone, once it has
been established by the initial excess of convergence. The relative warmth of
subsiding air means that air pressure falls more slowly with increasing height
than is the case in the surrounding cooler air masses, with the result that the
upward doming of isobaric surfaces increases with height, often into the upper
troposphere.''
Easterly winds of the tropical oceans, both north and south of the equator,
have long been known as trade winds. The word ''trade'' is probably derived
from an obsolete form, meaning ''track'', and refers to the persistent direction of