Geography Reference
In-Depth Information
circulations. The diabatic heating associated with tropical precipitation not only
drives a local response in the atmospheric circulation, but through excitation of
equatorial waves may also induce a remote response. Thus, there is a strong inter-
action among cumulus convection, mesoscale, and large-scale circulations, which
is of primary importance for understanding tropical motion systems. Furthermore,
the distribution of diabatic heating in the tropics is strongly influenced by sea sur-
face temperature (SST) variations, and these in turn are strongly influenced by the
motion of the atmosphere.
An understanding of tropical circulations thus requires consideration of equato-
rial wave dynamics, interactions of cumulus convection and mesoscale circulations
with large-scale motions, and air-sea interactions. Detailed treatment of all these
topics is beyond the scope of an introductory text. Nevertheless, because the tropics
play a fundamental role in the general circulation of the atmosphere, and coupling
between tropical and middle latitudes is an important consideration in extratropical
extended-range forecasting, some discussion of the tropics is required even in a
text with an extratropical emphasis.
Of course, it is not always possible to distinguish clearly between tropical and
extratropical systems. In subtropical regions (
30
◦
latitude), circulation systems
characteristic of both tropical and extratropical regions may be observed depending
on the season and geographical location. To keep the discussion in this chapter as
simple as possible, we focus primarily on the zone well equatorward of 30
◦
latitude
where the influence of middle latitude systems should be minimum.
∼
11.1
THE OBSERVED STRUCTURE OF LARGE-SCALE TROPICAL
CIRCULATIONS
Due to the nature of their energy sources, as well as the smallness of the Coriolis
parameter, large-scale equatorial motion systems have several distinctive charac-
teristic structural features that are quite different from those of midlatitude systems.
Many of these can be understood in terms of the equatorial wave modes discussed
in Section 11.3. Before discussing equatorial wave theory, however, it is useful to
review some of the major observed circulation features of the tropical atmosphere.
11.1.1
The Intertropical Convergence Zone
Traditionally, the tropical general circulation was thought to consist of a thermally
direct Hadley circulation in which air in the lower troposphere in both hemispheres
moved equatorward toward the
intertropical convergence zone
(ITCZ) where by
continuity considerations it was forced to rise uniformly and move poleward, thus
transporting heat away from the equator in the upper troposphere in both hemi-
spheres. This simple model of large-scale overturning is not, however, consistent
with the observed vertical profile of equivalent potential temperature (θ
e
).As
indicated in Fig. 11.1 the mean tropical atmosphere is conditionally stable above
