Geography Reference
In-Depth Information
The low-level convergence and upper-level divergence over the continent con-
stitute a secondary circulation that concentrates cyclonic vorticity at the lower
levels and anticyclonic vorticity at the upper levels. Thus, the vorticity adjusts
toward geostrophic balance. From Fig. 11.9 it is clear that a positive correlation
exists between the vertical motion and the temperature field. Therefore, the mon-
soon circulation converts eddy potential energy to eddy kinetic energy, just as
midlatitude baroclinic eddies do.
Unlike the case of baroclinic eddies, however, the primary energy cycle of the
monsoons does not involve the zonal mean potential or kinetic energy. Rather,
eddy potential energy is generated directly by diabatic heating (latent and radia-
tive heating); the eddy potential energy is converted to eddy kinetic energy by a
thermally direct secondary circulation; and the eddy kinetic energy is frictionally
dissipated. (A portion of the eddy kinetic energy may be converted to zonal kinetic
energy.) In a dry atmosphere, monsoon circulations would still exist; however,
because the diabatic heating would then be confined to a shallow layer near the
surface, they would be much weaker than the observed monsoons. The presence
of cumulus convection and its concomitant latent heat release greatly amplifies
the eddy potential energy generation and makes the summer monsoons among the
most important features of the global circulation.
In the winter season the thermal contrast between the land and the sea reverses
so that the circulation is just opposite to that shown in Fig. 11.9. As a result the
continents are cool and dry, and the precipitation is found over the relatively warm
oceans.
11.1.5
The Walker Circulation
The pattern of diabatic heating in the equatorial regions exhibits strong departures
from zonal symmetry. These are caused by longitudinal variations in sea surface
temperature due mainly to the effects of wind-driven ocean currents. Such SST
variations produce zonally asymmetric atmospheric circulations, which in some
regions dominate over the Hadley circulation. Of particular significance is the east-
west overturning along the equator, which is shown schematically in Fig. 11.10.
Several overturning cells are indicated, which are associated with diabatic heating
over equatorial Africa, Central and South America, and the Maritime Continent
(i.e., the Indonesian region). The dominant cell in both zonal scale and ampli-
tude, however, is that in the equatorial Pacific. This cell is referred to as Walker
circulation , after G. T. Walker who first documented the surface pressure pattern
associated with it.
As suggested by Fig. 11.10, this pressure pattern consists of low surface pressure
in the western Pacific and high surface pressure in the eastern Pacific. The resulting
westward-directed pressure gradient force drives mean surface easterlies in the
equatorial Pacific, which are much stronger than the zonal-mean surface easterlies,
Search WWH ::




Custom Search