Geography Reference
In-Depth Information
CHAPTER 10
The General Circulation
In its broadest sense the
general circulation
of the atmosphere is usually
considered to include the totality of motions that characterizes the global scale
atmospheric flow. Specifically, the study of the general circulation is concerned
with the dynamics of climate—that is, with the temporally averaged structures of
the fields of wind, temperature, humidity, precipitation, and other meteorological
variables. The general circulation may thus be considered to consist of the flow
averaged in time over a period sufficiently long to remove the random variations
associated with individual weather systems, but short enough to retain monthly
and seasonal variations.
In the past, both observational and theoretical studies of the general circulation
concentrated on the dynamics of the zonally averaged flow. The time-averaged cir-
culation is, however, highly dependent on longitude due to longitudinally asym-
metric forcing by orography and land-sea heating contrasts. The longitudinally
dependent components of the general circulation may be separated into
quasi
-
stationary circulations
, which vary little in time,
monsoonal circulations
, which
are seasonally reversing, and various subseasonal and interannual components,
which together account for
low-frequency variability
. A complete understanding
of the physical basis for the general circulation requires an explanation not only
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