Geoscience Reference
In-Depth Information
most important controls on atmospheric circulation described below. In approximate
descending order of importance, these controls can be classified as follows.
Planetary interrelationship
Latitudinal differences in solar energy input
The Earth rotates around an axis that is (when averaged over the whole year)
perpendicular to the plane in which the Earth moves around the Sun. Consequently,
the solar energy received per unit area is, on average, greatest at the equator and
least at the poles. This difference causes atmospheric circulations that transfer
energy within the moving atmosphere from low to high latitude.
Seasonal perturbations
Because the axis of rotation of the Earth on any particular day is not perpendicular
to the plane in which the Earth moves around the Sun (see Fig. 5.7 and Fig. 9.1),
the latitude where there is most and least solar radiation changes with season. This
results in persistent regular seasonal changes in circulation patterns.
Daily perturbations
The rotation of the Earth means there is a regular diurnal cycle in the longitude at
which there is maximum input of solar radiation. At any latitude, the magnitude
and timing of this daily cycle of energy changes with season.
Persistent perturbations
Contrast in ocean to continent surface exchanges
On average about half of the solar energy reaching the Earth enters the atmosphere
from the Earth's surface, mainly in the form of surface latent and sensible heat
fluxes. Because water is freely available at the ocean surface but not necessarily at
continental surfaces, there is a characteristic difference in the relative magnitude
of these two different fluxes for these two surfaces. The average aerodynamic
roughness of oceans is also less than that of continents. This contrast in surface
exchanges of energy and momentum influences regional hydroclimate.
Continental topography
Atmospheric circulation mainly occurs in the troposphere. In some locations the
height of continental topography is of the same order as the depth of the troposphere
and this can influence regional flow patterns to some extent, particularly when
topography is organized in mountain chains that lie roughly perpendicular to
atmospheric flow (e.g., the Rockies and Andes).
