Geoscience Reference
In-Depth Information
2
H
L
Φ
+ 2d
Φ
1
3
Φ
+ d
Φ
4
Φ
L
H
Warmer surface
Cooler surface
Figure 7.1 A thermally direct circulation system.
resulting release of latent heat into the middle troposphere further increases the
air temperature, reducing density (Eq. 5.1) and adding buoyancy, so the parcels
continue to rise. In this way, the latent heat flux transports heat vertically. With
respect to latitude, however, the latent heat flux works “backward,” with cool-
ing by evaporation over the cooler surface (Region 4) and the release of latent
heat over the warmer surface (Region 1).
Note how the sensible and latent heat fluxes work together. The release of
latent heat in the midtroposphere converts latent to sensible heat, feeding into
the horizontal upper-tropospheric sensible heat flux. The overall result is the
net transport of heat to cooler regions and higher elevations.
THE HADLEY CIRCULATION
A thermally direct circulation known as the
Hadley circulation
covers the full
depth of the troposphere and approximately half the earth's surface area, be-
tween 30°N and 30°S. It is a large-scale, overturning circulation (
Fig. 7.1)
that
exists only in the zonal mean, as discussed below. The surface temperature
gradient that drives the Hadley circulation is meridional, so the circulation is
fundamentally due to the latitudinal dependence of the solar heating and, ulti-
mately, the (approximately) spherical shape of the planet. Hadley circulations
occur on other planets as well, most notably Mars, Jupiter, and Venus.
Climatological meridional (dark contours) and vertical
p
-velocity (light con-
tours) winds are together in
Figure 7.2.
Both variables are averaged over the
full year and across longitudes to form the zonal mean. Arrows indicate the
direction of the low, and regions of high and low geopotential heights (high
and low relative to the same level) are indicated. Note the similarity to
Figure
7.1.
The warmer surface near the equator is co-located with the region of rising
