Geoscience Reference
In-Depth Information
equatorial regions. The uneven solar heating results in a decrease in tropospheric
temperatures from south to north and a corresponding poleward decrease in the
height of pressure surfaces, inducing a pressure gradient. The atmosphere works
to reduce these gradients. In lower latitudes, where the Coriolis parameter is fairly
small, the poleward atmospheric energy transport is associated with the thermally
direct Hadley circulation cells. In the middle and higher latitudes, the time-mean
tropospheric flow is primarily westerly (west to east), representing an approximate
balance between the pressure gradient and Coriolis forces (geostrophic balance).
The poleward energy transport is primarily accomplished via baroclinic eddies
(associated with surface cyclones and anticyclones) and long waves that represent
disturbances in the westerly flow. The differential solar heating also results in den-
sity gradients in the ocean, resulting in a net poleward transport of oceanic sensible
heat, which plays a major role in low latitudes.
Estimates of the poleward transports by the atmosphere and ocean required to
account for the TOA radiation imbalance shown in
Figure 3.1
have been provided in
numerous studies. The general approach has been to calculate the required transport
from satellite-derived TOA radiation fluxes, the atmospheric transport from global
reanalyses and then estimate the ocean transport as a residual. At present, direct
estimates of ocean transport from ocean velocity and temperature are considered
unreliable. Here we examine estimates of the annual mean atmospheric and ocean
transports from the study of K. Trenberth and J. Caron (
2001
). They made use of
the ERBE radiation data and atmospheric transports calculated from the NCEP/
NCAR and ECMWF ERA-15 atmospheric reanalyses (see
Chapter 9
). Instead of
obtaining the ocean transports as a residual, they used surface heat fluxes inferred
from the atmospheric energy budget. Their estimates also contain adjustments for
mass and energy balance. Details are provided in that paper and a companion study
by Trenberth, Caron, and D. Stepaniak (
2001
).
The resulting estimates of the required annual transport and the atmospheric and
oceanic contributions are given in
Figure 3.3
. The poleward ocean transport domi-
nates only between 0° and 17°N. At 35°N, close to where the total peak transport
occurs in both hemispheres, the atmospheric transport accounts for about 78 percent
of the total in the Northern Hemisphere and 92 percent in the Southern Hemisphere.
In general, a greater proportion of the required total transport is contributed by the
atmosphere than the ocean as compared with older estimates.
Thermodynamically, the atmosphere can hence be viewed as the principal engine
which pumps heat from equatorial sources to sinks in the Arctic and Antarctic. If
there was no meridional energy exchange, the polar regions would be much colder,
and the equatorial regions much warmer, than observed. Put differently, the polar
regions, while certainly cold, are not nearly as cold as would be expected on the
basis of their annual incoming solar radiation. Because the earth's rotational axis is
inclined 23.5° with respect to its orbital plane, we experience seasonality in the lat-
itudinal distribution of incoming (and net) solar radiation, expressed most strongly
in the polar regions (see
Figure 2.1
for effects on day length). During the winter of
each hemisphere, the incoming solar radiation in the polar regions is small or zero,
