Geography Reference
In-Depth Information
It would be possible to utilize the complete spherical coordinate version of the
TEM equations for this analysis, but we are concerned primarily with the angular
momentum balance for a zonal ring of air extending from the surface to the top of
the atmosphere. In that case it proves simpler to use the conventional Eulerian mean
formulation. It also proves convenient to use a special vertical coordinate, called
the
sigma coordinate
, in which the surface of the earth is a coordinate surface.
Because the average rotation rate of the earth is itself observed to be very close to
constant, the atmosphere must also on the average conserve its angular momentum.
The atmosphere gains angular momentum from the earth in the tropics where the
surface winds are easterly (i.e., where the angular momentum of the atmosphere is
less than that of the earth) and gives up angular momentum to the earth in middle
latitudes where the surface winds are westerly. Thus, there must be a net poleward
transport of angular momentum within the atmosphere; otherwise the torque due
to surface friction would decelerate both easterlies and westerlies. Furthermore,
the angular momentum given by the earth to the atmosphere in the belt of easterlies
must just balance the angular momentum given up to the earth in the westerlies if
the global angular momentum of the atmosphere is to remain constant.
In the equatorial regions the poleward angular momentum transport is divided
between the advection of absolute angular momentum by the poleward flow in
the axially symmetric Hadley circulation and transport by eddies. In midlatitudes,
however, it is primarily the eddy motions that transport momentum poleward, and
the angular momentum budget of the atmosphere must be qualitatively as shown
in Fig. 10.10.
As Fig. 10.10 suggests, there is a maximum poleward flux of angular momentum
at about 30
◦
latitude and a maximum horizontal flux convergence at about 45
◦
. This
maximum in the flux convergence is a reflection of the strong energy conversion in
upper-level westerlies and is the mechanism whereby the atmosphere can maintain
Eddy flux
200
(Divergence)
(Convergence)
600
Easterlies
Westerlies
1000
0¡
15¡
30¡
45¡
60¡
90¡
Latitude
Fig. 10.10
Schematic mean angular momentum budget for the atmosphere-earth system.
