Page 316 - Fundamentals of Air Pollution 3E

272 17. The Physics of the Atmosphere

35° and 55°. A high index thus represents strong zonal flow; a low index
indicates weak zonal flow. A low index is frequently accompanied by closed
circulations which provide a greater degree of meridional flow. In keeping
with the transient behavior of the mid-latitude circulation, the zonal index
varies irregularly, cycling from low to high in periods ranging from 20 to
60 days.
The jet stream is caused by strong temperature gradients, so it is not
surprising that it is frequently above the polar front, which lies in the
convergence zone between the mid-latitude loop of the general circulation
and the loop nearest the poles (Fig. 17-25). The positions of both the polar
front and the jet stream are quite variable, shifting poleward with surface
surges of warm air and moving toward the equator with outbreaks of
cold air.

C. Polar Region

The circulation cells nearest the poles include rising air along the polar
front, movement toward the poles aloft, sinking in the polar regions causing
subsidence inversions, and flow toward the equator near the earth's surface.
These motions contribute to the heat balance as the moisture in the air
rising over the polar front condenses, releasing the heat that was used to
evaporate the water nearer the equator. Also, the equatorward-moving air
is cold and will be warmed as it is moved toward the tropics.

D. Other Factors
Of considerable usefulness in transporting heat toward the poles are the
ocean currents. They are particularly effective because of the high heat
content of water. Significant poleward-moving currents are the Brazil, Kuro-
shio, and Gulf Stream currents. Currents returning cold water toward the
equator are the Peru and California currents.
The pressure pattern changes from winter to summer in response to
temperature changes. Because most of the Southern Hemisphere consists
of ocean, the summer-to- winter temperature differences are moderated.
However, the increased land mass in the Northern Hemisphere allows
high continental temperatures in summer, causing small equator-to-pole
temperature differences; cooling over the continents in winter produces
more significant equator-to-pole temperature differences, increasing the
westerly winds in general and the jet stream in particular.

REFERENCES

1. Battan, L. J. "Fundamentals of Meteorology." Prentice-Hall, Englewood Cliffs, NJ, 1979.
2. Byers, H. R., "General Meteorology," 4th ed. McGraw-Hill, New York, 1974.