Environmental Engineering Reference
In-Depth Information
without any significant increase in temperature. The ice plateau of Antarctica also stays
very cool, so the temperature gradients do not change very much from winter to summer.
The position of the boundary between the westerlies and easterlies (of both the upper
and the surface winds) varies throughout the year. From December to February the polar
vortex of the winter (northern) hemisphere expands, pushing the belts southwards so that,
at the surface, the boundaries are at about 30° N and 35° S. As the year progresses, the
other polar vortex begins to expand as winter sets in over the southern hemisphere. The
boundaries eventually reach about 35° N and 30° S by June to August. The separation
between the two systems is not vertical. As a result, some parts of the tropics have
easterlies in the lower atmosphere and westerlies above. Only over a small area of the
globe do easterlies occur at all levels, whereas westerlies extend throughout the
atmosphere over a large proportion of Earth.
ROSSBY WAVES
The pattern of easterlies and westerlies in the upper atmosphere is only part of the total
picture. In addition to the marked zonal flows there are less apparent but none the less
important meridional flows. In the circumpolar areas, for example, there occur wave-like
patterns of flow called Rossby waves (after C. F. Rossby, a Swedish meteorologist) that
play a vital role in the energy exchange between the temperate and polar areas.
It is not easy to detect these meridional flows within the pattern of strong zonal
circulation by normal methods of depicting winds. The normal methods usually show
average conditions, so that processes which balance each other, flowing northwards for
six months, perhaps, then southwards for the next six months, are lost. Yet that is what
happens in the case of the Rossby waves. At a particular location southerly flows may
last for a few days, to be followed by more northerly winds as the wave progresses
eastwards.
In order to see these waves it is necessary to use a rather different technique of
presenting atmospheric circulation. Instead of mapping the actual wind directions or
speeds, the height at which a particular pressure surface is reached can be plotted. This
may seem a strange way of depicting winds, but, as we know, the geostrophic winds blow
parallel to the isobars, at a speed inversely proportional to the distance between the
isobars. Similarly the winds blow parallel to the contours of the pressure surface. Where
the contours are close together, the winds are rapid. Irregularities in the pressure surface
indicate local patterns of wind movement.
Figure 6.17 shows a pressure surface (500 hPa) map for January. The projection of the
map may make it difficult to appreciate the direction of flow immediately. What is clear
is that the flow is not perfectly circular around the North Pole. Areas occur, even on this
monthly chart, where the mean flow has a northward component, and other areas occur
where it is southwards. Effectively the air is flowing in a series of waves around the pole,
carrying warmer air northwards on parts of its track and cold air southwards elsewhere.
These are the Rossby waves. In January the most prominent features of the waves are the
pronounced troughs in the pressure surface near 80° W and 140° E, with a weaker trough
between 10° E and 60° E. In July the circulation is less intense and the troughs less well
marked.
