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air to the north and warmer air to the south. Then, small eddy perturbations
(waves) develop in the zonal flow initiated, for example, by uneven heating at
the surface or within the atmosphere. In midlatitudes, these perturbations are
unstable, meaning that they grow with time as the energy of the zonal mean
flow is converted into eddy energy. The instability mechanism is known as
baroclinic instability
, and it occurs because of the strong meridional tempera-
ture gradients that are typical of middle latitudes (
Figs. 2.6,
2.7,
and
2.9
). The
waves continue to grow, and they “break” to form cutoff highs and lows. As
depicted in
Figure 7.7b,
at the end of the index cycle warmer air has moved
poleward and cooler air has moved equatorward, both contributing to the
poleward transport of heat (and momentum) to balance the radiation deficit/
surplus
(Fig. 5.9
). The index cycle is more active during the winter, when me-
ridional temperature gradients are larger.
7.3 EXERCISES
7.1. Verify that Eqs. 7.5 and 7.6 satisfy Eq. 7.7.
7.2. State one way in which the Hadley and Walker circulations are similar, and
one way in which they are different.
7.3. Consider a layer of the atmosphere extending from the surface to 800 hPa
located on the equator. Within this layer, at what rate is the Hadley
circulation transporting mass, and in what direction is this mass flux during
boreal winter? During austral winter?
