Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 243

Tropopause, declining
in height poleward

Polar front
jet stream

Polar front
jet stream

Polar
cell

Arctic
front

N

HP

LP

polar
easterlies

Mid-latitude
cyclones

Mid-latitude
westerlies

HP

Hadley
cell

Rossby waves
in jet stream

LP NE trades

Fig. 6.10 The observed General Atmospheric Circulation, involving convective cells with Coriolis turning, jet stream, Rossby waves, and
associated frontal systems.

6.1.6 Mid-latitude circulation and climates

Higher-frequency Rossby waves superimposed on the
long-Rossby waves represent junctions between warm
equatorial air and cold polar air with large temperature
gradients across them, known as fronts (Fig. 6.10). Fronts
slope gently upward from low to high latitudes and are the
sites of what is termed “slantwise convection,” that is, the
forced upwelling of warm low-latitude air over sinking
cold polar air; they are a form of rotating density current
(Section 4.12). The intersection of a front with the earth's
surface is not simple for there are often smaller “parasitic”
waves and fronts superimposed that are shed off by the
vorticity of the major Rossby waves. These moving air
masses comprise stable air of contrasting temperature and
pressure separated by the frontal surfaces. They dominate
the weather and climate of mid-latitudes, giving rise to a
more-or-less predictable sequence of weather, but which
travel at more-or-less unpredictable rates, much to the
chagrin of forecasters.
Important climatic variability in the northern hemi-
sphere mid-latitudes (and probably elsewhere through
some teleclimatic connection, probably from warming of
the tropical Indo-Pacific oceans) seems to be correlated
with what has become known as the Northern Hemisphere
Annular Mode ( NAM ), also known as the North Atlantic
Oscillation ( NAO ). It is represented as the difference in
sea level pressure between the Azores High (descending
low-latitude air) and the Iceland Low (see weather chart of
Fig. 3.21). High-index time periods (large statistically sig-
nificant variations in pressure) are marked by anomalously

So far we have implied that the equatorial air masses that
cool-convect down to the subtropical surface flow back west
to form the trade winds. In fact, the poleward horizontal
pressure gradient and the resulting thermal wind ensures
that a substantial poleward-moving component comes into
contact with equator-moving cool polar air, the polar east-
erlies, at mid-latitudes 40-55 . The two masses meet along

what is known as the polar front , where Coriolis deflection
leads to the formation of a zone of westerly winds these lat-
itudes. The westerly winds so characteristic of mid-latitude
climates are really the low-altitude remnants of the much
stronger polar front jet stream wind (see Section 6.1.2).
Observations indicate not only that the jet streams encircle
the globe, but that the seasonal-averaged winds vary in
strength and direction because of two to four wave-like bil-
lows that occur with wavelengths of several thousands of
kilometers. These planetary long waves are often seen as sea-
sonal-permanent features of the atmospheric circulation on
mean pressure maps. They are termed Rossby long waves
(Fig. 6.10) and serve to transfer momentum and heat across
the mid-latitudes. Rossby waves owe their origin to differ-
ential heating of major continental land masses and sea sur-
faces in the lower atmosphere region below the jet stream.
This engenders a wave-like diversion of jet stream flow
around the isobars of the resulting pressure anomalies, the
waves themselves traveling much more slowly than the air in
the jet stream itself.