Geoscience Reference
In-Depth Information
4.2
The Stratospheric Circulation
4.2.1
Mean Thermal Structure
The Arctic stratosphere extends upward from about 10 km to 40-50 km altitude. It
is a very dry, statically stable region in which ozone dominates the absorption of
solar ultraviolet radiation and emission of infrared radiation. In the troposphere (the
region between the stratosphere and the surface, which contains about 80 percent of
the atmospheric mass), the mean thermal structure is maintained by an approximate
balance between infrared radiative cooling to space, surface radiative heating, and
the subsequent vertical transport of sensible and latent heat by turbulent processes;
and large-scale horizontal heat transport by synoptic-scale disturbances. The dom-
inance of surface radiative heating results in surface and tropospheric temperatures
that are highest in the equatorial regions and decrease toward the poles. As the
troposphere is primarily heated from below, there is also a rapid decrease in tem-
perature with altitude (however, the potential temperature - the temperature that
an air parcel would have if raised or lowered to the 1000 hPa surface - increases
with height). There are of course prominent seasonal and regional departures, such
as strong low-level temperature inversions in the Arctic during winter (
Chapter 5
).
By contrast, for mean conditions in the stratosphere, the infrared cooling to space
is balanced primarily by radiative heating owing to absorption of ultraviolet radi-
ation by ozone. As a result of ozone absorption, the mean temperature increases
upward in the stratosphere (hence potential temperature increases strongly with
height, meaning very stable conditions). Above roughly 50 km, we enter the meso-
sphere, where temperature again decreases with height because of reduced ozone
absorption. In the stratosphere above about 30 hPa (roughly 25 km), the tempera-
ture decreases from the summer pole to the winter pole, in qualitative accord with
radiative equilibrium conditions (in June, total solar radiation at the top of the atmo-
sphere is greatest at the North Pole and least at the South Pole). Winter temperatures
at around 30 km in the Arctic are below −70°C, whereas in midsummer - with
twenty-four-hour sunlight - they exceed −40°C (Andrews, Holton, and Leovy,
1987
). However, in the lower stratosphere, the temperature is strongly influenced by
upper-tropospheric processes. This region is characterized by a mean temperature
minimum at the equator and maxima at the summer pole and in the mid-latitudes of
the winter hemisphere (Holton,
1992
).
4.2.2
Mean Circulation and Dynamics
Figure 4.2
provides mean geopotential height fields at the 30 hPa level for the four
mid-season months. In winter, as illustrated by January, the height field points to
strong westerlies in the middle and upper stratosphere flowing around a deep cold
vortex. Above 40 km (3 hPa, not shown), there is a polar night westerly jet stream in
mid-latitudes. Peak mean zonal winds of 80 m s
−1
actually occur around 60-70 km
