Geoscience Reference
In-Depth Information
Figure 4.2.
Fields of mean 30 hPa geopotential height (gpm) for the four mid-season
months over the 1970-1999 period, based on NCEP/NCAR data (by the authors).
altitude (0.3 - 0.1 hPa). The winter mean stratospheric vortex is broadly symmetric,
but less so than its Antarctic counterpart. There are troughs over eastern Asia and
eastern North America and a weak ridge located over western North America. In
spring, illustrated by the April field, the vortex center has shifted well off the pole to
north-central Eurasia. In summer and above about 20 km (as seen in the July field),
the cyclonic vortex has broken down. There is polar easterly flow around a highly
symmetric warm polar anticyclone. Summer sees an easterly jet of about 60 m s
−1
at 0.1 hPa located around 50-60°N. The October field illustrates the transition back
toward winter conditions.
If there were no transports arising from the breaking of atmospheric waves propa-
gating upward from the troposphere, the zonal mean temperature of the stratosphere
would relax to a radiatively determined state, with the temperature distribution cor-
responding to an annually varying thermal equilibrium that follows the annual cycle
in solar heating. The circulation would hence represent a zonal-mean zonal flow in
balance with the meridional temperature gradient (a thermal wind balance), with
