Geoscience Reference
In-Depth Information
vorticity (the sum of relative and planetary vorticity) and the gradient of potential
temperature. The PV maximum shown in
Figure 4.3
arises from the low density of
the air at stratospheric levels (specific volume is large), the strong increase of poten-
tial temperature with height (strong stability) and the positive absolute vorticity.
4.2.3
Sudden Stratospheric Warmings
An important aspect of variability in the stratospheric circulation is sudden strato-
spheric warmings. Scherhag (
1960
) first noted these in the 1950s and they were sub-
sequently found to be a characteristic late winter phenomenon (Hare,
1961
; Wilson
and Godson,
1962
,
1963
; Labitzke,
1968
,
1981
). V. Limpasuvan, D. Thompson, and
D. Hartmann (
2004
) provide a valuable composite analysis of the lifecycle of sud-
den stratospheric warming events in the Northern Hemisphere from the Transformed
Eulerian Mean (TEM) framework. The reader is referred to that paper and extensive
references therein for details.
The TEM framework is very useful for diagnosing the influence of eddies on the
zonal mean circulation. It is useful to first consider the TEM framework in the con-
text of the troposphere. Briefly, tropospheric eddies transport heat poleward. This
acts to reduce the zonal mean temperature gradient in the extratropics and hence the
strength of the zonal mean winds. Eddies also transport westerly zonal momentum
poleward, tending to increase the zonal mean winds in the extratropics. In the TEM
framework, the net effect of these processes at a given latitude can be expressed in
terms of the divergence of the Eliassen-Palm (EP) flux. The EP flux has vertical
and meridional components. The vertical component of the EP flux is related to the
strength of the meridional eddy temperature flux (v'T') while the meridional com-
ponent is related to strength of the eddy momentum flux (u'v'), where the primes
indicate departures from the zonal mean and the overbars indicate the zonal mean.
For long-term mean winter conditions, through much of the depth of the atmo-
sphere and north of about 30
o
N, the EP flux divergence is negative (i.e., convergent).
It can be shown that EP flux convergence represents a westward force on the zonal
wind - that is, it induces slowing of the winds (Holton,
1992
). In a steady state
condition, the zonal wind and temperature are constant. Slowing of the zonal mean
wind by the EP flux convergence is balanced by the effects of a residual circula-
tion in the meridional plane that is in a thermally direct sense (clockwise as viewed
in a mean cross section with the North Pole to the right). This residual circulation
(which is part of the total zonal mean meridional circulation) is highly efficient in
maintaining thermal wind balance in large-scale atmospheric motions and keeping
the ageostophic flow small in comparison with the geostrophic flow. The Coriolis
force on the residual poleward meridional wind speeds up the zonal winds aloft.
The Coriolis force on the residual equatorward wind reduces the zonal winds near
the surface where the EP flux is divergent. Through continuity, there must be atten-
dant vertical motions. A residual downward motion (adiabatic warming) balances
radiative cooling in higher latitudes while upward motion (adiabatic cooling) in
lower latitudes balances radiative warming. Departures from the steady state, seen
