Geography Reference
In-Depth Information
warming of the polar stratosphere, which can quickly reverse the meridional tem-
perature gradient and (through thermal wind balance) create a circumpolar easterly
current. Warmings of 40 K in a few days have occurred at the 50-hPa level.
Numerous observational studies of sudden warmings confirm that enhanced
propagation of planetary waves from the troposphere, primarily zonal wave num-
bers 1 and 2, is essential for the development of the warmings. Since major warm-
ings are observed primarily in the Northern Hemisphere, it is logical to conclude
that enhanced wave propagation into the stratosphere is due to topographically
forced waves, which are much stronger in the Northern Hemisphere than in the
Southern Hemisphere. Even in the Northern Hemisphere, however, it is apparently
only in certain winters that conditions are right to produce sudden warmings.
It is generally accepted that sudden warmings are an example of transient mean-
flow forcing due to planetary wave driving. Section 10.2.3 showed that in order for
planetary waves to decelerate the zonal-mean circulation there must be a nonzero
equatorward eddy potential vorticity flux (i.e., a net EP flux convergence). We
further showed that for steady nondissipative waves, the divergence of the EP flux
vanishes. For normal stationary planetary waves in the stratospheric polar night jet,
this constraint should be at least approximately satisfied, as radiative and frictional
dissipation are rather small. Thus, the strong interaction between the waves and the
mean flow that occurs during the course of a sudden warming must be associated
with wave transience (i.e., change of amplitude with respect to time) and wave
dissipation. Most of the dramatic mean-flow deceleration that occurs during a
sudden warming is caused by amplification of quasi-stationary planetary waves in
the troposphere followed by propagation into the stratosphere.
This process can be understood by considering the interaction between zonal
mean and eddy potential vorticities. We assume that on the short time scales charac-
teristic of sudden warmings, diabatic processes can be neglected. We also assume
that the eddy motion is approximately governed by the linearized eddy poten-
tial vorticity equation and that the zonal mean is changed only by the eddy flux
convergence. The eddies and mean flow are thus related by the quasi-linear system
∂
∂t
+
q
+
∂
∂x
∂q
∂y
=−
v
S
u
(12.23)
where again
ρ
0
N
2
f
0
ρ
0
∂ψ
∂z
∂
∂z
q
≡∇
2
ψ
+
(12.24)
and an eddy damping term, -S
, has been added to represent the effects of both
mechanical and thermal dissipation. (For constant and equal Rayleigh friction
