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as time changes in the zonal wind and temperature, arise from imbalances between
the EP flux divergence/convergence and associated residual circulations. The EP
flux divergence pattern is weaker in summer.
The mean circulation and thermal structure of the winter stratosphere examined
earlier can be similarly understood from the presence of mean EP flux convergence
and meridional plane circulations associated with the upward propagation of plan-
etary waves. In the extratropics, for example, eddy forcing maintains the observed
stratospheric temperature above its radiative equilibrium. Hence there is net radi-
ative cooling that is balanced by downward motion (adiabatic warming) associ-
ated with the residual circulation. In the tropics, the temperature is below radiative
equilibrium, consistent with upward motion (adiabatic cooling). In turn, sudden
stratospheric warmings (which, as the name implies, are transient events) can be
understood from the anomalous upward propagation of planetary waves.
As outlined by Limpasuvan et al. (
2004
), when an anomalous vertically prop-
agating planetary wave enters the stratosphere, it imparts an anomalous EP flux
convergence. The decelerated westerly flow is brought back toward balance by the
Coriolis force acting on the poleward mean residual flow anomaly. This induces adi-
abatic temperature changes that also try to bring the flow back to balance. Through
continuity, the poleward mean meridional flow across the axis of the EP flux forc-
ing requires sinking motion (adiabatic warming) below and poleward of the forc-
ing region. In turn, there is rising motion (adiabatic warming) below and equator-
ward of the forcing region. These meridional-plane circulations quickly weaken the
meridional temperature gradient, and give rise to rapid high-latitude stratospheric
warming.
In extreme cases, stratospheric temperatures can rise 50K and the circumpo-
lar vortex can reverse to easterly flow over the span of a few days. The presence
of vertically propagating waves, while necessary for sudden stratospheric warm-
ings, is not a sufficient condition. It appears that the stratospheric flow needs to be
“preconditioned,” such that wave activity is focused toward the polar vortex. The
preconditioning occurs when the vortex is poleward of its climatological position
(Limpasuvan et al.,
2004
).
Figure 4.4
illustrates changes in 10 hPa temperature associated with a major
warming event that took place between late December 1984 and early January
1985. Temperature fields are given for five-day averages from December 17-21,
1984, December 27-31, 1984, and January 6-10, 1985. These are identified by K.
Kodera and M. Chiba (
1995
) as representing, respectively, the prewarming, warm-
ing, and postwarming phases of the event. The prewarming stage shows a vortex
center located well off the pole over the Norwegian Sea, with minimum temper-
atures of about −72°C. During the warming phase, the vortex breaks down into
four centers. The postwarming phase shows additional breakdown. The contrast in
high latitude (e.g., north of 70°N) temperatures between the prewarming and the
postwarming phases is readily apparent. Between the warming and postwarming
phases, the stratospheric zonal-mean zonal winds reversed at 70°N (Kodera and
Chiba,
1995
).
