Abstract:
The leading modes of month-to-month
variability in the Northern and Southern Hemispheres are examined by comparing
a 100-year run of the Geophysical Fluid Dynamics Laboratory general circulation
model with the NCEP/NCAR reanalyses of observations. The model simulation
is a control experiment in which the sea surface temperatures are fixed
to the climatological annual cycle with out any interannual variability.
The leading modes describe nearly zonally symmetric, or annular, expansion
and contraction of the polar vortex as the mid-latitude jet shifts equatorward
and poleward. This fluctuation is strongest during the winter months.
The structure and amplitude of the simulated mode is very similar to that
derived from observations, indicating that the mode arises from the internal
dynamics of the atmosphere.
Dynamical diagnosis of both observations
and simulation indicates that variations in the zonal flow are forced by
eddy fluxes in the free troposphere, while the Coriolis acceleration associated
with the mean meridional circulation maintains the surface wind anomalies
against friction. High frequency transients contribute most to the total
eddy forcing in the Southern Hemisphere. In the Northern Hemisphere, stationary
waves provide most of the eddy momentum fluxes, although high frequency
transients also make an important contribution. The behavior of the stationary
waves can be partly explained with index of refraction arguments. When
the tropospheric westerlies are dis placed poleward, Rossby waves are refracted
equatorward, inducing poleward momentum fluxes and reinforcing the high
latitude westerlies. Planetary Rossby wave refraction can also explain
why the stratospheric polar vortex is stronger when the tropospheric westerlies
are displaced poleward. When planetary wave activity is refracted equatorward,
it is less likely to propagate into the stratosphere and disturb the polar
vortex.
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