J. Anstey
Modelling the quasi-biennial oscillation in atmospheric general
circulation models
The quasi-biennial oscillation (QBO) dominates the interannual
variability of the tropical troposphere, but its representation in
general circulation models (GCMs) is often poor or nonexistent. We use a
large ensemble of experiments with a GCM that resolves the middle
atmosphere to test the model parametric sensitivity of the QBO.
Sufficiently fine vertical resolution and sufficiently strong
parameterized non-orographic gravity wave drag are required for the GCM
to spontaneously develop QBO-like oscillations of the tropical
stratospheric wind, consistent with previous studies. Controlled
experiments in which the zonal-mean state is artificially constrained
show that the response of eastward-propagating resolved waves to changes
in vertical resolution in the lowermost stratosphere (roughly 20-25 km)
is the key factor leading the model to develop a realistic QBO. At
coarse vertical resolution, parameterized wave forcing is unable to
emulate the effect of the resolved waves for a wide variety of parameter
settings, and hence does not induce a QBO in spite of the fact that it
dominates the overall momentum budget of the oscillation.
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