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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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