Inferences about QBO Dynamics from the Atmospheric “Tape Recorder” Effect

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Abstract
Observations of ascending water vapor anomalies in the tropical lower stratosphere—the so-called tape recorder effect—have been used to infer profiles of vertical mean motion, vertical constituent diffusivity, and lateral in-mixing rate in this region. The magnitude of vertical wave flux required to drive the quasi-biennial oscillation (QBO) in the presence of mean upwelling, along with other related aspects of QBO dynamics, is examined in light of the tape recorder results using a simple one-dimensional model. As found in a previous study, it is necessary that wave fluxes significantly exceed the “classic” values associated with long-period Kelvin and Rossby-gravity waves; the extra fluxes are presumably associated with a continuous spectrum of short-period gravity and inertia–gravity waves. Larger wave fluxes, when used in connection with traditional wave-transport parameterizations developed for the QBO, require a larger vertical diffusivity of momentum in order to prevent the formation of unrealistically strong vertical wind shear. The profile of constituent diffusivity derived from the tape recorder effect, however, is much smaller everywhere than the momentum diffusivity assumed in previous QBO modeling studies. Realistic shear is obtained in the model using a momentum-conserving “shear adjustment” scheme representing the effect of unresolved shear instabilities and other processes not included in the wave-transport parameterization. This device, together with a QBO amplitude profile based on the equatorial-wave phase speeds, motivates an (otherwise inviscid) analytic QBO solution in the underdamped, quasi-compressible case. The simple analytic solutions replicate most aspects of the numerical solution, display a similar dependence on wave flux at the lower boundary, and provide reasonably accurate estimates of QBO period in the inviscid limit.

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