A Partial Theory of the 26-Month Oscillation of the Zonal Wind in the Equatorial Stratosphere

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Abstract
Attempted explanations of the 26-month zonal wind oscillation in the equatorial stratosphere are critically examined. Explanations in terms of subharmonic response, ‘vacillation,’ or a resonant mode are rejected. Some indirect astronomical and terrestrial evidence of a fluctuation of solar ultraviolet emission is noted, and, in view of the inadequacy of other explanations, appears to be a probable cause. A simple derivation is given of the thermal and geostrophic wind fields which propagate downward by eddy and radiative transfer through the equatorial stratosphere from a layer somewhere above 25 km which is periodically heated, presumably by a solar source of 26-month period. The wind field is shown to lag the temperature field by about 3 months in the vicinity of the equator, in agreement with available observations. The thermal and geostrophic wind disturbances propagate downward about 1 km mo−1 for an effective conductivity (sum of eddy and radiative conductivities) of the order of 104 cm2 sec−1, a value which is consistent with values derived from vertical diffusion of artificial radioactivity. The 26-month zonal wind oscillation is therefore described as a forced geostrophic wave motion produced by fluctuating solar emission. Its large amplitude over the equator traces to the greater sensitivity of the equatorial stratosphere to fluctuations of solar emission, to the large geostrophic wind for a given pressure gradient force, and finally to the protection, provided by large static and baroclinic stabilities in the equatorial stratosphere, from rapid heat and momentum transfers.

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