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James R. Holton and Hsiu-Chi Tan

Abstract

Monthly mean Northern Hemisphere 50 mb geopotential heights for a 16-year period (1962-77) are composited with respect to the phase of the equatorial quasi-biennial oscillation (QBO). The observed zonal mean geopotential height at high latitudes is significantly lower during the westerly phase of the equatorial QBO than during the easterly phase in all months composited.

For this 16-year sample we find that in early winter (November-December) the amplitude of planetary wavenumber 1 is nearly 40% stronger during the easterly phase of the equatorial QBO. In late winter (January-March) the amplitude of planetary wavenumber 2, on the other hand, is nearly 60% stronger during the westerly phase of the equatorial QBO. Data from an additional 6-year sample show a similar wavenumber 1 signal during the November-December period. However, an additional 4-year sample does not support our conclusions concerning wavenumber 2 during the January-March period. Composites based on zonal wind data from a longitudinal network of Southern Hemisphere stations show a significant QBO in the zonal wind component in the spring but not in the summer.

We hypothesize that shifts in the latitude of the zero mean zonal wind line (critical line) associated with the equatorial QBO may be responsible for the planetary wave portion of the extratropical 50 mb QBO.

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