The Influence of the Equatorial Quasi-Biennial Oscillation on the Global Circulation at 50 mb

Abstract

Monthly mean Northern Hemisphere 50 mb geopotential heights for a16-year period (1962-77) are composited with respect to the phaseof the equatorial quasi-biennial oscillation (QBO). The observedzonal mean geopotential height at high latitudes is significantlylower during the westerly phase of the equatorial QBO than duringthe 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 duringthe easterly phase of the equatorial QBO. In late winter (January-March)the amplitude of planetary wavenumber 2, on the other hand, is nearly60% stronger during the westerly phase of the equatorial QBO. Datafrom an additional 6-year sample show a similar wavenumber 1 signalduring the November-December period. However, an additional 4-yearsample does not support our conclusions concerning wavenumber 2 duringthe January-March period. Composites based on zonal wind data froma longitudinal network of Southern Hemisphere stations show a significantQBO in the zonal wind component in the spring but not in the summer.We hypothesize that shifts in the latitude of the zero mean zonalwind line (critical line) associated with the equatorial QBO maybe responsible for the planetary wave portion of the extratropical50 mb QBO.