Intraseasonal Tropical—Extra-Tropical Interactions Observed in the Stratosphere

Abstract

Using 34 years (July 1957–June 1991) of 50 hPa northern hemisphere geopotential height data, Φ 50 and observed tropical winds, u QBO , an attempt is made to isolate spatial patterns of interannual variability using Empirical Orthogonal Function (EOF) analysis. The dominant variability of Φ 50 in all months is zonally symmetric, representing changes in the latitudinal gradient of Φ 50 and hence in the 50 hPa northern hemisphere geostrophic wind, u G50 . EOF analysis of combined fields of u G50 and u QBO is performed, in an attempt to isolate relationships between the quasi-biennial oscillation (QBO) and the extratropics. A two-way relationship is found. There is a QBO signal in the extratropics, with enhanced mid-latitude westerlies in winters when the QBO has its west phase. A component of annual variability is evident in the tropics, with additional easterly forcing there during the west phase of the QBO (which we attribute to the enhanced equatorward propagation of planetary waves), which tends to cancel a restoration of westerly winds by mean meridional advection, leading to the observed `delay in onset’ of the easterly phase of the QBO.