Quasi-biennial Modulation of Planetary-Wave Fluxes in the Northern Hemisphere Winter

Abstract

Using 25 years of National Meteorological Center (NMC) data for 1964–88 the relation between tropical and extratropical quasi-biennial oscillations (QBOs) was examined for zonally averaged quantities and planetary-wave Eliassen–Palm fluxes in the Northern Hemisphere winter. The extratropical QBO discussed by Holton and Tan existed in both temporal halves of the dataset. Autocorrelation analysis demonstrated that it was an important mode of interannual variability in the extratropical winter stratosphere. Correlation with the tropics was strongest when 40-mb equatorial winds were used to define the tropical QBO. Easterly phase at 40 mb implied a weaker than normal polar night jet and warmer than normal polar temperature and vice versa. An opposite relationship was obtained using 10-mb equatorial winds. The association between tropical and extra-tropical QBOs was observed in about 90% of the winters and was statistically significant. It is shown that planetary-wave Eliassen–Palm fluxes were generally consistent with the extratropical QBO. These fluxes were more (less) convergent in the midlatitude (subtropical) upper stratosphere in the 40-mb east (= easterly) phase category relative to the west category. The composite difference in flux divergence was a dipole, the location of which coincided with the observed mean zonal wind anomaly. The difference was strongest in early- to midwinter. However, composites of planetary-wave life cycles were similar in the two phase categories, with only slightly more events, slightly larger events, and larger mean flow response in the east category. There was very good correlation between planetary-wave flux convergence and observed mean flow tendencies on a daily basis, but the tendencies were smaller in magnitude.