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John Hampson and Peter Haynes

Abstract

This paper investigates the occurrence of phase alignment of the tropical stratospheric quasi-biennial oscillation (QBO) with the annual cycle. First, updating previous studies, observational results are shown for NCEP reanalysis data and Singapore radiosondes: both datasets show strong phase alignment of the QBO at 24.5 km. Phase alignment is investigated in a 3D mechanistic stratospheric model including explicit large-scale planetary waves, forced by a lower boundary geopotential anomaly, and a simple equatorial wave parameterization. The model simulates a QBO-like oscillation, with the period depending on the lower boundary momentum flux of the parameterized waves. Phase alignment is manifested in two different ways. First, simulated oscillations of both integer and noninteger year periods are shown to lock on to a certain phase of the annual cycle. Second, when the magnitude of the lower boundary momentum flux is varied about a range implying oscillation period close to 2 yr, the period of the resulting oscillation is exactly 2 yr for a finite range of such magnitude. Analysis of the 3D model results suggest that the the phase alignment is due largely to the annual cycle in tropical upwelling. This hypothesis is supported by simulations with a 1D equatorial model including both parameterized waves and seasonally varying upwelling. The oscillations in this model show significant phase alignment when the upwelling parameters are tuned to correspond to the 3D model, although the phase alignment is weaker than that seen in the 3D model.

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John Hampson and Peter Haynes

Abstract

The work described here examines the influence of the equatorial quasi-biennial oscillation (QBO) on the extratropics in a zonally truncated 3D mechanistic stratospheric model. Model results show that the extratropical response to the QBO depends critically on the phase alignment of the QBO with the annual cycle: the signal of extratropical response varies by a factor of 8 between the phase alignment that gives minimum response and that which gives maximum response. Model simulations in which the time and height structure of the QBO are varied suggest that, in this zonally truncated model, the equatorial height of 21–23 km is most influential for the extratropical response and that late autumn/early winter is the time at which the QBO has the most influence over the extratropical circulation. The correlation coefficient between the QBO (measured by zonal wind) and the extratropics (measured by zonal wind or potential temperature) is as high as 0.95. The correlation coefficient is largest for simulations with lower boundary wave forcing weaker than that which gives largest extratropical interannual variability. For stronger extratropical wave forcing, the correlation coefficient is slightly smaller, but the regression coefficient of the linear term in a least squares fit is significantly larger.

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