One-to-Two Month Oscillations in the Stratosphere during Southern Winter

Abstract

Stratospheric disturbances on the 35?60 day time scale are investigated with particular emphasis on the Southern Hemisphere. The data used are stratospheric brightness temperatures from 90 to 1.5 hPa covering seven 6-month southern winter periods (from April 1980 to March 1987). Global time-lag correlation plots are constructed from which tropical/extratropical connections, three-dimensional wave structure, and propagation characteristics are studied. Horizontal correlation patterns at 90 hPa reveal a strong connection between the Indonesian tropics and the winter extratropics. Vertical correlation patterns in the southern winter extratropics reveal westward tilt with height and vertical propagation of 35?60 day zonal wavenumber 1 perturbations, from tropospheric regions up to great heights, at least as high as the stratopause region. Disturbances are found to propagate from 90 hPa to 1.5 hPa in generally 6 to 9 days. In contrast, the vertical correlation plots in the tropics indicate little or no vertical propagation or westward tilt with height. Statistical coherence studies provide supporting evidence that the observed extratropical disturbances may result from tropical forcing, with the ensuing connection favoring a period close to 50 days. The observations are in qualitative agreement with model calculations featuring a tropical forcing and suggest, at least for the disturbances captured by the broad vertical weighting functions of the satellite instruments used, that the low frequency disturbances move out of the tropical latitudes before propagating vertically to the stratopause region.