OBSERVATIONAL EVIDENCE OF PREFERRED FLOW REGIMES IN THE NORTHERN-HEMISPHERE WINTER STRATOSPHERE

Ten years of stratospheric geopotential height data are analyzed in an attempt to determine whether there are preferred flow regimes in the Northern Hemisphere winter stratosphere. The data are taken from Strat ospheric Sounding Units on board NOAA satellites. The probability dens ity estimate of the amplitude of the wavenumber 1 10-mb height is foun d to be bimodal. The density distribution is composed of a dominant la rge-amplitude mode and a less frequent low-amplitude mode. When the wa venumber 1 10-mb height data are projected onto the phase plane define d by the 10-mb zonal-mean winds and wavenumber 1 100-mb heights, three preferred regimes are evident. The small-amplitude mode separates int o a strong zonal wind-weak wave regime and a weak zonal wind-weak wave regime. The large-amplitude mode is an intermediate zonal wind-strong wave regime. Transitions between the large-amplitude regime and the w eak zonal wind-weak wave regime are found to be associated with major stratospheric warmings. The clustering of the stratospheric data into the preferred flow regimes is interpreted in light of the bifurcation properties of the Holton and Mass model. The interannual variability o f the Northern Hemisphere winter stratosphere is interpreted in terms of the relative frequency of the observed preferred regimes.