Stratospheric bimodality: Can the QBO explain the regime behaviour of the NH winter vortex?

The stratospheric Northern Hemisphere (NH) winter mean vortex alternates between a strong and a weak state which is manifested in a statistically significant bimodal distribution. In the end of the 1970s a regime change took place increasing the probability of the strong phase relative to the weak phase (Christiansen 2003). In this paper we show that the regime behavior of the extra-tropical vortex can be explained by its connection to the quasi-biennial oscillation (QBO), i.e., the Holton-Tan relationship. The winter means of the stratospheric equatorial wind between 70 and 8 hPa show a bimodal structure. This bimodality originates from the strong oscillatory nature of the QBO together with an only weak variability of its amplitude. We show that westerly/easterly phases of the QBO correspond to strong/weak phases of the vortex. The change of the vortex in the late 1970ies can be related to a change in the phase of the QBO. Careful consideration of the statistical significance shows that this change in the QBO can be explained by a random process simply related to the annual sampling of the quasi biannual variability. Thus, no additional physical explanation of the regime behavior and regime change of the vortex may be necessary. Finally, we relate the bimodality of the winter QBO to previous findings of phase-locking between the QBO and the annual-cycle.