Model studies of the interannual variability of the northern‐hemisphere stratospheric winter circulation: The role of the quasi‐biennial oscillation

A series of experiments are described that examine the sensitivity of the northern‐hemisphere winter evolution to the equatorial quasi‐biennial oscillation (QBO). The prime tool for the experiments is a stratosphere‐mesosphere model. The model is integrated over many years with the modelled equatorial winds relaxed towards observed values in order to simulate a realistic QBO. In experiment A the equatorial winds are relaxed towards Singapore radiosonde observations in the height region 16‐32 km. In contrast to previous modelling studies, the Holton‐Tan relationship (warm/cold winters associated with easterly/westerly QBO winds in the lower stratosphere) is absent. However, in a second experiment (run B) in which the equatorial winds are relaxed towards rocketsonde data over the extended height range 16‐58 km, a realistic Holton‐Tan relationship is reproduced. A series of further studies are described that explore in more detail the sensitivity to various equatorial height regions and to the bottom‐boundary forcing. The experiments suggest that the evolution of the northern‐hemisphere winter circulation is sensitive to equatorial winds throughout the whole depth of the stratosphere and not just to the lower‐stratospheric wind direction as previously assumed.