The influence of the equatorial upper stratosphere on stratospheric sudden warmings

A stratosphere mesosphere model is used in a set of idealized experiments to investigate the sensitivity of the Northern Hemisphere winter stratospheric flow to the equatorial zonal winds in the upper and the lower stratosphere. Sensitivity is found to the lower stratospheric winds ( below 40 km) only in the early part of the model integration as the model relaxes towards a perpetual winter state. On the other hand the model showed significant sensitivity to the upper stratospheric winds throughout the major part of the integration. This has possible implications for the influence of the quasi-biennial oscillation (QBO) and solar cycle on the northern winter stratospheric flow. It suggests that the lower stratospheric QBO may influence the early winter, but later in the winter when the flow is highly non-linear the greatest influence may come from variations in the semi-annual oscillation (SAO) which dominates the equatorial winds in the upper stratosphere. Possible sources of variations in the SAO include both the influence of the QBO and of the 11-year solar cycle.