Sudden stratospheric warming events in a simple stratosphere-mesosphere model

Equatorial winds in the stratosphere are known to influence the frequency of stratospheric mid-winter sudden warmings. Sudden warmings, in turn, influence the Earth's climate both through their direct influence on polar temperatures and through the temperature dependence of ozone depletion in the lower stratosphere. The conventional (Holton-Tan) explanation for the equatorial influence on sudden warmings is in terms of the equatorial winds in the lower stratosphere (20-30 km) acting as a wave-guide for mid-latitude planetary wave propagation. Gray et al (2001) correlated equatorial rocketsonde wind data with polar temperature to show that in addition to the recognised Holton-Tan relationship, a reasonably strong correlation of polar temperatures with equatorial winds in the upper stratosphere was found. This was associated with the strength and vertical extent of the westerly phase of the semi-annual oscillation in the upper stratosphere or possibly the descending incoming phase QBO, but the exact mechanism of influence is unclear.

The purpose of this study is to further investigate these results and elucidate the mechanism in terms of how the westerly phase of the SAO may influence wave propagation in early winter. A simple Stratosphere- Mesosphere model is used in a perpetual winter state and ensemble integrations are carried out. We relax towards various profiles of equatorial winds, and although the pattern that emerges is complicated, the model results do support the findings of Gray et al (2001): the equatorial upper stratosphere plays an important role in the formation of warming events. In order to gain a clearer picture of the exact mechanisms involved, both refractive index and EP flux are used as diagnostic tools for the various integrations, along with synoptic PV maps to chart of the development of warming events.