An alternative (or additional?) perspective is provided by the work of Gray et al. (2001) who have demonstrated, using rocketsonde and satellite data that temperature anomalies in the tropical upper stratosphere (potentially related to solar activity) are correlated with subsequent temperature anomalies in the polar lower stratosphere. They also demonstrated, using a mechanistic model of the middle atmosphere that zonal wind anomalies in the sub-tropical upper stratosphere can influence the timing and amplitude of sudden stratospheric warmings – events during the polar winter in which enhanced planetary wave activity disturbs the cold polar vortex. An example is shown in Figure 39: the unperturbed simulations (right) show random timing of warming events but when a westerly wind anomaly in forced near the stratopause in the sub-tropics of the winter hemisphere the warmings become more organised into specific time periods.
Gray (2003) pointed out that such a relationship between equatorial winds in the upper stratosphere and the timing of sudden stratospheric warmings could help to explain the interaction between the solar cycle and QBO influences on polar temperatures (as identified by Labitzke and van Loon, 1992, see Figure 15). Modelling evidence to support this idea has been provided by Matthes et al. (2004) who found they could reproduce the observed solar cycle/QBO polar temperature relationship if typical QBO wind profiles were imposed through the depth of the tropical stratosphere.
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