6.3 Dynamical mechanisms in the middle atmosphere

While some of the similarities between the observational analyses of solar influence and modelling studies of UV effects are intriguing they do not explain the mechanisms whereby the influence takes place. Kodera (1995) has argued that changes in the winter polar stratosphere brought about by anomalous solar heating may influence the passage of upward propagating planetary waves and thus their deposition of momentum. This argument has been developed by Kodera and Kuroda (2002Jump To The Next Citation Point) who suggest that the momentum deposition will influence the strength of the mean overturning of the stratosphere (also known as the Brewer–Dobson circulation) and thus tropical lower stratospheric temperatures (see cartoon in Figure 38View Image).
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Figure 38: Illustrating the proposed mechanism whereby solar heating around the stratopause may influence the atmosphere below. (a) The solar heating anomalies change the strength of the polar stratospheric jet (U); this influences the path of upward propagating planetary waves which deposit their zonal momentum on the poleward side of the jet. (b) The effect of this is to weaken the Brewer–Dobson circulation and thus to warm the tropical lower stratosphere. From (Kodera and Kuroda, 2002).

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 39View Image: 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.

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Figure 39: Temperature of the NH polar lower stratosphere evolving through the winter as calculated in a stratosphere-mesosphere mechanistic model. Each panel shows the results from twenty different simulations for each of which the initial conditions were slightly altered. Right: control situation; Left: anomalous westerly momentum applied in the winter sub-tropics near the stratopause. From Gray et al. (2004).

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 15View Image). 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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