18.104.22.168 Wind Speed
Confidence in future changes in windiness in Europe remains relatively low. Several model studies (e.g., Zwiers and Kharin, 1998; Knippertz et al., 2000; Leckebusch and Ulbrich, 2004; Pryor et al., 2005a; van den Hurk et al., 2006) have suggested increased average and/or extreme wind speeds in northern and/or central Europe, but some studies point in the opposite direction (e.g., Pryor et al., 2005b). The changes in both average and extreme wind speeds may be seasonally variable, but the details of this variation appear to be model-dependent (e.g., Räisänen et al., 2004; Rockel and Woth, 2007).
A key factor is the change in the large-scale atmospheric circulation (Räisänen et al., 2004; Leckebusch et al., 2006). Simulations with an increased north-south pressure gradient across northern Europe (e.g., top row of Figure 11.6) tend to indicate stronger winds in northern Europe, because of both the larger time-averaged pressure gradient and a northward shift in cyclone activity. Conversely, the northward shift in cyclone activity tends to reduce windiness in the Mediterranean area. On the other hand, simulations with little change in the pressure pattern tend to show only small changes in the mean wind speed (bottom row of Figure 11.6). Most of the MMD-projected pressure changes fall between the two PRUDENCE simulations shown in Figure 11.6, which suggests that the most likely outcome for windiness might be between these two cases.
Extreme wind speeds in Europe are mostly associated with strong winter cyclones (e.g., Leckebush and Ullbrich, 2004), the occurrence of which is only indirectly related to the time-mean circulation. Nevertheless, models suggest a general similarity between the changes in average and extreme wind speeds (Knippertz et al., 2000; Räisänen et al., 2004). A caveat to this conclusion is that, even in most RCMs, the simulated extremes of wind speed over land tend to be too low (see Section 11.3.2).