Observed trends report 1.6
- Severe windstorms around the UK have become more frequent in the past few decades, although not above that seen in the 1920s.
There is considerable interest in possible trends in severe wind storms around the UK, but these are difficult to identify, due to low numbers of such storms, their decadal variability, and by the unreliability and lack of representativity of direct wind speed observations. In UKCIP02, we showed how the frequency of severe gales over the past century, although showing an increase over the past decade or so, did not support any relationship with man-made warming. Alexander et al. (2005) presented an analysis whereby a severe storm event is characterised by a rapid change in (MSLP) (specifically ±10hPa in a 3h period); this is different from the severe gales in UKCIP02. They found a significant increase in the number of severe storms over the UK as a whole since the 1950s. This analysis is being extended back in time using newly-digitised MSLP data from as many as possible long-period observing sites in the UK and Ireland, and some preliminary results are shown in Figure 1.14 (Allan et al. in preparation). It appears that an equally stormy period to those in the most recent full decade (1990s) was experienced in the 1920s. Similar conclusions are drawn in IPCC AR4 (Chapter 3, para 220.127.116.11 and Fig. 3.41).
Whereas it is not our purpose here to discuss detailed links between the NAO and storminess, it will be immediately apparent that the two stormiest periods in Figure 1.14, in the 1920s and 1990s, coincide with decades of sustained positive NAO index, whereas the least stormy decade, the 1960s, is a time when the smoothed NAO index was most negative (see Figure 1.13). Although work by Gillett et al. (2003) has shown that man-made factors have had a detectable influence on sea-level pressure distributions (and hence atmospheric circulation patterns) over the second half of the 20th century, there continues to be little evidence that the recent increase in storminess over the UK is related to man-made climate change.