3.8.4 Evidence for Changes in Extratropical Storms and Extreme Events
3.8.4.1 Extratropical Cyclones
Intense extratropical cyclones are often associated with extreme weather, particularly with severe windstorms. Significant increases in the number or strength of intense extratropical cyclone systems have been documented in a number of studies (e.g., Lambert, 1996; Gustafsson, 1997; McCabe et al., 2001; Wang et al., 2006a) with associated changes in the preferred tracks of storms as described in Section 3.5.3. As with tropical cyclones, detection of long-term changes in cyclone measures is hampered by incomplete and changing observing systems. Some earlier results have been questioned because of changes in the observation system (e.g., Graham and Diaz, 2001).
Results from NRA and ERA-40 show that an increase in the number of deep cyclones is apparent over the North Pacific and North Atlantic (Graham and Diaz, 2001; Gulev et al., 2001), with statistically significant winter increases over both ocean basins (Simmonds and Keay, 2002; Wang et al., 2006a). Geng and Sugi (2001) found that cyclone density, deepening rate, central pressure gradient and translation speed have all been increasing in the winter North Atlantic. Caires and Sterl (2005) compared global estimates of 100-year return values of wind speed and SWH in ERA-40, with linear bias corrections based on buoy data, for three different 10-year periods. They showed that the differences in the storm tracks can be attributed to decadal variability in the NH, linked to changes in global circulation patterns, most notably to the NAO (see also Section 3.5.6).
Using NCEP-2 reanalysis data, Lim and Simmonds (2002) showed that for 1979 to 1999, increasing trends in the annual number of explosively developing (deepening by 1 hPa per hour or more) extratropical cyclones are significant in the SH and over the globe (0.56 and 0.78 more systems per year, respectively), while the positive trend did not achieve significance in the NH. Simmonds and Keay (2002) obtained similar results for the change in the number of cyclones in the decile for deepest cyclones averaged over the North Pacific and over the North Atlantic in winter over the period 1958 to 1997.
As noted in Sections 3.5.3 and 3.5.7, the time-dependent biases in the reanalysis cause uncertainties in the trends reported above. Besides reanalyses, station data may also be used to indicate evidence for changes in extratropical cyclone activity. Instead of direct station wind measurements, which may suffer from a lack of consistency of instrumentation, methodology and exposure, values based on pressure gradients have been derived that are more reliable for discerning long-term changes. Alexandersson et al. (2000) used station pressure observations for 21 stations over northwestern Europe back to 1881, from which geostrophic winds were calculated using ‘pressure-triangle’ methods. They found a decline of storminess expressed by the 95th and 99th percentiles from high levels during the late 19th century to a minimum around 1960 and then a quite rapid increase to a maximum around 1990, followed again by a decline (Figure 3.41). Positive NAO winters are typically associated with more intense and frequent storms (see Section 3.6.4). Similar results were obtained by Schmith et al. (1998) using simpler indices based on pressure tendency. Bärring and von Storch (2004), using both pressure tendencies and the number of very low pressure values, confirmed these results on the basis of two especially long station series in southern Sweden dating back to about 1800. Studies of rapid pressure changes at stations indicate an increase in the frequency, duration and intensity of winter cyclone activity over the lower Canadian Arctic and in the number and intensity of severe storms over the southern UK since the 1950s, but a decrease over southern Canada and Iceland (Wang et al., 2006b; Alexander et al., 2005). Besides a northward shift of the storm track (see 3.5.3), the station pressure data for parts of the North Atlantic region show a modest increase in severe storms in recent decades. However, decadal-scale fluctuations of similar magnitude have occurred earlier in the 19th and 20th centuries.
Direct surface wind measurements have, however, been used in a few studies. An analysis of extreme pressure differences and surface winds (Salinger et al., 2005) showed a significant increasing trend over the last 40 years in westerly wind extremes over the southern part of New Zealand and the oceans to the south. The trends are consistent with the increased frequency of El Niño events in recent decades, associated with Pacific decadal variability (see Section 3.6.3). While the zonal pressure gradient and extreme westerly wind frequency have both increased over southern New Zealand, the frequency of extreme easterly winds has also increased there, suggesting more variability in the circulation generally. However, trends in pressure differences (based on the ERA-40, NRA and station data) are not always consistent with changes in surface windiness (e.g., Smits et al., 2005). Based on observed winds at 10 m height over the Netherlands, Smits et al. (2005) found a decline in strong (greater than about 8 on the Beaufort scale) wind events over the last 40 years. Differences cannot entirely be explained by changes in surface aerodynamic roughness, and they concluded that inhomogeneities in the reanalyses are the cause. However, local differences can be important and intensity and severity of storms may not always be synonymous with local extreme surface winds and gusts.