IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3Over North America, a declining trend in 50th and 90th percentile windspeeds has been reported for much of the United States over 1973 to2005 (Pryor et al., 2007) and in 10-m hourly wind data over 1953-2006over western and most of southern Canada (Wan et al., 2010). Anincreasing trend has been reported in average winds over Alaska over1955-2001 by Lynch et al. (2004) and over the central Canadian Arcticin all seasons and in the Maritimes in spring and autumn by Wan et al.(2010) as well as in annual maximum winds in a regional reanalysisover the southern Maritimes from 1979-2003 (Hundecha et al., 2008).Over China, negative trends have been reported in 10-m monthly meanand 95th percentile winds over 1969-2005 (Guo et al., 2011), in dailymaximum wind speeds over 1956-2004 by Jiang et al. (2010a), and in2-m average winds over the Tibetan plateau from 1966-2003 (Y. Zhanget al., 2007), confirming earlier declining trends in mean and strong10-m winds reported by Xu et al. (2006). Over Europe, Smits et al. (2005)found declining trends in extreme winds (those occurring on average10 and 2 times per year) in 10-m anemometer data over 1962-2002.Pirazolli and Tomasin (2003) reported a generally declining trend inboth annual mean and annual maximum winds from 1951 to the mid-1970s and an increasing trend since then, from observations in thecentral Mediterranean region. Similar to the mostly declining trendsfound in Northern Hemisphere studies of surface wind observations,Vautard et al. (2010) also found mostly declining trends in surface windobservations across the continental northern mid-latitudes and astronger decline in extreme winds compared to mean winds in surfacewind measurements. In the Southern Hemisphere, McVicar et al. (2008)reported declines in 2-m mean wind speed over 88% of Australia(significant over 57% of the country) over 1975-2006 and positive trendsover about 12% of the mainland interior and southern and easterncoastal regions including Tasmania. In Antarctica, increasing trends inmean wind speeds have been reported over the second half of the 20thcentury (Turner et al., 2005). With the exception of the robust declines inwind reported over China, studies in most areas are too few in numberto draw robust conclusions on wind speed change and even fewerstudies have addressed extreme wind change. Some studies reportopposite trends between anemometer winds and reanalysis data sets insome areas (Smits et al., 2005; McVicar et al., 2008; Vautard et al., 2010);however, comparisons of surface anemometer data at 10 m or lowerwith reanalysis-derived 10-m data that do not resolve complex surfacefeatures is problematic.Trends in extreme winds have also been inferred from trends in particularphenomena. With regards to tropical cyclones (Section 3.4.4.), nostatistically significant trends have been detected in the overall globalannual number although a trend has been reported in the intensity ofthe strongest storms since 1980 [but there is low confidence that anyobserved long-term (i.e., 40 years or more) increases in tropical cycloneactivity are robust, after accounting for past changes in observingcapabilities; see Section 3.4.4]. In the mid-latitudes, studies have usedproxies for wind such as pressure tendencies or geostrophic windscalculated from triangles of pressure (geo-winds) over Europe (e.g.,Barring and von Storch, 2004; Matulla et al., 2008; Allan et al., 2009;Barring and Fortuniak, 2009; X.L. Wang et al., 2009b) and Australia (e.g.,Alexander and Power, 2009; Alexander et al., 2011). For Europe, thesestudies suggest that storm activity was higher around 1900 and in the1990s and lower in the 1960s and 1970s, although X.L. Wang et al.(2009b) note that seasonal trends behave differently than annual trends.In general, long-term trends differ between the different availablestudies as well as studies that focus on the period for which reanalysisdata exist (e.g., Raible, 2007; Leckebusch et al., 2008; Della-Marta et al.,2009; Nissen et al., 2010), and strong inter-decadal variability is alsooften reported (e.g., Allan et al., 2009; X.L. Wang et al., 2009b; Nissen etal., 2010). Over southeast Australia, a decline in storm activity sincearound 1885 has been reported (Alexander and Power, 2009; Alexander etal., 2011). See Section 3.4.5 for more discussion of extratropical cyclones.Regarding other phenomena associated with extreme winds, such asthunderstorms, tornadoes, and mesoscale convective complexes, studiesare too few in number to assess the effect of their changes on extremewinds. As well, historical data inhomogeneities mean that there is lowconfidence in any observed trends in these small-scale phenomena.The AR4 reported for the mid-latitudes that trends in the Northern andSouthern Annular Modes, which correspond to sea level pressure reductionsover the poles, are likely related in part to human activity, and this inturn has affected storm tracks and wind patterns in both hemispheres(Hegerl et al., 2007). The relationship between mean and severe windsand natural modes of variability has been investigated in several post-AR4 studies. On the Canadian west coast, Abeysirigunawardena et al.(2009) found that higher extreme winds tend to occur during the negative(i.e., cold) ENSO phase. The generally increasing trend in mean windspeeds over recent decades in Antarctica is consistent with the changein the nature of the Southern Annular Mode toward its high index state(Turner et al., 2005). Donat et al. (2010b) concluded that 80% of stormdays in central Europe are connected with westerly flows that occurprimarily during the positive phase of the NAO. Declining trends in windover China have mainly been linked to circulation changes due to aweaker land-sea thermal contrast (Xu et al., 2006; Jiang et al., 2010a;Guo et al., 2011). Vautard et al. (2010) attribute the slowdown in mid tohigh percentiles of surface winds over most of the continental northernmid-latitudes to changes in atmospheric circulation (10-50%) and anincrease in surface roughness due to biomass increases (25-60%),which are supported by RCM simulations. X.L. Wang et al. (2009a)formally detected a link between external forcing and positive trends inthe high northern latitudes and negative trends in the northern midlatitudesusing a proxy for wind (geostrophic wind energy) in the borealwinter. Trends in mean and annual maximum winds in the centralMediterranean region were found to be positively correlated withtemperature but not with the NAO index (Pirazzoli and Tomasin, 2003).Nissen et al. (2010) used cyclone tracking to identify associated strongwinds in reanalysis data from 1957 to 2002 and found a positive trendin the central Mediterranean region and southern Europe and a negativetrend over the western Mediterranean region.Projections of wind speed changes and particularly wind extremeswere not specifically addressed in the AR4 although references to windspeed were made in relation to other variables and phenomena such as150