IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentDJFJJADJFJJA-10 -50 5% Change10Figure 3-8 | Averaged changes from a 19-member ensemble of CMIP3 GCMs in the mean of the daily averaged 10-m wind speeds (top) and 99th percentile of the daily averaged10-m wind speeds (bottom) for the period 2081-2100 relative to 1981-2000 (% change) for December to February (left) and June to August (right) plotted only where more than66% of the models agree on the sign of the change. Black stippling indicates areas where more than 90% of the models agree on the sign of the change. Red stippling indicatesareas where more than 66% of models agree on a small change between ±2%. Adapted from McInnes et al. (2011); for more details see Appendix 3.A.mid-latitude storm tracks, tropical cyclones, and ocean waves(Christensen et al., 2007; Meehl et al., 2007b). Meehl et al. (2007b)projected a likely increase in tropical cyclone extreme winds in thefuture and provided more evidence for a projected poleward shift of thestorm tracks and associated changes in wind patterns. Since the AR4,new studies have focused on future changes in winds. Gastineau andSoden (2009) reported a decrease in 99th-percentile winds at 850 hPain the tropics and an increase in the extratropics in a 17-member multimodelensemble over 2081-2100 relative to 1981-2000. McInnes et al.(2011) presented spatial maps of multi-model agreement in mean and99th-percentile 10-m wind change between 1981-2000 and 2081-2100in a 19-member ensemble (see Figure 3-8). These show an increase inmean winds over Europe, parts of Central and North America, the tropicalSouth Pacific, and the Southern Ocean. Mean wind speed declines occuralong the equator reflecting a slowdown in the Walker circulation(Collins et al., 2010) (and in the vicinity of the subtropical ridge in bothhemispheres which, together with the strengthening of winds furtherpoleward, reflect the contraction toward the poles of the mid-latitudestorm tracks; see Section 3.4.5). Seasonal differences are also apparentwith more extensive mean wind increases in the Arctic and parts of thenorthern Pacific in DJF and decreases over most of the northern Pacificin JJA. The 99th-percentile wind changes show declines over most oceanareas except the northern Pacific and Arctic and Southern Ocean southof 40°S in DJF, the south Pacific between about 10 and 25°S in JJA, andthe Southern Ocean south of 50°S in JJA. Increases in 99th-percentilewinds occur over the Arctic and large parts of the continental area in theNorthern Hemisphere in DJF and in Africa, northern Australia, andCentral and South America in JJA. Despite the projections displayed inFigure 3-8, the relatively few studies of projected extreme winds,combined with shortcomings in the simulation of extreme winds andthe different models, regions, and methods used to develop projectionsof this quantity, means that we have low confidence in projections ofchanges in strong winds.Regional increases in winter wind storm risk over Europe due tochanges in storm tracks are also supported by a number of regionalstudies (e.g., Pinto et al., 2007b; Debernard and Roed, 2008; Leckebuschet al., 2008; Sterl et al., 2009; Donat et al., 2010a,b, 2011). However, GCMsat their current resolution are unable to resolve small-scale phenomenasuch as tropical cyclones, tornadoes, and mesoscale convective complexesthat are associated with particularly severe winds, although as noted byMcInnes et al. (2011) these winds would typically be more extreme than99th percentile. There is evidence to suggest an increase in extremewinds from tropical cyclones in the future (see Section 3.4.4). Anincrease in atmospheric greenhouse gas concentrations may causesome of the atmospheric conditions conducive to tornadoes such asatmospheric instability to increase due to increasing temperature andhumidity, while others such as vertical shear to decrease due to areduced pole-to-equator temperature gradient (Diffenbaugh et al.,2008), but the literature on these phenomena is extremely limited at151