IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentbased on a study with reanalyses. The low-latitude (south of 45°N)decrease was also noted by Zou et al. (2006), who reported a decreasein the number of severe storms for mainland China based on an analysisof extremes of observed 6-hourly pressure tendencies over the last 50years.Alexander and Power (2009) showed that the number of observedsevere storms at Cape Otway (south-east Australia) has decreasedsince the mid-19th century, strengthening the evidence of a southwardshift in Southern Hemisphere storm tracks previously noted usingreanalyses (Fyfe, 2003; Hope et al., 2006; Wang et al., 2006). Frederiksenand Frederiksen (2007) linked the reduction in cyclogenesis at 30°Sand southward shift to a decrease in the vertical mean meridionaltemperature gradient. Using reanalyses, both Pezza et al. (2007) andLim and Simmonds (2009) have confirmed previous studies showing atrend toward more intense low-pressure systems. However, the trend ofa decreasing number of cyclones seems to depend on the choice ofreanalysis and pressure level (Lim and Simmonds, 2009), emphasizingthe weaker consistency among reanalysis products for the SouthernHemisphere extratropical cyclones. Recent studies support the notionof more cyclones around Antarctica when the SAM (see Section 3.4.3)is in its positive phase and a shift of cyclones toward mid-latitudeswhen the SAM is in its negative phase (Pezza and Simmonds, 2008).Additionally, more intense (and fewer) cyclones seem to occur whenthe PDO (see Section 3.4.3) is strongly positive and vice versa (Pezza etal., 2007).In conclusion, it is likely that there has been a poleward shift in themain northern and southern storm tracks during the last 50 years. Thereis strong agreement with respect to this change between severalreanalysis products for a wide selection of cyclone parameters andcyclone identification methods and European and Australian pressurebasedstorminess proxies are consistent with a poleward shift over thelast 50 years, which indicates that the evidence is robust. Advances havebeen made in documenting the observed decadal and multi-decadalvariability of extratropical cyclones using proxies for storminess. So therecent poleward shift should be seen in light of new studies with longertime spans that indicate that the last 50 years coincide with relativelylow cyclonic activity in northern coastal Europe in the beginning of theperiod. Several studies using reanalyses suggest an intensification ofhigh-latitude cyclones, but there is still insufficient knowledge of howchanges in the observational systems are influencing the cycloneintensification in reanalyses so even in cases of high agreement amongthe studies the evidence cannot be considered to be robust, thus wehave only low confidence in these changes. Other regional changes inintensity and the number of cyclones have been reported. However, thelevel of agreement between different studies using different trackingalgorithms, different reanalyses, or different cyclone parameters is stilllow. Thus, we have low confidence in the amplitude, and in someregions in the sign, of the regional changes.Regarding possible causes of the observed poleward shift, the AR4concluded that trends over recent decades in the Northern andSouthern Annular Modes, which correspond to sea level pressurereductions over the poles, are likely related in part to human activity,but an anthropogenic influence on extratropical cyclones had not beenformally detected, owing to large internal variability and problems dueto changes in observing systems (Hegerl et al., 2007). Anthropogenicinfluences on these modes of variability are also discussed in Section3.4.3.Seasonal global sea level pressure changes have been shown to beinconsistent with simulated internal variability (Giannini et al., 2003;Gillett et al., 2005; Gillett and Stott, 2009; X.L. Wang et al., 2009a), butchanges in sea level pressure in regions of extratropical cyclones (midandhigh latitudes) have not formally been attributed to anthropogenicforcings (Gillett and Stott, 2009). However, the trend pattern inatmospheric storminess as inferred from geostrophic wind energy andocean wave heights has been found to contain a detectable response toanthropogenic and natural forcings with the effect of external forcingsbeing strongest in the winter hemisphere (X.L. Wang et al., 2009a).Nevertheless, the models generally simulate smaller changes thanobserved and also appear to underestimate the internal variability,reducing the robustness of their detection results. New idealized studieshave advanced the physical understanding of how storm tracks mayrespond to changes in the underlying surface conditions, indicating thata uniform SST increase weakens (reduced cyclone intensity or numberof cyclones) and shifts the storm track poleward and strengthened SSTgradients near the subtropical jet may lead to a meridional shift in thestorm track either toward the poles or the equator depending on thelocation of the SST gradient change (Deser et al., 2007; Brayshaw et al.,2008; Semmler et al., 2008; Kodama and Iwasaki, 2009), but the averageglobal cyclone activity is not expected to change much under moderategreenhouse gas forcing (O’Gorman and Schneider, 2008; Bengtsson et al.,2009). Studies have also emphasized the important role of stratosphericchanges (induced by ozone or greenhouse gas changes) in explaininglatitudinal shifts in storm tracks and several mechanisms have beenproposed (Son et al., 2010). This has particularly strengthened theunderstanding of the Southern Hemisphere changes. According to Fogtet al. (2009) both coupled climate models and observed trends in theSAM were found to be outside the range of internal climate variabilityduring the austral summer. This was mainly attributed to stratosphericozone depletion (see Section 3.4.3).In summary, there is medium confidence in an anthropogenic influenceon the observed poleward shift in extratropical cyclone activity. It hasnot formally been attributed. However indirect evidence such as globalanthropogenic influence on the sea level pressure distribution and trendpatterns in atmospheric storminess inferred from geostrophic wind andocean wave heights has been found. While physical understanding ofhow anthropogenic forcings may influence extratropical cyclone stormtracks has strengthened, the importance of the different mechanisms inthe observed shifts is still unclear.The AR4 reported that in a future warmer climate, a consistent projectionfrom the majority of the coupled atmosphere-ocean GCMs is fewer165