IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentgenesis, track, and intensity evolution. Of particular concern is the limitedability of global models to accurately simulate upper-tropospheric wind(Cordero and Forster, 2006; Bender et al., 2010), which modulates verticalwind shear and tropical cyclone genesis and intensity evolution. Thusthere is low confidence in projections of changes in tropical cyclonegenesis, location, tracks, duration, or areas of impact, and existingmodel projections do not show dramatic large-scale changes in thesefeatures.In summary, there is low confidence that any observed long-term(i.e., 40 years or more) increases in tropical cyclone activity arerobust, after accounting for past changes in observing capabilities.The uncertainties in the historical tropical cyclone records, theincomplete understanding of the physical mechanisms linkingtropical cyclone metrics to climate change, and the degree oftropical cyclone variability provide only low confidence for theattribution of any detectable changes in tropical cyclone activityto anthropogenic influences. There is low confidence in projectionsof changes in tropical cyclone genesis, location, tracks, duration,or areas of impact. Based on the level of consistency amongmodels, and physical reasoning, it is likely that tropical cyclonerelatedrainfall rates will increase with greenhouse warming. Itis likely that the global frequency of tropical cyclones will eitherdecrease or remain essentially unchanged. An increase in meantropical cyclone maximum wind speed is likely, although increasesmay not occur in all tropical regions. While it is likely that overallglobal frequency will either decrease or remain essentiallyunchanged, it is more likely than not that the frequency of the mostintense storms will increase substantially in some ocean basins.3.4.5. Extratropical CyclonesExtratropical cyclones (synoptic-scale low-pressure systems) existthroughout the mid-latitudes in both hemispheres and mainly developover the oceanic basins in the proximity of the upper-tropospheric jetstreams, as a result of flow over mountains (lee cyclogenesis) or throughconversions from tropical to extratropical systems. It should be notedthat regionalized smaller-scale mid-latitude circulation phenomena suchas polar lows and mesoscale cyclones are not treated in this section (butsee Sections 3.3.3 and 3.4.3). Extratropical cyclones are the main polewardtransporter of heat and moisture and may be accompanied by adverseweather conditions such as windstorms, the buildup of waves and stormsurges, or extreme precipitation events. Thus, changes in the intensity ofextratropical cyclones or a systematic shift in the geographical locationof extratropical cyclone activity may have a great impact on a widerange of regional climate extremes as well as the long-term changes intemperature and precipitation. Extratropical cyclones mainly form andgrow via atmospheric instabilities such as a disturbance along a zone ofstrong temperature contrast (baroclinic instabilities), which is a reservoirof available potential energy that can be converted into the kinetic energyassociated with extratropical cyclones. Intensification of the cyclonesmay also take place due to processes such as release of energy due tophase changes of water (latent heat release) (Gutowski et al., 1992;Wernli et al., 2002). Why should we expect climate change to influenceextratropical cyclones? A simplified line of argument would be that boththe large-scale low and high level pole to equator temperature gradientsmay change (possibly in opposite directions) in a climate change scenarioleading to a change in the atmospheric instabilities responsible forcyclone formation and growth (baroclinicity). These changes may beinduced by a variety of mechanisms operating in different parts of theatmospheric column ranging from changing surface conditions (Deser etal., 2007; Bader et al., 2011) to stratospheric changes (Son et al., 2010).In addition, changes in precipitation intensities within extratropicalcyclones may change the latent heat release. According to theories onwave-mean flow interaction, changes in the extratropical storm tracks arealso associated with changes in the large-scale flow (Robinson, 2000;Lorenz and Hartmann, 2003). A latitudinal shift of the upper troposphericjet would be accompanied by a latitudinal shift in the extratropicalstorm track. It is, however, still unclear to what extent a latitudinal shiftin the jet changes the total storm track activity rather than shifting itlatitudinally (Wettstein and Wallace, 2010). Even within the very simplifiedoutline above the possible impacts of climate change on extratropicalcyclone development are many and clearly not trivial.When validated using reanalyses with similar horizontal resolution,climate models are found to represent the general structure of thestorm track pattern well (Bengtsson et al., 2006; Greeves et al., 2007;Ulbrich et al., 2008; Catto et al., 2010). However, using data from fivedifferent coupled models, the rate of transfer of zonal available potentialenergy to eddy available potential energy in synoptic systems was foundto be too large, yielding too much energy and an overactive energy cycle(Marques et al., 2011). Models tend to have excessively zonal stormtracks and some show a poor extension of the storm tracks into Europe(Pinto et al., 2006; Greeves et al., 2007; Orsolini and Sorteberg, 2009).It has also been noted that representation of cyclone activity maydepend on the physics formulations and the horizontal resolution of themodel (Jung et al., 2006; Greeves et al., 2007).Paleoclimatic proxies for extratropical cyclone variability are still few,but progress is being made in using coastal dune field development andsand grain content of peat bogs as proxies for storminess. Publicationscovering parts of western Europe indicate enhanced sand movement inEuropean coastal areas during the Little Ice Age (Wilson et al., 2004; deJong et al., 2006, 2007; Clemmensen et al., 2007; Clarke and Rendell, 2009;Sjogren, 2009). It should be noted that sand influx is also influenced bysediment availability, which is controlled mainly by the degree ofvegetation cover and the moisture content of the sediment (Li et al.,2004; Wiggs et al., 2004). Intense cultivation, overgrazing, and forestdisturbance make soils more prone to erosion, which can lead toincreased sand transport even under less windy conditions. Thus theinformation gained from paleoclimatic proxies to put the last 100 yearsof extratropical cyclone variability in context is limited.Century-long time-series of estimates of extremes in geostrophic winddeduced from triangles of pressure stations, pressure tendencies from163