IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3this time. There is thus low confidence in projections of changes in suchsmall-scale systems because of limited studies, inability of climate modelsto resolve these phenomena, and possible competing factors affectingfuture changes. Confidence in the extreme wind changes is thereforelower in the regions most influenced by these phenomena irrespectiveof whether there is high agreement between GCMs on the sign of thewind speed change.In addition to studies using GCMs there have also been several recentstudies employing RCMs. Those focusing on Europe (e.g., Beniston et al.,2007; Rockel and Woth, 2007; Haugen and Iversen, 2008; Rauthe et al.,2010) also provide a general picture of an increasing trend in extremewinds over northern Europe despite a range of different downscalingmodels used, the different GCMs in which the downscaling is undertaken,and different metrics used to quantify extreme winds. Small-scale polarlows that typically form north of 60°N have been found to decline infrequency in RCM simulations downscaled from a GCM under differentemission scenarios and this is related to greater stability over the regiondue to mid-troposphere temperatures warming faster than sea surfacetemperatures over the region (Zahn and von Storch, 2010). In other partsof the world there have been very few studies. Over China, Jiang et al.(2010b) projected decreases in annual and winter mean wind speedbased on two RCMs that downscale two different GCMs. Over NorthAmerica, statistical downscaling of winds from four GCMs over fiveairports in the northwest United States indicated declines in summerwind speeds and less certain changes in winter (Sailor et al., 2008).A number of recent studies have addressed observed changes inwind speed across different parts of the globe, but due to thevarious shortcomings associated with anemometer data and theinconsistency in anemometer and reanalysis trends in some regions,we have low confidence in wind trends and their causes at thisstage. We also have low confidence in how the observed trends inmean wind speed relate to trends in extreme winds. The fewstudies of projected extreme winds, combined with shortcomingsin the simulation of extreme winds and the different models,regions, and methods used to develop projections of this quantity,mean that we have low confidence in projections of changes inextreme winds (with the exception of changes associated withtropical cyclones; Section 3.4.4). There is low confidence inprojections of small-scale phenomena such as tornadoesbecause competing physical processes may affect future trendsand because climate models do not simulate such phenomena.3.4. Observed and Projected Changes inPhenomena Related to Weather andClimate Extremes3.4.1. MonsoonsChanges in monsoon-related extreme precipitation and winds due toclimate change are not well understood. Generally, precipitation is themost important variable, but it is also a variable associated with largeruncertainties in climate simulations and projections (Wang et al., 2005;Kang and Shukla, 2006). Changes in monsoons should be better depictedby large-scale dynamics, circulation, or moisture convergence morebroadly than via precipitation only. However, few studies have focusedon observed changes in the large-scale and regional monsoon circulations.Hence, in this section, we focus mostly on monsoon-induced changesin total and seasonal rainfall, with most discussions of intense rainfallcovered in Section 3.3.2.Modeling experiments to assess paleo-monsoons suggest that in thepast, during the Holocene due to orbital forcing on a millennial timescale, there was a progressive southward shift of the NorthernHemisphere summer position of the ITCZ around 8,000 years ago. Thiswas accompanied by a pronounced weakening of the monsoon rainfallsystems in Africa and Asia and increasing dryness on both continents,while in South America the monsoon was weaker and drier than in thepresent, as suggested both by models and paleoclimatic indicators(Wanner et al., 2008).The delineation of the global monsoon has been mostly performedusing rainfall data or outgoing longwave radiation (OLR) fields (Kim etal., 2008). Lau and Wu (2007) identified two opposite time evolutions inthe occurrence of rainfall events in the tropics: a negative trend inmoderate rain events and a positive trend in heavy and light rainevents. Positive trends in intense rain were located in deep convectivecores of the ITCZ, South Pacific Convergence Zone, Indian Ocean, andmonsoon regions.In the Indo-Pacific region, covering the southeast Asian and northAustralian monsoon, Caesar et al. (2011) found low spatial coherence intrends in precipitation extremes across the region between 1971 and2003. In the few cases where statistically significant trends in precipitationextremes were identified, there was generally a trend towards wetterconditions, in common with the global results of Alexander et al. (2006).Liu et al. (2011) reported a decline in recorded precipitation events inChina over 1960-2000, which was mainly accounted for by a decreasein light precipitation events, with intensities of 0.1-0.3 mm day -1 . Someof the extreme precipitation appeared to be positively correlated with aLa Niña-like sea surface temperature (SST) pattern, but withoutsuggesting the presence of a trend. With regard to wind changes, Guoet al. (2011) analyzed near-surface wind speed change in China and itsmonsoon regions from 1969 to 2005 and showed a statistically significantweakening in annual and seasonal mean wind.For the Indian monsoon, Rajeevan et al. (2008) showed that extremerain events have an increasing trend between 1901 and 2005, but thetrend is much stronger after 1950. Sen Roy (2009) investigated changesin extreme hourly rainfall in India, and found widespread increases inheavy precipitation events across India, mostly in the high-elevationregions of the northwestern Himalaya as well as along the foothills of theHimalaya extending south into the Indo-Ganges basin, and particularlyduring the summer monsoon season during 1980-2002.152