IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentIn the African monsoon region, Fontaine et al. (2011) investigatedrecent observed trends using high-resolution gridded precipitation(period 1979-2002), OLR, and reanalyses. Their results revealed a rainfallincrease in North Africa since the mid-1990s. Over the longer term,however, Zhou et al. (2008a,b) and Wang and Ding (2006) reported anoverall decreasing long-term trend in global land monsoon rainfallduring the last 54 years, which was mainly caused by decreasing rainfallin the North African and South Asian monsoons.For the American monsoon regions, Cavazos et al. (2008) reportedincreases in the intensity of precipitation in the mountain sites of thenorthwestern Mexico section of the North American monsoon over the1961-1998 period, apparently related to an increased contribution fromheavy precipitation derived from tropical cyclones. Arriaga-Ramírez andCavazos (2010) found that total and extreme rainfall in the monsoonregion of western Mexico and the US southwest presented a statisticallysignificant increase during 1961-1998, mainly in winter. Groisman andKnight (2008) found that consecutive dry days (see Box 3-3 for definition)have significantly increased in the US southwest. On the other hand,increases in heavy precipitation during 1960-2000 in the South Americanmonsoon have been documented by Marengo et al. (2009a,b) andRusticucci et al. (2010). Studies using circulation fields such as 850 hPawinds or moisture flux have been performed for the South Americanmonsoon system for assessments of the onset and end of the monsoon,and indicate that the onset exhibits a marked interannual variabilitylinked to variations in SST anomalies in the eastern Pacific and tropicalAtlantic (Gan et al., 2006; da Silva and de Carvalho, 2007; Raia andCavalcanti, 2008; Nieto-Ferreira and Rickenbach, 2011).Attributing the causes of changes in monsoons is difficult in partbecause there are substantial inter-model differences in representingAsian monsoon processes (Christensen et al., 2007). Most modelssimulate the general migration of seasonal tropical rain, although theobserved maximum rainfall during the monsoon season along the westcoast of India, the North Bay of Bengal, and adjoining northeast India ispoorly simulated by many models due to limited resolution. Bollasina andNigam (2009) show the presence of large systematic biases in coupledsimulations of boreal summer precipitation, evaporation, and SST in theIndian Ocean. Many of the biases are pervasive, being common to mostsimulations.The observed negative trend in global land monsoon rainfall is betterreproduced by atmospheric models forced by observed historical SSTthan by coupled models without explicit forcing by observed oceantemperatures (Kim et al., 2008). This trend in the east Asian monsoon isstrongly linked to the warming trend over the central eastern Pacific andthe western tropical Indian Ocean (Zhou et al., 2008b). For the westAfrican monsoon, Joly and Voldoire (2010) explore the role of Gulf ofGuinea SSTs in its interannual variability. In most of the studied CMIP3simulations, the interannual variability of SST is very weak in the Gulf ofGuinea, especially along the Guinean Coast. As a consequence, theinfluence on the monsoon rainfall over the African continent is poorlyreproduced. It is suggested that this may be due to the counteractingeffects of the Pacific and Atlantic basins over the last decades. Thedecreasing long-term trend in north African summer monsoon rainfall maybe due to the atmosphere response to observed SST variations (Hoerlinget al., 2006; Zhou et al., 2008b; Scaife et al., 2009). A similar trend inglobal monsoon precipitation in land regions is reproduced in CMIP3models’ 20th-century simulations when they include anthropogenicforcing, and for some simulations natural forcing (including volcanicforcing) as well, though the trend is much weaker in general, with theexception of one model (HadCM3) capable of producing a trend ofsimilar magnitude (Li et al., 2008). The decrease in east Asian monsoonrainfall also seems to be related to tropical SST changes (Li et al., 2008),and the less spatially coherent positive trends in precipitation extremesin the southeast Asian and north Australian monsoons appear to bepositively correlated with a La Niña-like SST pattern (Caesar et al., 2011).A variety of factors, natural and anthropogenic, have been suggested aspossible causes of variations in monsoons. Changes in regional monsoonsare strongly influenced by the changes in the states of dominant patternsof climate variability such as ENSO, the Pacific Decadal Oscillation (PDO),the Northern Annular Mode (NAM), the Atlantic Multi-decadal Oscillation(AMO), and the Southern Annular Mode (SAM) (see also Sections 3.4.2and 3.4.3). Additionally, model-based evidence has suggested that landsurface processes and land use changes could in some instancessignificantly impact regional monsoons. Tropical land cover change inAfrica and southeast Asia appears to have weaker local climatic impactsthan in Amazonia (Voldoire and Royer, 2004; Mabuchi et al., 2005a,b).Grimm et al. (2007) and Collini et al. (2008) explored possible feedbacksbetween soil moisture and precipitation during the early stages of themonsoon in South America, when the surface is not sufficiently wet, andsoil moisture anomalies may thus also modulate the development ofprecipitation. However, the influence of historical land use on themonsoon is difficult to quantify, due both to the poor documentation ofland use and difficulties in simulating the monsoon at fine scales. Theimpact of aerosols (black carbon and sulfate) on changes in rainfallvariability and amounts in monsoon regions has been discussed byMeehl et al. (2008), Lau et al. (2006), and Silva Dias et al. (2002). Thesestudies suggest that there are still large uncertainties and a strongmodel dependency in the representation of the relevant land surfaceprocesses and the role of aerosol direct forcing, and resulting interactions(e.g., in the case of land use forcing; Pitman et al., 2009).Regarding projections of change in the monsoons, the AR4 (Christensen etal., 2007) concluded: “There is a tendency for monsoonal circulations toresult in increased precipitation due to enhanced moisture convergence,despite a tendency towards weakening of the monsoonal flowsthemselves. However, many aspects of tropical climatic responses remainuncertain.” Held and Soden (2006) demonstrate that an increase in thehydrological cycle is accompanied by a global weakening of the largescalecirculation. As global warming is projected to lead to fasterwarming over land than over the oceans (e.g., Meehl et al., 2007b;Sutton et al., 2007), the continental-scale land-sea thermal contrast, amajor factor affecting monsoon circulations, will become stronger insummer. Based on this projection, a simple scenario is that the summer153