IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3monsoon will be stronger and the winter monsoon will be weaker in thefuture than now. However, model results derived from the analyses of15 CMIP3 global models are not as straightforward as implied by thissimple consideration (Tanaka et al., 2005), as they show a weakening ofthese tropical circulations by the late 21st century compared to the late20th century. In turn, such changes in circulation may lead to changesin precipitation associated with monsoons. For instance, the monsoonalprecipitation in Mexico and Central America is projected to decrease inassociation with increasing precipitation over the eastern equatorialPacific through changes in the Walker circulation and local Hadleycirculation (e.g., Lu et al., 2007). Furthermore, observations and modelssuggest that changes in monsoons are related at least in part tochanges in observed SSTs, as noted above.At regional scales, there is little consensus in GCM projections regardingthe sign of future change in monsoon characteristics, such as circulationand rainfall. For instance, while some models project an intensedrying of the Sahel under a global warming scenario, others project anintensification of the rains, and some project more frequent extremeevents (Cook and Vizy, 2006). Increases in precipitation are projected inthe Asian monsoon (along with an increase in interannual seasonaveragedprecipitation variability), and in the southern part of the westAfrican monsoon, but with some decreases in the Sahel in northernsummer. In the Australian monsoon in southern summer, an analysis byMoise and Colman (2009) from the entire ensemble mean of CMIP3simulations suggested no changes in Australian tropical rainfall duringthe summer and only slightly enhanced interannual variability.A study of 19 CMIP3 global models reported a projected increase inmean south Asian summer monsoon precipitation of 8% and a possibleextension of the monsoon period (Kripalani et al., 2007). A study(Ashfaq et al., 2009) from the downscaling of the National Center forAtmospheric Research (NCAR) CCSM3 global model using the RegCM3regional model suggests a weakening of the large-scale monsoon flowand suppression of the dominant intra-seasonal oscillatory modes withoverall weakening of the south Asian summer monsoon by the end ofthe 21st century, resulting in a decrease in summer precipitation insoutheast Asia.Kitoh and Uchiyama (2006) used 15 models under the A1B scenario toanalyze the changes in intensity and duration of precipitation in theBaiu-Changma-Meiyu rain band at the end of the 21st century. Theyfound a delay in early summer rain withdrawal over the region extendingfrom the Taiwan province of China, and across the Ryukyu Islands to thesouth of Japan, contrasted with an earlier withdrawal over the YangtzeBasin. They attributed this feature to El Niño-like mean state changesover the monsoon trough and subtropical anticyclone over the westernPacific region (Meehl et al., 2007b). A southwestward extension of thesubtropical anticyclone over the northwestern Pacific Ocean associatedwith El Niño-like mean state changes and a dry air intrusion in the midtropospherefrom the Asian continent to the northwest of Japan providesfavorable conditions for intense precipitation in the Baiu season inJapan (Kanada et al., 2010a). Kitoh et al. (2009) projected changes inprecipitation characteristics during the east Asian summer rainy season,using a 5-km mesh cloud-resolving model embedded in a 20-km meshglobal atmospheric model with CMIP3 mean SST changes. The frequencyof heavy precipitation was projected to increase at the end of the 21stcentury for hourly as well as daily precipitation. Further, extreme hourlyprecipitation was projected to increase even in the near future (2030s)when the temperature increase is still modest, even though uncertaintiesin the projection (and even the simulation) of hourly rainfall are still high.Climate change scenarios for the 21st century show a weakening of theNorth American monsoon through a weakening and poleward expansionof the Hadley cell (Lu et al., 2007). The expansion of the Hadley cell iscaused by an increase in the subtropical static stability, which pushespoleward the baroclinic instability zone and hence the outer boundaryof the Hadley cell. Simple physical arguments (Held and Soden, 2006)predict a slowdown of the tropical overturning circulation under globalwarming. A few studies (e.g., Marengo et al., 2009a) have projected overthe period 1960-2100 a weak tendency for an increase in dry spells. Theprojections show an increase in the frequency of rainfall extremes insoutheastern South America by the end of the 21st century, possibly dueto an intensification of the moisture transport from Amazonia by a morefrequent/intense low-level jet east of the Andes in the A2 emissionsscenario (Marengo et al., 2009a; Soares and Marengo, 2009).There are many deficiencies in model representation of the monsoonsand the processes affecting them, and this reduces confidence in theirability to project future changes. Some of the uncertainty in global andregional climate change projections in the monsoon regions results fromthe limits in the model representation of resolved processes (e.g., moistureadvection), the parameterizations of sub-grid-scale processes (e.g.,clouds, precipitation), and model simulations of feedback mechanismsat the global and regional scale (e.g., changes in land use/cover; seealso Section 3.1.4). Kharin et al. (2007) made an intercomparison ofprecipitation extremes in the tropical region in all AR4 models withobserved extremes expressed as 20-year return values. They found verylarge disagreement in the tropics suggesting that some physicalprocesses associated with extreme precipitation are not well representedby the models due to model resolution and physics. Shukla (2007) notedthat current climate models cannot even adequately predict the meanintensity and the seasonal variations of the Asian summer monsoon. Thisreduces confidence in the projected changes in extreme precipitationover the monsoon regions. Many of the important climatic effects of theMadden-Julian Oscillation (MJO, a natural mode of the climate systemoperating on time scales of about a month), including its impacts onrainfall variability in the monsoons, are still poorly simulated bycontemporary climate models (Christensen et al., 2007).Current GCMs still have difficulties and display a wide range of skill insimulating the subseasonal variability associated with the Asian summermonsoon (Lin et al., 2008b). Most GCMs simulate westward propagationof the coupled equatorial easterly waves, but relatively poor eastwardpropagation of the MJO and overly weak variances for both the easterlywaves and the MJO. Most GCMs are able to reproduce the basic154