IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentcharacteristics of the precipitation seasonal cycle associated withthe South American Monsoon System (SAMS), but there are largediscrepancies in the South Atlantic Convergence Zone represented bythe models in both intensity and location, and in its seasonal evolution(Vera et al., 2006). In addition, models exhibit large discrepancies in thedirection of the changes associated with the summer (SAMS) precipitation,which makes the projections for that region highly uncertain. Lin et al.(2008a) show that the coupled GCMs have significant problems anddisplay a wide range of skill in simulating the North American monsoonand associated intra-seasonal variability.Most of the models reproduce the monsoon rain belt, extending fromsoutheast to northwest, and its gradual northward shift in early summer,but overestimate the precipitation over the core monsoon regionthroughout the seasonal cycle and fail to reproduce the monsoonretreat in the fall. The AR4 assessed that models fail in representing themain features of the west African monsoon although most of them dohave a monsoonal climate albeit with some distortion (Christensen etal., 2007). Other major sources of uncertainty in projections of monsoonchanges are the responses and feedbacks of the climate system toemissions as represented in climate models. These uncertainties areparticularly related to the representation of the conversion of emissionsinto concentrations of radiatively active species (i.e., via atmosphericchemistry and carbon cycle models) and especially those derived fromaerosol products of biomass burning, which can affect the onset of therainy season (Silva Dias et al., 2002). The subsequent response of thephysical climate system complicates the nature of future projections ofmonsoon precipitation. Moreover, the long-term variations of modelskill in simulating monsoons and their variations represent an additionalsource of uncertainty for the monsoon regions, and indicate that theregional reliability of long climate model runs may depend on the timeslice for which the output of the model is analyzed.The AR4 (Hegerl et al., 2007) concluded that the currentunderstanding of climate change in the monsoon regions remainsone of considerable uncertainty with respect to circulation andprecipitation. With a few exceptions in some monsoon regions,this has not changed. These conclusions have been based on veryfew studies, there are many issues with model representation ofmonsoons and the underlying processes, and there is littleconsensus in climate models, so there is low confidence inprojections of changes in monsoons, even in the sign of the change.However, one common pattern is a likely increase in extremeprecipitation in monsoon regions (see Section 3.3.2), though notnecessarily induced by changes in monsoon characteristics, andnot necessarily occurring in all monsoon regions.3.4.2. El Niño-Southern OscillationThe El Niño-Southern Oscillation (ENSO) is a natural fluctuation of theglobal climate system caused by equatorial ocean-atmosphere interactionin the tropical Pacific Ocean (Philander, 1990). The term ‘SouthernOscillation’ refers to a tendency for above-average surface atmosphericpressures in the Indian Ocean to be associated with below-averagepressures in the Pacific, and vice versa. This oscillation is associatedwith variations in SSTs in the east equatorial Pacific. The oceanic andatmospheric variations are collectively referred to as ENSO. An El Niñoepisode is one phase of the ENSO phenomenon and is associated withabnormally warm central and east equatorial Pacific Ocean surfacetemperatures, while the opposite phase, a La Niña episode, is associatedwith abnormally cool ocean temperatures in this region. Both phasesare associated with a characteristic spatial pattern of droughts andfloods. An El Niño episode is usually accompanied by drought insoutheastern Asia, India, Australia, southeastern Africa, Amazonia, andnortheast Brazil, with fewer than normal tropical cyclones aroundAustralia and in the North Atlantic. Wetter than normal conditionsduring El Niño episodes are observed along the west coast of tropicalSouth America, subtropical latitudes of western North America, andsoutheastern America. In a La Niña episode the climate anomalies areusually the opposite of those in an El Niño. Pacific islands are stronglyaffected by ENSO variations. Recent research (e.g., Kenyon and Hegerl,2008; Ropelewski and Bell, 2008; Schubert et al., 2008a; Alexander et al.,2009; Grimm and Tedeschi, 2009; Zhang et al., 2010) has demonstratedthat different phases of ENSO (El Niño or La Niña episodes) also areassociated with different frequencies of occurrence of short-term weatherextremes such as heavy rainfall events and extreme temperatures. Therelationship between ENSO and interannual variations in tropical cycloneactivity is well known (e.g., Kuleshov et al., 2008). The simultaneousoccurrence of a variety of climate extremes in an El Niño episode (or aLa Niña episode) may provide special challenges for organizations copingwith disasters induced by ENSO (see also Section 3.1.1). Monitoring andpredicting ENSO can lead to disaster risk reduction through early warning(see Case Study 9.2.11).The AR4 noted that orbital variations could affect the ENSO behavior(Jansen et al., 2007). Cane (2005) found that a relatively simple coupledmodel suggested that systematic changes in El Niño could be stimulatedby seasonal changes in solar insolation. However, a more comprehensivemodel simulation (Wittenberg, 2009) has suggested that longtermchanges in the behavior of the phenomenon might occur evenwithout forcing from radiative changes. Vecchi and Wittenberg (2010)concluded that the “tropical Pacific could generate variations in ENSOfrequency and intensity on its own (via chaotic behavior), respond toexternal radiative forcings (e.g., changes in greenhouse gases, volcaniceruptions, atmospheric aerosols, etc), or both.” Meehl et al. (2009a)demonstrate that solar insolation variations related to the 11-yearsunspot cycle can affect ocean temperatures associated with ENSO.ENSO has varied in strength over the last millennium with strongeractivity in the 17th century and late 14th century, and weaker activityduring the 12th and 15th centuries (Cobb et al., 2003; Conroy et al.,2009). On longer time scales, there is evidence that ENSO may havechanged in response to changes in the orbit of the Earth (Vecchi andWittenberg, 2010), with the phenomenon apparently being weakeraround 6,000 years ago (according to proxy measurements from corals155