IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3and climate model simulations; Rein et al., 2005; Brown et al., 2006;Otto-Bliesner et al., 2009), and model simulations suggest that it wasstronger at the last glacial maximum (An et al., 2004). Fossil coralevidence indicates that the phenomenon continued to operate duringthe last glacial interval (Tudhope et al., 2001). Thus the paleoclimaticevidence indicates that ENSO can continue to operate, although alteredperhaps in intensity, in very different background climate states.The AR4 noted that the nature of ENSO has varied substantially over theperiod of instrumental data, with strong events from the late 19thcentury through the first quarter of the 20th century and again after1950. An apparent climate shift around 1976-1977 was associated witha shift to generally above-normal SSTs in the central and eastern Pacificand a tendency toward more prolonged and stronger El Niño episodes(Trenberth et al., 2007). Ocean temperatures in the central equatorialPacific (the so-called NINO3 index) suggest a trend toward more frequentor stronger El Niño episodes over the past 50 to 100 years (Vecchi andWittenberg, 2010). Vecchi et al. (2006) reported a weakening of theequatorial Pacific pressure gradient since the 1960s, with a sharp dropin the 1970s. Power and Smith (2007) proposed that the apparentdominance of El Niño during the last few decades was due in part to achange in the background state of the Southern Oscillation Index (SOI,the standardized difference in surface atmospheric pressure betweenTahiti and Darwin), rather than a change in variability or a shift to morefrequent El Niño events alone. Nicholls (2008) examined the behavior ofthe SOI and another index, the NINO3.4 index of central equatorialPacific SSTs, but found no evidence of trends in the variability or thepersistence of the indices [although Yu and Kao (2007) reported decadalvariations in the persistence barrier, the tendency for weaker persistenceacross the Northern Hemisphere spring], nor in their seasonal patterns.There was a trend toward what might be considered more ‘El Niño-like’behavior in the SOI (and more weakly in NINO3.4), but only through theperiod March to September and not in November to February, the seasonwhen El Niño and La Niña events typically peak. The trend in the SOIreflected only a trend in Darwin pressures, with no trend in Tahitipressures. Apart from this trend, the temporal/seasonal nature of ENSO hasbeen remarkably consistent through a period of strong global warming.There is evidence, however, of a tendency for recent El Niño episodes tobe centered more in the central equatorial Pacific than in the east Pacific(Yeh et al., 2009), and for these central Pacific episodes to be increasingin intensity (Lee and McPhaden, 2010). In turn, these changes mayexplain changes that have been noted in the remote influences of thephenomenon on the climate over Australia and in the mid-latitudes(Wang and Hendon, 2007; Weng et al., 2009). For instance, Taschetto etal. (2009) demonstrated that episodes with the warming centered in thecentral Pacific exhibit different patterns of Australian rainfall variationsrelative to the east Pacific-centered El Niño events.The possible role of increased greenhouse gases in affecting the behaviorof ENSO over the past 50 to 100 years is uncertain. Yeh et al. (2009)suggested that changes in the background temperature associated withincreases in greenhouse gases should affect the behavior of El Niño,such as the location of the strongest SST anomalies, because El Niñobehavior is strongly related to the average ocean temperature gradientsin the equatorial Pacific. Some studies (e.g., Q. Zhang et al., 2008) havesuggested that increased activity might be due to increased CO 2 ;however, no formal attribution study has yet been completed and someother studies (e.g., Power and Smith, 2007) suggest that changes in thephenomenon are within the range of natural variability (i.e., that nochange has yet been detected, let alone attributed to a specific cause).Global warming is projected to lead to a mean reduction in the zonalmean wind across the equatorial Pacific (Vecchi and Soden, 2007b).However, this change should not be described as an ‘El Niño-like’ averagechange even though during an El Niño episode these winds also weaken,because there is only limited correspondence between these changes inthe mean state of the equatorial Pacific and an El Niño episode. TheAR4 determined that all models exhibited continued ENSO interannualvariability in projections through the 21st century, but the projectedbehavior of the phenomenon differed between models, and it wasconcluded that “there is no consistent indication at this time ofdiscernible changes in projected ENSO amplitude or frequency in the21st century” (Meehl et al., 2007b). Models project a wide variety ofchanges in ENSO variability and the frequency of El Niño episodes as aconsequence of increased greenhouse gas concentrations, with a rangebetween a 30% reduction to a 30% increase in variability (van Oldenborghet al., 2005). One model study even found that although ENSO activityincreased when atmospheric CO 2 concentrations were doubled orquadrupled, a considerable decrease in activity occurred when CO 2 wasincreased by a factor of 16 times, much greater than is possible throughthe 21st century (Cherchi et al., 2008), suggesting a wide variety ofpossible ENSO changes as a result of CO 2 changes. The remote impacts,on rainfall for instance, of ENSO may change as CO 2 increases, even ifthe equatorial Pacific aspect of ENSO does not change substantially. Forinstance, regions in which rainfall increases in the future tend to showincreases in interannual rainfall variability (Boer, 2009), without anystrong change in the interannual variability of tropical SSTs. Also, sincesome long-term projected changes in response to increased greenhousegases may resemble the climate response to an El Niño event, this mayenhance or mask the response to El Niño events in the future (Lau et al.,2008b; Müller and Roeckner, 2008).One change that models tend to project is an increasing tendency for ElNiño episodes to be centered in the central equatorial Pacific, ratherthan the traditional location in the eastern equatorial Pacific. Yeh et al.(2009) examined the relative frequency of El Niño episodes simulated incoupled climate models with projected increases in greenhouse gasconcentrations. A majority of models, especially those best able tosimulate the current ratio of central Pacific locations to east Pacificlocations of El Niño events, projected a further increase in the relativefrequency of these central Pacific events. Such a change would also haveimplications for the remote influence of the phenomenon on climate awayfrom the equatorial Pacific (e.g., Australia and India). However, even theprojection that the 21st century may see an increased frequency of centralPacific El Niño episodes, relative to the frequency of events locatedfurther east (Yeh et al., 2009), is subject to considerable uncertainty. Of156