Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeorbital-time-scale variations were interannualto millennial time-scale variations, many abruptin nature (Mayewski et al., 2004; Viau et al.,2006), arising from variations in solar output,volcanic aerosols, and internally generatedcovariations among the different componentsof the climate system. On longer, or “orbital”time scales, the ice sheets, biogeochemicallydetermined greenhouse gas concentrations, anddust and aerosol loading should be regardedas internal components of the climate system,but over the past 11,000 years, they changedslowly enough relative to other components ofthe climate system, such as the atmosphere andsurface ocean, that they are most appropriatelyconsidered as external controls of regional-scaleclimate variations (Saltzman, 2002).2.1.1 Coupled Ocean-AtmosphereForcing of North AmericanHydrological VariabilityThe standard approach that uses models todemonstrate a link between SSTs and observedclimate variability involves forcing an AtmosphericGeneral Circulation Model (AGCM)with observed SSTs as a lower boundarycondition (see Hoerling et al., 2008 (CCSPSAP 1.3, Box 3.2) for further discussion of thisapproach). Ensembles of simulations are usedwith different initial conditions such that theinternally generated atmospheric weather inthe ensemble members is uncorrelated from onemember to the next and, after averaging overthe ensemble, the part of the model simulationcommon to all—the part that is SST forced—is isolated. The relative importance of SSTanomalies in different ocean basins can beassessed by specifying observed SSTs only insome areas and using climatological SSTs (orSSTs computed with a mixed layer (ML) ocean)elsewhere.Schubert et al. (2004a,b) performed a climatemodel simulation from 1930 to 2004, whichsuggested that both a cold eastern equatorialPacific and a warm subtropical Atlantic werethe underlying forcing for drought over NorthAmerica in the 1930s. Seager et al. (2005b) andHerweijer et al. (2006) performed ensemblesthat covered the entire period of SST observationssince 1856. These studies conclude thatcold eastern equatorial Pacific SST anomaliesin each of the three 19th century droughts, theDust Bowl, and the 1950s drought were theprime forcing factors. Seager (2007) has madethe same case for the 1998–2002 period ofthe current drought, suggesting a supportingrole for warm subtropical Atlantic in forcingdrought in the West. During the 1930sand 1950s droughts, the Atlantic was warm,whereas, the 19th century droughts seem to bemore solely Pacific driven. Results for the DustBowl drought are shown in Figure 3.3, and timeseries of modeled and observed precipitationover the Great Plains are shown in Figure 3.4.Hoerling and Kumar (2003) instead emphasizethe combination of a La Niña-like state and awarm Indo-west Pacific Ocean in forcing the1998–2002 period of the most recent drought.On longer time scales, Huang et al. (2005) haveshown that models forced by tropical PacificSSTs alone can reproduce the North Americanwet spell between the 1976–77 and 1997–98 ElNiños. The Dust Bowl drought was unusual inthat it did not impact the Southwest. Rather, itcaused reduced precipitation and high temperaturesin the northern Rocky Mountain Statesand the western Canadian prairies, a spatialpattern that models generally fail to simulate(Seager et al., 2007b).The SST anomalies prescribed in the climatemodels that result in reductions in precipitationare small, no more than a fraction of adegree Celsius. These changes are an orderof magnitude smaller than the SST anomaliesassociated with interannual El Niño/SouthernOscillation (ENSO) events or Holocene SSTvariations related to insolation (incoming solarradiation) variations (~0.50 °C; Liu et al., 2003,2004). It is the persistence of the SST anomaliesand associated moisture deficits that createserious drought conditions. In the Pacific, theSST anomalies presumably arise naturally fromENSO-like dynamics on time scales of a year toa decade (Newman et al., 2003). The warm SSTanomalies in the Atlantic that occurred in the1930s and 1950s (and in between), and usuallyreferred to as part of an Atlantic MultidecadalOscillation (AMO; Kushnir, 1994; Enfield etal., 2001), are of unknown origin. Kushnir(1994), Sutton and Hodson (2005), and Knightet al. (2005) have linked them to changes inthe Meridional Overturning Circulation (seeChapter 4), which implies that a strongeroverturning and a warmer North Atlantic Ocean75