Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Programthe moisture decrease in the midcontinentneeded for these vegetation changes is about350 millimeters per year (mm y –1 ), or about1 millimeter per day (mm d –1 ), or levels between50 and 80 percent of the present-day values.As is the case for the African humid period,the effective-moisture variations recorded bypaleoenvironmental data from the midcontinentof North America provide a target forsimulation by climate models, and also as wasthe case for Africa, those simulations haveevolved over time toward models with increasedcoupling among systems. The first generation ofsimulations with AGCMs featured models thatwere of relatively coarse spatial resolution, hadfixed SSTs, and land cover that was specifiedto match that of the modern day. These simulations,focusing on 6 ka, revealed some likelymechanisms for developing dry conditions inthe midcontinent, such as the impact of theinsolation forcing on surface energy and waterbalances and the direct and indirect effects ofinsolation on atmospheric circulation (Webbet al., 1993b; Bartlein et al., 1998; Webb etal., 1998). However, the specific simulationsof precipitation or precipitation minus evapotranspiration(P–E) indicated little change inmoisture or even increases in some regions.Given the close link between SST variationsand drought across North America at present,and the inability of these early simulations tosimulate such mechanisms because they hadfixed SSTs, this result is not surprising.What can be regarded as the current-generationsimulations for 6 ka include those done with fullycoupled AOGCMs (FOAM and CSM 1, Harrisonet al., 2003; CCSM 3, Otto-Bliesner et al.,2006), and an AGCM with a mixed-layerocean (CCM 3.10, Shin et al., 2006). Thesesimulations thereby allow the influence of SSTvariations to be registered in the simulatedclimate either implicitly, by calculating themin the ocean component of the models (FOAM,CSM 1, CCSM 3), or explicitly, by imposingthem either as present-day long-term averages,or as perturbations of those long-term averagesintended to represent extreme states of, forexample, ENSO (CCM 3.10). The trade-off betweenthese approaches is that the fully coupled,implicit approach will reflect the impact of thelarge-scale controls of climate (e.g., insolation)on SST variability (if the model simulates thejoint response of the atmosphere and oceancorrectly), while the explicitly specified AGCMapproach allows the response to a hypotheticalstate of the ocean to be judged.These simulations produce generally dry conditionsin the interior of North America duringthe growing season (and an enhancement ofthe North American monsoon), but as was thecase for Africa, the magnitude of the moisturechanges is not as large as that recorded bythe paleoenvironmental data (with maximumprecipitation-rate anomalies on the order of0.5 mm d –1 , roughly half as large as it wouldneed to be to match the paleoenvironmental observations).Despite this, the simulations revealsome specific mechanisms for generating thedry conditions; these include (1) atmosphericcirculation responses to the insolation andSST forcing/feedback that favor a “package”of circulation anomalies that include expansionof the subtropical high-pressure systems insummer, (2) the development of an upper-levelridge and large-scale subsidence over centralNorth America (a circulation feature that favorsdrought at the present), and (3) changes insurface energy and water balances that lead toreinforcement of this circulation configuration.Analyses of the 6 ka simulated and present-day“observed” (i.e., reanalysis data) circulationwere used by Harrison et al. (2003) to describethe linkage that exists in between the uplift thatoccurs in the Southwestern United States andNorthern Mexico as part of the North Americanmonsoon system, and subsidence on the GreatPlains and Pacific Northwest (Higgins et al.,1997; see also Vera et al., 2006).Chapter 3102