Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeThe summertime establishment of the upperlevelridge, the related subsidence over themiddle of the North American continent, andthe onshore flow and uplift in the SouthwesternUnited States and Northern Mexico areinfluenced to a large extent by the topographyof western North America, which is greatlyoversimplified in GCMs (see Fig. 4 in Bartleinand Hostetler, 2004). This potential “built-in”source of mismatch between the paleoclimaticsimulations and observations can be reduced bysimulating climate with regional climate models(RCMs). Summer (June, July, and August)precipitation and soil moisture simulated usingRegCM3 (Diffenbaugh et al., 2006) is shown inFigure 3.14, which illustrates moisture anomaliesthat are more comparable in magnitudeto those recorded by the paleoenvironmentaldata than are the GCM simulations. RegCMas applied in these simulations has a spatialresolution of 55 km, which resolves climaticallyimportant details of the topography of theWestern United States. In these simulations,the “lateral boundary conditions” or inputs tothe RCM, were supplied by a simulation usingan AGCM (CAM 3), that in turn used the SSTssimulated by the fully coupled AOGCM simulationfor 6 ka (and present) by Otto-Bliesneret al. (2006). These SSTs were also supplieddirectly to RegCM3. The simulations thusreveal the impact of the insolation forcing, aswell as the influence of the insolation-relatedchanges on interannual variability in SSTs (overthe 30 years of each simulation). The resultsclearly show the suppression of precipitationover the midcontinent and enhancement overthe Southwestern United States and NorthernMexico, and the contribution of the precipitationanomaly to that of soil moisture (Fig. 3.14). Incontrast to the GCM simulations, the inclusionof 6 ka SST variability in the RCM simulationsreduces slightly the magnitude of the moistureanomalies, but overall these anomalies are closeto those inferred from paleoenvironmental observationsand reinforce the conceptual modellinking the North American midcontinentalHolocene drought to increased subsidence (seealso Shinker et al., 2006; Harrison et al., 2003).The potential of vegetation feedback to amplifythe middle Holocene drought has not been asintensively explored as it has for Africa, butFigure 3.14. Regional climate model (RegCM3) simulations of precipitation rate (A, B) and soilmoisture (C, D) for 6,000 years before present (6 ka) (Diffenbaugh et al., 2006, land grid pointsonly). RegCM is run using lateral boundary conditions supplied by CAM3, the atmospheric componentof CCSM3. In panels A and C, the CAM3 boundary conditions included 6 ka-insolation,and time-varying sea-surface temperatures (SSTs) provided by a fully coupled Atmosphere-OceanGeneral Circulation Model (AOGCM) simulation for 6 ka using CCSM3 (Otto-Bliesner et al., 2006).In panels B and D, the CAM3 boundary conditions included 6-ka insolation, and time-varying SSTsprovided by a fully coupled CCSM simulation for the present. The differences between simulationsreveal the impact of the insolation-forced differences in SST variability between 6 ka and present.mm, millimeters; mm/d, millimeters per day.103