Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Programchange for which the ultimate controls areknown and (2) to use the mechanistic aspectsof the models and simulations produced withthem to explain the patterns and variationsrecorded by the data. Mismatches between thesimulations and observations can arise fromone or more sources, including inadequaciesof the climate models, misinterpretation ofthe paleoenvironmental data, and incompletenessof the experimental design (i.e., failureto include one or more controls or processesthat influenced the real climate) (Peteet, 2001;Bartlein and Hostetler, 2004).In general, the simulations done as part ofPMIP, as well as others, show a clear amplificationof the African-Asian monsoon during theearly and middle part of the Holocene, but onethat is insufficient to completely explain themagnitude of the changes in lake status, and theextent of the observed northward displacementof the vegetation zones into the region nowoccupied by desert (Joussaume et al., 1999;Kohfeld and Harrison, 2000). The initial PMIPsimulations were “snapshot” or “time-slice”simulations of the conditions around 6 ka, andas a consequence are able to only indirectlycomment on the mechanisms involved in theabrupt beginning and end of the humid period.In addition, the earlier simulations wereperformed using AGCMs, with present-dayland-surface characteristics, which thereforedid not adequately represent the full influenceof the ocean or terrestrial vegetation on thesimulated climate.As a consequence, climate-simulation exercisesthat focus on the African monsoon or the Africanhumid period have evolved over the pastdecade or so toward models and experimentaldesigns that (1) include interactive couplingamong the atmosphere, ocean, and terrestrialbiosphere and (2) feature transient, or timeevolvingsimulations that, for example, allowexplicit examination of the timing and rate ofthe transition from a green to a brown Sahara.Two classes of models have been used, including(1) Atmosphere Ocean General CirculationModels with interactive oceans (AOGCMs), AtmosphereTerrestrial Vegetation General CirculationModels (AVGCMs), or both (AOVGCMs)that typically have spatial resolutions of a fewdegrees of latitude and longitude and (2) coarserresolution EMICs, or Earth-System Models ofIntermediate Complexity, that include representationof components of the climate system thatare not amenable to simulation with the higherresolution GCMs. (See Claussen, 2001, andBartlein and Hostetler, 2004, for a discussionof the taxonomy of climate models.)The coupled AOGCM simulations have illuminatedthe role that sea-surface temperatureslikely played in the amplification of the monsoon.Driven by both the insolation forcing andby ocean-atmosphere interactions, the pictureemerges of a role for the oceans in modulatingthe amplified seasonal cycle of insolation duringthe early and mid-Holocene in such a way as toincrease the summertime temperature contrastbetween continent and ocean that drives themonsoon, thereby strengthening it (Kutzbachand Liu, 1997; Zhao et al., 2005). In addition,there is an apparent role for teleconnectionsfrom the tropical Pacific in determining thestrength of the monsoon, in a manner similarto the “atmospheric bridge” teleconnectionbetween the tropical Pacific ocean and climateelsewhere at present (Shin et al., 2006; Zhao etal., 2007; Liu and Alexander, 2007).The observation of the dramatic vegetationchange motivated the development of simulationswith coupled vegetation components,first by asynchronously coupling equilibriumglobal vegetation models (EGVMs, Texier et al.,1997), and subsequently by using fully coupledAOVGCMs (e.g., Levis et al., 2004; Wohlfahrtet al., 2004; Gallimore et al., 2005; Braconnotet al., 2007a,b; Liu et al., 2007). These simulations,which also included investigation of thesynergistic effects of an interactive ocean andvegetation on the simulated climate (Wohlfahrtet al., 2004), produced results that stillunderrepresented the magnitude of monsoonenhancement, but to a lesser extent than theearlier AGCM or AOGCM simulations. Thesesimulations also suggest the specific mechanismsthrough which the vegetation and therelated soil-moisture conditions (Levis et al.,2004; Liu et al., 2007) influence the simulatedmonsoon.The EMIC simulations, run as transient or continuous(as opposed to time-slice) simulationsover the Holocene, are able to explicitly revealChapter 396