Book 2.indb - US Climate Change Science Program

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The U.S. Climate Change Science ProgramChapter 3Figure 3.3. The observed (top left) and modeled precipitation anomalies during the Dust Bowl (1932 to 1939)relative to an 1856 to 1928 climatology. Observations are from Global Historical Climatology Network (GHCN).The modeled values are model ensemble means from the ensembles with global sea-surface temperature (SST)forcing (GOGA), tropical Pacific forcing (POGA), tropical Pacific forcing and a mixed layer ocean elsewhere(POGA-ML), tropical Atlantic forcing (TAGA), and forcing with land and atmosphere initialized in January 1929from the GOGA run and integrated forward with the 1856–1928 climatological SST (COGA). The model is theNCAR CCM3. Units are millimeters (mm) per month. From Seager et al. (2007b); used with permission, copyright2008, American Meterological Society.76
Abrupt Climate ChangeFigure 3.4. (top) The precipitation anomaly (in millimeters per month) over the Great Plains (30°N.–50°N.,90°W.–110°W.) for the period 1856 to 2000 from the POGA-ML ensemble mean with only tropical Pacificsea-surface temperature (SST) forcing and from gridded station data. (bottom) Same as above but with GOGAensemble mean with global SST forcing. All data have been 6-year low-pass filtered. The shading encloses theensemble members within plus or minus of 2 standard deviations of the ensemble spread at any time. FromSeager et al. (2005b). GHCN, Global Historical Climatology Network.would induce a drying in southwestern NorthAmerica. However, others have argued thatthe AMO-related changes in tropical AtlanticSSTs are actually locally forced by changesin radiation associated with aerosols, risinggreenhouse gases, and solar irradiance (Mannand Emanuel, 2005).The dynamics that link tropical Pacific SSTanomalies to North American hydroclimate arebetter understood and, on long time scales, appearas analogs of higher frequency phenomenaassociated with ENSO. The influence is exertedin two ways: first, through propagation of Rossbywaves from the tropical Pacific polewardsand eastwards to the Americas (Trenberth et al.,1998) and, second, through the impact that SSTanomalies have on tropospheric temperatures,the subtropical jets, and the eddy-driven meanmeridional circulation (Seager et al., 2003,2005a,b; Lau et al., 2006). During La Niñas,both mechanisms force air to descend overwestern North America, which suppressesprecipitation. Although models, and analysis ofobservations (Enfield et al., 2001; McCabe etal., 2004; Wang et al., 2006), support the ideathat warm subtropical North Atlantic SSTs cancause drying over western North America, thedynamics that underlay this have not been soclearly diagnosed and explained within modelexperiments.The influence of Pacific SSTs on the NorthAmerican monsoon has been documented atinterannual and decadal time scales. In particular,a time-evolving teleconnection response inthe early part of the summer appears to influencethe strength and position of the monsoonridge. In contrast to winter precipitation/ENSOrelationships, La Niña-like conditions in theeastern and central tropical Pacific favor a wetand early monsoon and corresponding dry andhot conditions in the Central United States(Castro et al., 2001; Schubert et al., 2004a;Castro et al., 2007b). In contrast, El Niño-likeconditions favor a dry and delayed monsoonand corresponding wet and cool conditions inthe Central United States (op. cit.).2.1.2 Land Surface Feedbacks onHydroclimate VariabilityThe evidence that multiyear North Americandroughts appear systematically together withtropical SST anomalies and that atmospheric77
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TABLE OF CONTENTSAuthor Team for th
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The U.S. Climate Change Science Pro
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PREFACEReport Motivation and Guidan
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1CHAPTERAbrupt Climate ChangeIntrod
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Abrupt Climate Change-35Arctic temp
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Abrupt Climate ChangeFigure 1.2. Po
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Abrupt Climate Changewell below sea
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Abrupt Climate Changeheterogeneous
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Abrupt Climate Changeapart from dro
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Abrupt Climate Change6. Abrupt Chan
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Abrupt Climate Changeintermediate c
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202REFERENCESChapter 1 ReferencesAl
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U.S. Climate Change Science Program
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