Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3Figure 3.9. Reconstructed Palmer Drought Severity Index (PDSI) averaged over three regions of theWestern and Central United States: the Northwest (A), the Southwest (B), and the Great Plains (C). Timeis in calendar years A.D. See Figure 3.7 for the geographic locations of these regions. These series are allbased on the updated version of North American Drought Atlas (NADA). Unlike the drought area index inFigure 3.8, where more-positive numbers mean larger areas affected by drought, more-negative numbersmean drier conditions here in accordance with the original PDSI scale devised by Palmer (1965).3.4 Possible Causes of the MedievalMegadroughtsThe causes of the Medieval megadroughts arenow becoming unraveled and appear to havesimilar origin to the causes of modern droughts,which is consistent with the similar spatial patternsof Medieval and modern droughts (Herweijeret al., 2007). Cobb et al. (2003) have usedmodern and fossil coral records from Palmyra,a small island in the tropical Pacific Ocean,to reconstruct eastern and central equatorialPacific SSTs for three time segments withinthe Medieval period. These results indicate thatcolder—La Niña-like—conditions prevailed,which would be expected to induce drought overwestern North America. Graham et al. (2007)used these records, and additional sedimentrecords in the west Pacific, to create an idealizedpattern of Medieval tropical Pacific SSTwhich, when it was used to force an AGCM, didcreate a drought over the Southwest. Adoptinga different approach, Seager et al. (2008) usedthe Palmyra modern and fossil coral records toreconstruct annual tropical Pacific SSTs for the90