Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeon (Kaplan et al., 1998; Rayner et al., 2003).A mix of station data and tree ring analyseshas been used to identify six serious multiyeardroughts in western North America during thishistorical period (Fye et al., 2003; Herweijeret al., 2006). Of these, the most famous is the“Dust Bowl” drought that included most of the1930s decade. The other two in the 20th centuryare the severe drought in the Southwest from thelate 1940s to the late 1950s and the drought thatbegan in 1998 and is ongoing. Three droughtsin the mid to late 19th century occurred (withapproximate dates) from 1856 to 1865, from1870 to 1876, and from 1890 to 1896.In all of these droughts, dry conditions prevailedover most of western North Americafrom northern Mexico to southern Canada andfrom the Pacific Coast to the Mississippi Riverand sometimes farther east, with wet conditionsfarther north and farther south. The pattern ofthe Dust Bowl drought seemed unique in thatthe driest conditions were in the central andnorthern Great Plains and that dry conditionsextended into the Pacific Northwest, whileanomalies in the Southwest were modest.Early efforts used observations to link thesedroughts to mid-latitude ocean variability.Since the realization of the powerful impactsof El Niño on global climate, studies haveincreasingly linked persistent, multiyear NorthAmerican droughts with tropical Pacific SSTsand persistent La Niña events (Cole and Cook,1998; Cole et al., 2002; Fye et al., 2004). Thiscan be appreciated through the schematic mapsshown in Figure 3.2 that show the teleconnectionpatterns of temperature and precipitationover North America commonly associated withthe warm and cold phases of the ENSO cycleover the tropical Pacific. Warm ENSO episodes(El Niños) result in cool-wet conditions fromthe Southwestern over to the SoutheasternUnited States during the winter season. Incontrast, cold ENSO episodes (La Niñas) resultin the development of warm-dry (i.e., drought)conditions over the same U.S. region, againprimarily for the winter season. In contrast, theimportance of ENSO on summer season climateis much stronger elsewhere in the world, likeover Southeast Asia and Australasia. However,new research suggests a teleconnection betweenPacific SSTs and the North American monsoonas well (e.g., Castro et al., 2007b). The NorthAmerican monsoon (June through September)is a critical source of precipitation for much ofMexico (up to 70% of the annual total) and theSouthwestern United States (30%–50%). Themeans whereby tropical SST anomalies impactclimate worldwide are reasonably well understood.The SST anomalies lead to anomaliesin the patterns and magnitude of convectiveheating over the tropical oceans which driveatmospheric circulation anomalies that aretransmitted around the world via stationaryRossby waves. The stationary waves thenalso subsequently impact the propagation oftransient eddies thereby altering the patterns ofstorm tracks, which feeds back onto the meanflow. For a review see Trenberth et al. (1998).On longer time scales during the Holocene(roughly the past 11,000 years), climaticvariations in general, and hydrologic changesin particular, exceeded in both magnitude andduration those of the instrumental period or ofthe last millennium. In the mid-continent ofNorth America, for example, between about8,000 and 4,000 years ago, forests were replacedby steppe as the prairie expanded eastward, andsand dunes became activated across the GreatPlains. These Holocene paleoclimatic variationsoccurred in response to the large changes in thecontrols of global and regional climates thataccompanied deglaciation, including changesin ice-sheet size (area and elevation), thelatitudinal and seasonal distribution of insolation,and atmospheric composition, includinggreenhouse gases and dust and mineral aerosols(Wright et al., 1993). Superimposed on theseSince the realizationof the powerfulimpacts of El Niñoon global climate,studies haveincreasingly linkedpersistent, multiyearNorth Americandroughts withtropical Pacific SSTsand persistentLa Niña events.73