Book 2.indb - US Climate Change Science Program

Abrupt Climate Changebecause (1) the temperature change will bezonally uniform and extend into the subtropics(Schneider, 1977) and (2) the result will requirea balancing change in zonal winds that willpotentially interact with transient eddies tocreate hemispherically and zonally symmetriccirculation and hydroclimate changes.In the tropics, the precipitation anomaly patternassociated with North American droughts isvery zonally asymmetric with reduced precipitationover the cold waters of the easternand central equatorial Pacific and increasedprecipitation over the Indonesian region. Thecooler troposphere tends to increase convectiveinstability (Chiang and Sobel, 2002), andprecipitation increases in most tropical locationsoutside the Pacific with the exception ofcoastal East Africa, which dries, possibly as aconsequence of cooling of the Indian Ocean(Goddard and Graham, 1999).North American droughts are therefore aregional realization of persistent near-globalatmospheric circulation and hydroclimaticanomalies orchestrated by tropical atmosphereoceaninteractions. During North Americandroughts, dry conditions are also expected inmid-latitude South America, wet conditions inthe tropical Americas and over most tropicalregions, and dry conditions again over EastAfrica. Subtropical to mid-latitude dryingshould extend across most longitudes andpotentially impact the Mediterranean region.However, the signal away from the tropics andthe Americas is often obscured by the impactof other climate phenomena such as the NorthAtlantic Oscillation (NAO) impact on precipitationin the Mediterranean region (Hurrell, 1995;Fye et al., 2006). In a similar fashion, theHolocene drought in the mid-continent of NorthAmerica (Sec. 4) can be shown to be embeddedin global-scale energy balance and atmosphericcirculation changes.2.2.1 The Perfect Ocean for Drought:Gradual Climate Change Resulting inAbrupt ImpactsThe study of the 1998–2002 droughts thatspread across the United States, Southern Europe,and Southwest Asia provides an exampleof a potential abrupt regime shift to one withmore persistent and/or more severe droughtin response to gradual changes in global orregional climate conditions. Research by Hoerlingand Kumar (2003) provides compellingevidence that these severe drought conditionswere part of a persistent climate state that wasstrongly influenced by the tropical oceans.From 1998 through 2002, prolonged belownormalprecipitation and above-normal temperaturescaused the United States to experiencedrought in both the Southwest and WesternStates and along the Eastern Seaboard. Thesedroughts extended across southern Europe andSouthwest Asia, with as little as 50% of theaverage rainfall in some regions (Fig. 3.5). TheHoerling and Kumar (2003) study used climatemodel simulations to assess climate response toaltered oceanic conditions during the 4-year interval.Three different climate models were runa total of 51 times, and the responses averagedto identify the common, reproducible elementof the atmosphere’s sensitivity to the ocean.Results showed that the tropical oceans had asubstantial effect on the atmosphere (Fig. 3.6).The combination of unprecedented warmsea-surface conditions in the western tropicalPacific and 3-plus consecutive years of cold LaNiña conditions in the eastern tropical Pacificshifted the tropical rainfall patterns into the farwestern equatorial Pacific.Over the 1998 through 2002 period, the coldeastern Pacific tropical sea-surface temperatures,though unusual, were not unprecedented.However, the warmth in the tropical IndianOcean and the west Pacific Ocean was unprecedentedduring the 20th century, and attributionstudies indicate this warming (roughly 1 °Csince 1950) is beyond that expected of naturalvariability. The atmospheric modeling resultssuggest an important role for tropical IndianOcean and the west Pacific Ocean sea-surfaceconditions in the shifting of westerly jets andstorm tracks to higher latitudes with a nearlycontinuous belt of high pressure and associateddrying in the lower mid-latitudes. The tropicalocean forcing of multiyear persistence ofatmospheric circulation not only increased therisk for severe and synchronized drying of themid-latitudes between 1998 and 2002 but maypotentially do so in the future, if such oceanconditions occur more frequently.83