Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeAge and an enhanced summer monsoon duringthe Medieval Warm Period (Gupta et al., 2003).These changes might also be associated with areduction of the AMOC during the Little IceAge (Lund et al., 2006).6.3 Possible Impacts During the20th CenturyInstrumental records in the 20th century canalso provide clues about possible AMOCimpacts. Instrumental records show significantlarge-scale multidecadal variations in the AtlanticSST. The observed detrended 20th centurymultidecadal SST anomaly averaged over theNorth Atlantic, often called the Atlantic MultidecadalOscillation (AMO) (Enfield et al., 2001;Knight et al., 2005), has significant regionaland hemispheric climate impacts (Enfield et al.,2001; Knight et al., 2006; Zhang and Delworth,2006; Zhang et al., 2007a). The warm AMOphases occurred during 1925–65 and the recentdecade since 1995, and cold phases occurredduring 1900–25 and 1965–95. The AMO indexis highly correlated with multidecadal variationsof the tropical North Atlantic (TNA) SSTand Atlantic hurricane activity (Goldenberg etal., 2001; Landsea, 2005; Knight et al., 2006;Zhang and Delworth, 2006; Sutton and Hodson,2007). The observed TNA surface warmingis correlated with above-normal Atlantic hurricaneactivity during the 1950–60s and therecent decade since 1995.While the origin of these multidecadal SSTvariations is not certain, one leading hypothesisinvolves fluctuations of the AMOC. Modelsprovide some support for this (Delworth andMann, 2000; Knight et al., 2005), with typicalAMOC variability of several Sverdrups onmultidecadal time scales, corresponding to5–10% of the mean in these models. Anotherhypothesis is that they are forced by changesin radiative forcing (Mann and Emanuel,2006). Delworth et al. (2007) suggest that bothprocesses—radiative forcing changes, alongwith internal variability, possibly associatedwith the AMOC—may be important. A veryrecent study (Zhang, 2007) lends support to thehypothesis that AMOC fluctuations are importantfor the multidecadal variations of observedTNA SSTs. Zhang (2007) finds that observedTNA SST is strongly anticorrelated with TNAsubsurface ocean temperature (after removinglong-term trends). This anticorrelation is adistinctive signature of the AMOC variations incoupled climate models; in contrast, simulationsdriven by external radiative forcing changes donot generate anticorrelated surface and subsurfaceTNA variations, lending support to the ideathat the observed TNA SST fluctuations maybe AMOC-induced.6.3.1 Tropical ImpactsEmpirical analyses have demonstrated alink between multidecadal fluctuations ofAtlantic sea surface temperatures and Sahelian(African) summer rainfall variations (Follandet al., 1986), in which an unusually warmNorth Atlantic is associated with increasedsummer rainfall over the Sahel. Studies withatmospheric general circulation models (e.g.,Giannini et al., 2003; Lu and Delworth, 2005)have shown that models, when given the observedmultidecadal SST variations, are ableto reproduce much of the observed Sahelianrainfall variations. However, these studies donot identify the source of the SST fluctuations.Recent work (Held et al., 2005) suggests thatincreasing greenhouse gases and aerosolsmay also be important factors in the late 20thcentury Sahelian drying.The source of the observed Atlantic multidecadalSST variations has not been firmlyestablished. One leading candidate mechanisminvolves fluctuations of the AMOC. Knightet al. (2006) have analyzed a 1,400-yearcontrol integration of the coupled climatemodel HADCM3 and found a clear relationshipbetween AMO-like SST fluctuations andsurface air temperature over North Americaand Eurasia, modulation of the vertical shearof the zonal wind in the tropical Atlantic,and large-scale changes in Sahel and Brazilrainfall. Linkages between the AMO andthese tropical variations were often based onstatistical analyses. Linkages between AMOCchanges and tropical conditions, emphasizingthe importance of changes in the atmosphericand oceanic energy budgets, are emphasized inCheng et al. (2007). To investigate the causallink between the AMO and other multidecadalvariability, Zhang and Delworth (2006) simulatedthe impact of AMO-like SST variationson climate with a hybrid coupled model. Theydemonstrated that many features of observed153