Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Observational analyses (Enfield et al., 2001)suggest that the AMO has a strong impact onthe multidecadal variability of U.S. rainfalland river flows. McCabe et al. (2004) furthersuggest that there is significant positive correlationbetween the AMO and the CentralU.S. multidecadal drought frequency, and thepositive AMO phase contributes to the droughtsobserved over the continental U.S. in the decadesince 1995.multidecadal climate variability in the 20th centurymay be interpreted—at least partially—asa response to the AMO. A warm phase of theAMO leads to a northward shift of the AtlanticITCZ, and thus an increase in the Sahelianand Indian summer monsoonal rainfall, aswell as a reduction in the vertical shear of thezonal wind in the tropical Atlantic region that isimportant for the development of Atlantic majorHurricanes (Fig. 4.15). Thus, the AMO createslarge-scale atmospheric circulation anomaliesthat would be favorable for enhanced tropicalstorm activity. The study of Black et al. (1999)using Caribbean sediment records suggests thata southward shift of the Atlantic ITCZ when theNorth Atlantic is cold—similar to what is seenin the models—has been a robust feature of theclimate system for more than 800 years, and issimilar to results from the last ice age.6.3.2 Impacts on North America andWestern EuropeThe recent modeling studies (Sutton and Hodson,2005, 2007) provide a clear assessment ofthe impact of the AMO over the Atlantic, NorthAmerica, and Western Europe (Fig. 4.16). Inresponse to a warm phase of the AMO, a broadarea of low pressure develops over the Atlantic,extending westward into the Caribbean andSouthern United States. The pressure anomalypattern denotes weakened easterly trade winds,potentially reinforcing the positive SST anomaliesin the tropical North Atlantic Ocean byreducing the latent heat flux. Precipitation isgenerally enhanced over the warmer Atlanticwaters and is reduced over a broad expanse ofthe United States. The summer temperatureresponse is clear, with substantial warmingover the United States and Mexico, with weakerwarming over Western Europe.6.3.3 Impacts on Northern HemisphereMean TemperatureKnight et al. (2005) find in the 1,400-year controlintegration of the HADCM3 climate modelthat variations in the AMOC are correlated withvariations in the Northern Hemisphere meansurface temperature on decadal and longertime scales. Zhang et al. (2007a) demonstratethat AMO-like SST variations can contributeto the Northern Hemispheric mean surfacetemperature fluctuations, such as the early 20thcentury warming, the pause in hemisphericscalewarming in the mid-20th century, and thelate 20th century rapid warming, in additionto the long-term warming trend induced byincreasing greenhouse gases.6.4 Simulated Impacts on ENSOVariabilityModeling studies suggest that changes in theAMOC can modulate the characteristics ofEl-Niño Southern Oscillation (ENSO). Timmermannet al. (2005a) found that the simulatedweakening of the AMOC leads to a deepeningof the tropical Pacific thermocline, and aweakening of ENSO, through the propagation ofoceanic waves from the Atlantic to the tropicalPacific. Very recent modeling studies (Dongand Sutton, 2007; Timmermann et al., 2007)found opposite results, i.e., the weakening of theAMOC leads to an enhanced ENSO variabilitythrough atmospheric teleconnections. Dong etal. (2006) also show that a negative phase ofthe AMO leads to an enhancement of ENSOvariability.6.5 Impacts on EcosystemsRecent coupled climate–ecosytem model simulations(Schmittner, 2005) find that a collapseof the AMOC leads to a reduction of NorthAtlantic plankton stocks by more than 50%, and154