Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 46. What Are the Globaland Regional Impacts of aChange in the OverturningCirculation?In this section we review some of the climaticimpacts of the AMOC over a range of timescales. While all of the impacts are not necessarilyabrupt, they indicate consistent physicalrelationships that might be anticipated withany abrupt change in the AMOC. We startwith evidence of the climatic impact of AMOCchanges during glacial periods. While AMOCchanges are not hypothesized to cause Ice Ages,there are indications of large AMOC changeswithin glacial periods, and these offer excellentopportunities to evaluate the global-scale climaticimpact of large AMOC changes. We thenmove on to possible impacts of AMOC changesduring the instrumental era. All of these resultspoint to global-scale, robust impacts of AMOCchanges on the climate system. In particular,a central impact of AMOC changes is to alterthe interhemispheric temperature gradient,thereby moving the position of the IntertropicalConvergence Zone (ITCZ). Such ITCZ changesinduce a host of regional climate impacts.6.1 Extra-Tropical Impacts duringthe Last Ice AgeDuring the last glacial period, records indicatethere were significant abrupt climate changeevents, such as the D-O oscillations and Heinrichevents discussed in detail in Section 4.These are thought to be associated with changesin the AMOC, and thus offer important insightsinto the climatic impacts of large changes in theAMOC. The paleoproxies from the BermudaRise (McManus et al., 2004) further indicatethat the AMOC was substantially weakenedduring the Younger Dryas cooling event andwas almost shut down during the latest Heinrichevent—H1. The AMOC transports a substantialamount of heat northward. A rapid shutdownof the AMOC causes a cooling in the NorthAtlantic and a warming in the South Atlantic,associated with the reduction of the northwardocean heat transport, as simulated by manyclimate models (Vellinga and Wood, 2002;Dahl et al., 2005; Zhang and Delworth, 2005;Stouffer et al., 2006).The cooling stadials of the Greenland D-Ooscillations were also synchronous with higheroxygen levels off the California coast (indicatingreduced upwelling and reduced CaliforniaCurrent) (Behl and Kennett, 1996), enhancedNorth Pacific intermediate-water formation,and the strengthening of the Aleutian Low(Hendy and Kennett, 2000). This teleconnectionis seen in coupled modeling simulations inwhich the AMOC is suppressed in response tomassive freshwater inputs (Mikolajewicz et al.,1997; Zhang and Delworth, 2005), i.e., coolingin the North Atlantic induced by a weakenedAMOC can lead to the strengthening of theAleutian Low and large-scale cooling in thecentral North Pacific.The millennial-scale abrupt climate changeevents found in Greenland ice cores have beenlinked to the millennial-scale signal seen inAntarctic ice cores (Blunier et al., 1998; Benderet al., 1999; Blunier and Brook, 2001). A veryrecent high resolution glacial climate record derivedfrom the first deep ice core in the Atlanticsector of the Southern Ocean region (DronningMaud Land, Antarctica) shows a one-to-onecoupling between all Antarctic warm events(i.e., the A events discussed in detail in Sec. 3)and Greenland D-O oscillations during the lastice age (EPICA Community Members, 2006).The amplitude of the Antarctic warm events isfound to be linearly dependent on the durationof the concurrent Greenland cooling events.Such a bipolar seesaw pattern was explained bychanges in the heat flux connected to the reductionof the AMOC (Manabe and Stouffer, 1988;Stocker and Johnsen, 2003; EPICA CommunityMembers, 2006).150