Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeof uncertainties, this approach should ultimatelylead to our best estimates of the large-scaleocean circulation and AMOC variability.Our assessment of the state of the AtlanticMOC has been focused on 24°N., owing tothe concentration of observational estimatesthere, which, in turn, is historically relatedto the availability of long-term, high-qualitywestern boundary current observations at thislocation. The extent to which AMOC variabilityat this latitude, apart from that due to localwind-driven (Ekman) variability, is linkedto other latitudes in the Atlantic remains animportant research question. Also importantare changes in the structure of the AMOC,which could have long-term consequencesfor climate independent of changes in overallAMOC strength. For example, changes in therelative contributions of Southern Hemispherewater masses that supply the upper ocean returnflow of the cell (i.e., relatively warm and saltyIndian Ocean thermocline water versus coolerand fresher Subantarctic Mode Waters and AntarcticIntermediate Waters) could significantlyimpact the temperature and salinity of the NorthAtlantic over time and feed back on the deepwater formation process.Natural variability of the AMOC is driven byprocesses acting on a wide range of time scales.On intraseasonal to intrannual time scales, thedominant processes are wind-driven Ekmanvariability and internal changes due to Rossbyor Kelvin (boundary) waves. On interannualto decadal time scales, both variability inLabrador Sea convection related to NAO forcingand wind-driven baroclinic adjustment of theocean circulation are implicated in models (e.g.,Boning et al., 2006). Finally, on multidecadaltime scales, there is growing model evidencethat large-scale observed interhemisphericSST anomalies are linked to AMOC variations(Knight et al., 2005; Zhang and Delworth,2006). Our ability to detect future changes andtrends in the AMOC depends critically on ourknowledge of the spectrum of AMOC variabilityarising from these natural causes. Theidentification, and future detection, of AMOCchanges will ultimately rely on building a betterunderstanding of the natural variability of theAMOC on the interannual to multidecadal timescales that make up the lower frequency end ofthis spectrum.4. What Is The EvidenceFor Past Changes In TheOverturning Circulation?Our knowledge of the mean state and variabilityof the AMOC is limited by the short duration ofthe instrumental record. Thus, in order to gaina longer term perspective on AMOC variabilityand change, we turn to geologic records frompast climates that can yield important insightson past changes in the AMOC and how theyrelate to climate changes. In particular, wefocus on records from the last glacial period,for which there is evidence of changes in theAMOC that can be linked to a rich spectrumof climate variability and change. Improvingour ability to characterize and understand pastAMOC changes will increase confidence inour ability to predict any future changes in theAMOC, as well as the global impact of thesechanges on the Earth’s natural systems.The last glacial period was characterized bylarge, widespread and often abrupt climatechanges at millennial time scales, many ofwhich have been attributed to changes in theAMOC and its attendant feedbacks (Broecker etal., 1985; Clark et al., 2002a, 2007; Alley, 2007).In the following, we first summarize varioustypes of evidence (commonly referred to asproxy records, in that they provide an indirectmeasure of the physical property of interest)used to infer changes in the AMOC. We thendiscuss the current understanding of changesin the AMOC during the following four timewindows (Fig. 4.7):In order to gaina longer termperspective onAMOC variabilityand change, we turnto geologic recordsfrom past climatesthat can yieldimportant insightson past changesin the AMOC andhow they relate toclimate changes.135