Book 2.indb - US Climate Change Science Program

Abrupt Climate Change10 °C on time scales of a decade or two havebeen attributed to these abrupt change events.In this chapter, we assess whether such anabrupt change in the AMOC is likely to occurin the future in response to increasinggreenhouse gases. Specifically, there has beenextensive discussion, both in the scientific andpopular literature, about the possibility of amajor weakening or even complete shutdownof the AMOC in response to global warming,along with rapid changes in land-based icesheets (see Chapter 2) and Arctic sea ice (seeBox 4.1). As will be discussed more extensivelybelow, global warming tends to weaken theAMOC both by warming the upper ocean in thesubpolar North Atlantic and through enhancingthe flux of freshwater into the Arctic and NorthAtlantic. Both processes reduce the density ofthe upper ocean in the North Atlantic, therebystabilizing the water column and weakeningthe AMOC. These processes could cause aweakening or shutdown of the AMOC thatcould significantly reduce the poleward transportof heat in the Atlantic, thereby possiblyleading to regional cooling in the Atlantic andsurrounding continental regions, particularlyWestern Europe.In this chapter, we examine (1) our present understandingof the mechanisms controlling theAMOC, (2) our ability to monitor the state of theAMOC, (3) the impact of the AMOC on climatefrom observational and modeling studies, and(4) model-based studies that project the futureevolution of the AMOC in response to increasinggreenhouse gases and other changes inatmospheric composition. We use these resultsto assess the likelihood of an abrupt change inthe AMOC. In addition, we note the uncertaintiesin our understanding of the AMOC and inour ability to monitor and predict the AMOC.These uncertainties form important caveatsconcerning our central conclusions.2. What Are the ProcessesThat Control theOverturning Circulation?We first review our understanding of thefundamental driving processes for the AMOC.We break this discussion into two parts: themain discussion deals with the factors that arethought to be important for the equilibriumstate of the AMOC, while the last part (Sec. 2.5)discusses factors of relevance for transientchanges in the AMOC.Like any other steady circulation pattern inthe ocean, the flow of the Atlantic MeridionalOverturning Circulation must be maintainedagainst the dissipation of energy on the smallestlength scales. We wish to determine whatprocesses provide the energy that maintainsthe steady state AMOC. In general, the energysources for the ocean are wind stress at thesurface, tidal motion, heat fluxes from theatmosphere, and heat fluxes through the oceanbottom.2.1 Sandström’s ExperimentWe consider the surface heat fluxes first. Theyare distributed asymmetrically over the globe.The ocean gains heat in the low latitudesclose to the Equator and loses heat in the highlatitudes toward the poles. Is this meridionalgradient of the surface heat fluxes sufficientfor driving a deep overturning circulation?The first one to think about this question wasthe Swedish researcher Sandström (1908).He conducted a series of tank experiments.His tank was narrow, but long and deep,thus putting the stress on a two-dimensionalcirculation pattern. He applied heat sourcesand cooling devices at different depths and observedwhether a deep overturning circulationdeveloped. If he applied heating and coolingboth at the surface of the fluid, then he couldsee the water sink under the cooling device.This downward motion was compensated bya slow, broadly distributed upward motion.The resulting overturning circulation ceasedThe ocean gains heatin the low latitudesclose to the Equatorand loses heat in thehigh latitudes towardthe poles.121