Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangePFigure 4.2. A schematic meridional section of the Atlantic Ocean representing a zonally averaged picture(from Kuhlbrodt et al., 2007). The AMOC is denoted by straight blue arrows. The background color shadingdepicts a zonally averaged density profile from observational data. The thermocline lies between the warmer,lighter upper layers and the colder, deeper waters. Short, wavy orange arrows indicate diapycnal mixing, i.e.,mixing along the density gradient. This mainly vertical mixing is the consequence of the dissipation of internalwaves (long orange arrows). It goes along with warming at depth that leads to upwelling (red arrows). Blackarrows denote wind-driven upwelling caused by the divergence of the surface winds in the Southern Oceantogether with the Drake Passage effect (explained in the text). The Deacon cell is a wind-driven regional recirculation.The surface fluxes of heat (red wavy arrows) and freshwater (green wavy arrows) are often subsumedas buoyancy fluxes. The heat loss in the northern and southern high latitudes leads to cooling and subsequentsinking, i.e., formation of the deep water masses North Atlantic Deep Water (NADW) and Antarctic BottomWater (AABW). The blue double arrows subsume the different deep water formation sites in the North Atlantic(Nordic Seas and Labrador Sea) and in the Southern Ocean (Ross Sea and Weddell Sea).wind-driven upwelling is a driver of the AMOC.Recent studies show a weaker sensitivity ofthe overturning with higher model resolution,casting light on the question as to how strongthe regional eddy-driven recirculation is(Hallberg and Gnanadesikan, 2006). This couldcompensate for the northward Ekman transportwell above the depth of Drake Passage, shortcircuitingthe return flow.As with the mixing energy budget, the estimatesof the available energy for wind-driven upwellingare fraught with uncertainty. The work doneby the surface winds on that part of the flow thatis balanced by the large-scale pressure gradientscan be used for wind-driven upwelling fromdepth. Estimates are between 1 TW (Wunsch,1998) and 2 TW (Oort et al., 1994).2.4 Two Drivers of the EquilibriumCirculationWe define a ‘driver’ as a process that suppliesenergy to maintain a steady-state AMOCagainst dissipation. We find that there are twodrivers that are physically quite different fromeach other. Mixing-driven upwelling (case 1 inFig. 4.3) involves heat flux through the oceanacross the surfaces of constant density to depth.The water there expands and then rises to thesurface. By contrast, wind-driven upwelling(case 2) means that the waters are pulled to thesurface along surfaces of constant density; thewater changes its density at the surface when itis in contact with the atmosphere. No interiorheat flux is required.125