Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4surface fluxes. A classical assumption is thatvertical mixing in the ocean transports heatdownward (Munk, 1966). This heat warms thewater at depth, decreasing its density and causingit to rise. In other words, vertical advectionw of temperature T and its vertical mixing,parameterized as diffusion with strength κ,are in balance:∂T∂ ∂Tw = ∂ z ∂ z ∂ z(where z denotes the vertical direction). Themixing due to molecular motion is far too smallfor this purpose: the respective mixing coefficientκ is on the order of 10 –7 m 2 s –1 . To achievethe observed upwelling of about 30 Sverdrups(Sv, where 1 Sv = 10 6 m 3 s –1 ), a vertical mixingwith a global average strength of κ = 10 –4 m 2 s –1is required (Munk and Wunsch, 1998; Ganachaudand Wunsch, 2000). This is presumablyaccomplished by turbulent mixing.2.2 Mixing Energy SourcesIn order to investigate whether there is enoughenergy available to drive this mixing, we turn tothe schematic overview presented in Figure 4.1.We have already mentioned the heat fluxesthrough the surface. They are essential becausethe AMOC is a thermally direct circulation.The other two relevant energy sources of theocean are winds and tides. The wind stressgenerates surface waves and acts on the largescalecirculation. Important for vertical mixingat depth are internal waves that are generatedin the surface layer and radiate through theocean. They finally dissipate by turbulence onthe smallest length scale and the water mixes.The interaction of tidal motion with the oceanbottom also generates internal waves, especiallywhere the topography is rough. Again, theseinternal waves break and dissipate, creatingturbulent mixing.Analysis of the mixing energy budget of theocean (Munk and Wunsch, 1998; Wunsch andFerrari, 2004) shows that the mixing energythat is available from those energy sources,about 0.4 TW, is just what is needed whenone assumes that all 30 Sv of deep water thatare globally formed are upwelled from depthby the advection-diffusion balance. However,the estimates of the magnitude of the termsin the mixing budget are highly uncertain. Onthe one hand, some studies suggest that lessthan these 0.4 TW are required (e.g., Hughesand Griffiths, 2006). On the other hand, themixing efficiency, a crucial parameter in thecomputation of this budget, might be smallerthan previously thought (Arneborg, 2002),which would increase the required energy.Therefore, it cannot be determined whether themixing energy budget is actually closed. Thismotivated the search for other possible drivingmechanisms for the AMOC.2.3 Wind-Driven Upwelling in theSouthern OceanToggweiler and Samuels (1993a, 1995, 1998)proposed a completely different driving mechanism.The surface wind forcing in the SouthernOcean leads to a northward volume transport.Due to the meridional shear of the winds, this“Ekman” transport is divergent south of 50 °S,and thus water needs to upwell from belowthe surface to fulfill continuity. The situationis special in the Southern Ocean in that itforms a closed circle around the Earth, withthe Drake Passage between South America asthe narrowest and shallowest (about 2,500 m)place (outlined dashed in Fig. 4.2). No net zonalpressure gradient can be maintained above thesill, and so no net meridional flow balancedby such a large-scale pressure gradient canexist. However, other types of flow are possible—wind-drivenfor instance. Accordingto Toggweiler and Samuels (1995) this DrakePassage effect means that the waters drawnupward by the Ekman divergence must comefrom below the sill depth, as only from there canthey be advected meridionally. Thus we havesouthward advection at depth, wind-driven upwellingin the Southern Ocean, and northwardEkman transport at the surface. The loop wouldbe closed by the deep water formation in thenorthern North Atlantic, as that is where deepwater of the density found at around 2,500 mdepth is formed.Evidence from observed tracer concentrationssupports this picture of the AMOC. A numberof studies (e.g., Toggweiler and Samuels, 1993b;Webb and Suginohara, 2001) question that deepupwelling occurs in a broad, diffuse manner,and rather point toward substantial upwellingof deep water masses in the Southern Ocean.From model studies it is not clear to what extent124