Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Figure 4.11. Time series of the strength of the Atlantic meridional overturning as simulatedby a suite of coupled ocean-atmosphere models using present-day boundary conditions, fromStouffer et al. (2006). The strength is listed along the y-axis in Sverdrups (Sv; 1 Sv = 10 6 m 3 s –1 ).Curves were smoothed with a 10-yr running mean to reduce high-frequency fluctuations.The numbers after the model names indicate the long-term mean of the Atlantic MOC.THC, Thermohaline Circulation.2007; see also Fig. 4.17). In addition, some ofthe 20th century simulations show substantialdrifts that might hinder predictions of futureAMOC strength (Randall et al., 2007).There are also substantial differences amongmodels in AMOC variability, which tends toscale with the mean strength of the overturning.Models with a more vigorous overturning tendto produce pronounced multidecadal variations,while variability in models with a weakerAMOC is more damped (Stouffer et al., 2006).Time series of the AMOC are too incompleteto give an indication of which mode is moreaccurate, although recent observations suggestthat the AMOC is highly variable on sub-annualtime scales (Sec. 3.3; Cunningham et al., 2007).Another useful model-data comparison can bemade for ocean heat transport in the Atlantic.A significant fraction of the northward heattransport in the Atlantic is due to the AMOC,with additional contributions from horizontalcirculations (e.g., Roemmich and Wunsch,1985). In the absence of variations in radiativeforcing, changes in ocean heat storage are smallwhen averaged over long periods. Under theseconditions, ocean heat transport must balancesurface heat fluxes, and the heat transporttherefore provides an indication of how wellsurface fluxes are simulated. There are severalcalculations of heat transport at 20–25°N. inthe Atlantic derived by combining hydrographicobservations in inverse models. Thesemethods yield estimates of about 1.3 Petawatts(PW; 1 PW = 10 15 Watts) with errors on theorder of about 0.2 PW (Ganachaud and Wunsch,2000; Stammer et al., 2003). While all modelsagree that heat transport in the Atlantic isnorthward at 20°N., the modeled magnitudevaries greatly (Fig. 4.12). Most models tend tounderestimate the ocean heat transport, withranges generally between 0.5 to 1.1 PW (Jia,2003; Stouffer et al., 2006). The mismatch isbelieved to result from two factors: (1) smallerthan observed temperature differences betweenthe upper and lower branches of the AMOC,with surface waters too cold and deep waterstoo warm, and (2) overturning that is too weak(Jia, 2003). The source of these model errorswill be discussed further.Schmittner et al. (2005) and Schneider et al.(2007) have proposed that the skill of a modelin producing the climatological spatial patternsof temperature, salinity, and pycnocline depthin the North Atlantic is another useful measureof model ability to simulate the overturningcirculation. These authors found that modelssimulate temperature better than salinity; theyattribute errors in the latter to biases in thehydrologic cycle in the atmosphere (Schneideret al., 2007). Large errors in pycnocline depthare probably the result of compounded errorsfrom both temperature and salinity fields. Also,errors over the North Atlantic alone tend to be146