Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Measurement ofthe MOC remainsa difficult challenge,and serious effortstoward quantifyingthe MOC, andmonitoring itschange, havedeveloped onlyrecently.set of high-latitude sinking regions with morebroadly distributed global upwelling patternsvia a complex interbasin circulation (Stommel,1958; Gordon, 1986). The general pattern ofthis circulation has been established for decadesbased on global hydrographic observations, andcontinues to be refined. However, measurementof the MOC remains a difficult challenge, andserious efforts toward quantifying the MOC,and monitoring its change, have developedonly recently.Current efforts to quantify the MOC usingocean observations rely on four main approaches:1. Static ocean “inverse” models utilizingmultiple hydrographic sections2. Analysis of individual transoceanichydrographic sections3. Continuous time-series observationsalong a transoceanic section, and4. Time-dependent ocean “state estimation”modelsWe describe, in turn, the fundamentals ofthese approaches and their assumptions, andthe most recent results on the Atlantic MOCthat have emerged from each one. In principle,the AMOC can also be estimated from oceanmodels driven by observed atmospheric forcingthat are not constrained by ocean observations,or by coupled ocean-atmosphere models. Thereare many examples of such calculations in theliterature, but we will restrict our review tothose estimates that are constrained in one wayor another by ocean observations.3.1 Ocean Inverse ModelsOcean “inverse” models combine several (twoor more) hydrographic sections bounding aspecified oceanic domain to estimate the totalocean circulation through each section. Theseare often referred to as “box inverse” modelsbecause they close off an oceanic “box” definedby the sections and adjacent continental boundaries,thereby allowing conservation statementsto be applied to the domain. The data used inthese calculations consist of profiles of temperatureand salinity at a number of discrete stationsdistributed along the sections. The modelsassume a geostrophic balance for the oceancirculation (apart from the wind-driven surfaceEkman layer) and derive the geostrophic velocityprofile between each pair of stations, relativeto an unknown reference constant, or “referencevelocity.” The distribution of this referencevelocity along each section, and therefore theabsolute circulation, is determined by specifyinga number of constraints on the circulationwithin the box and then solving a least-squares(or other mathematical optimization) problemthat best fits the constraints, within specifiederror tolerances. The specified constraintscan be many but typically include—aboveall—overall mass conservation within the box,mass conservation within specified layers,independent observational estimates of masstransports through parts of the sections (e.g.,transports derived from current meter arrays),and conservation of property transports (e.g.,salt, nutrients, geochemical tracers). Increasingly,the solutions may also be constrained byestimates of surface heat and freshwater fluxes.Once a solution is obtained, the transport profilethrough each section can be derived, and theAMOC (for zonal basin-spanning sections) canbe estimated.The most comprehensive and up-to-date inverseanalyses for the global time-mean ocean includethose by Ganachaud (2003a) and Lumpkin andSpeer (2007) (Fig. 4.4), based on the WorldOcean Circulation Experiment (WOCE)hydrographic data collected during the 1990s.The strength of the Atlantic MOC is given as18 ± 2.5 Sv by Lumpkin and Speer (2007) near24 °N., where it reaches its maximum value.The corresponding estimate from Ganachaud(2003a) is 16 ± 2 Sv, in agreement within theerror estimates. In both analyses the AMOCstrength is nearly uniform throughout theAtlantic from 20°S. to 45°N., ranging fromapproximately 14 to 18 Sv. These estimatesshould be taken as being representative of theaverage strength of the AMOC over the periodof the observations.An implicit assumption in these analyses is thatthe ocean circulation is in a “steady state” overthe time period of the observations, in the abovecases over a span of some 10 years. This islikely false, as estimates of relative geostrophictransports across individual repeated sectionsin the North Atlantic show typical variations of± 6 Sv (Lavin et al., 1998; Ganachaud, 2003a).128