Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeThis variability is accounted for in the inversemodels by allowing a relatively generous errortolerance on mass conservation, particularly inupper-ocean layers, which exhibit the strongesttemporal variability. While this is an acknowledgedweakness of the technique, it is offsetby the large number of independent sectionsincluded in these (global) analyses, which tendto iron out deviations in individual sectionsfrom the time mean. The overall error estimatesfor the AMOC resulting from these analysesreach about 10–15% of the AMOC magnitudein the mid-latitude North Atlantic, which atthe present time can probably be considered asthe best constrained available estimate of the“mean” current (1990s) state of the AtlanticAMOC. However, unless repeated over differenttime periods, these techniques are unable toprovide information on the temporal variabilityof the AMOC.3.2 Individual TransoceanicHydrographic SectionsHistorically, analysis of individual transoceanichydrographic sections has played a prominentrole in estimating the strength of the AMOC andthe meridional transport of heat of the oceans(Hall and Bryden, 1982). The technique is similarto that of the box inverse techniques exceptthat only a single overall mass constraint—thetotal mass transport across the section—is applied.Other constraints, such as the transportsof western boundary currents known fromother direct measurements, can also be usedwhere available. The general methodology issummarized in Box 4.2. Determination of theunknown “reference velocity” in the oceaninterior is usually accomplished either by assumingthat it is uniform across the section orby adjusting it in such a way (subject to overallmass conservation) that it satisfies other a70 º N4.0±0.764 º N56 º N48 º N16.3±2.717.0±4.336 º N24 º N18.0±2.511 º N0 º11 º S16.2±3.090 º W60 º W30 º W0 ºFigure 4.4. Schematic of the Atlantic MOC and major currents involved in the upper (red)and lower (blue) limbs of the AMOC, after Lumpkin and Speer (2007). The boxed numbersindicate the magnitude of the AMOC at several key locations, indicated by gray lines, alongwith error estimates. The red to green to blue transition on various curves denotes a cooling(red is warm, blue is cold) and sinking of the water mass along its path. Figure courtesy of R.Lumpkin, NOAA/AOML.129