Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Box 4.2. How Do We Measure the AMOC?Observational estimates of the AMOC require the measurement, or inference, of all components of the meridional circulationacross a basinwide section. In principle, if direct measurements of the meridional velocity profile are available atall locations across the section, the calculation of the AMOC is straightforward: the velocity is zonally integrated acrossthe section at each depth, and the resulting vertical transport profile is then summed over the northward-moving part ofthe profile (which is typically the upper ~1,000 m for the Atlantic) to obtain the strength of the AMOC.In practice, available methods for measuring the absolute velocity across the full width of a transbasin section are eitherprohibitively expensive or of insufficient accuracy to allow a reliable estimate of the AMOC. Thus, the meridional circulationis typically broken down into several discrete components that can either be measured directly (by current observations),indirectly (by geostrophic calculations based on hydrographic data), or inferred from wind observations (Ekmantransports) or mass-balance constraints.An illustration of this breakdown is shown in Box 4.2 Figure 1 for the specific situation of the subtropical Atlantic Oceannear 26ºN., where the RAPID-MOC array is deployed and where a number of basinwide hydrographic sections havebeen occupied. The measured transport components include (1) direct measurement of the flow though the Straits ofFlorida and (2) geostrophic mid-ocean flow derived from density profiles at the eastern and western sides of the ocean,relative to an unknown constant, or “reference velocity.” A third component is the ageostrophic flow in the surface layerdriven by winds (the Ekman transport), which can be estimated from available wind-stress products. The only remainingunmeasured component is the depth-independent (“barotropic”) mid-ocean flow, which is inferred by requiring anoverall mass balance across the section. Once combined, these components define the basinwide transport profile andthe AMOC strength.The above breakdown is effective because it takes advantage of the spatially integrating nature of geostrophic computationsacross the interior of the ocean and limits the need for direct velocity or transport measurements to narrow regions nearthe coastal boundaries where swiftcurrents may occur (in particular, inthe western boundary region). Theapplication is similar for individualhydrographic sections or mooreddensity arrays such as used inRAPID, except that the mooredarrays can provide continuous estimatesof the interior flow insteadof single snapshots in time. Eachlocation where the AMOC is tobe measured requires a samplingstrategy tuned to the section’stopography and known circulationfeatures, but the methodologyis essentially the same (Hall andBryden, 1982; Bryden et al., 1991;Cunningham et al., 2007). Inversemodels (see Sec. 3.1) follow a similarapproach but use a formalizedset of constraints with specifiederror tolerances (e.g., overall massbalance, western boundary currenttransports, property fluxes) tooptimally determine the referencevelocity distribution across a section(Wunsch, 1996).Box 4.2 Figure 1. Circulation components required to estimate the AMOC. The figuredepicts the approximate topography along 24–26ºN. and the strategy employed by the RAPIDmonitoring array. The transport of the western boundary current is continuously monitoredby a calibrated submarine cable across the Straits of Florida. Hydrographic moorings (depictedby white vertical lines) near the east and west sides of the basin monitor the (relative) geostrophicflow across the basin as well as local flow contributions adjacent to the boundaries.Ekman transport is estimated from satellite wind observations. A uniform velocity correctionis included in the interior ocean to conserve mass across the section. Figure courtesy of J.Hirschi, NOC, Southampton, U.K.130