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WIREs Clim Change
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Observing and modeling changes in the Atlantic MOC

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Abstract The Atlantic Meridional Overturning Circulation (MOC) is defined, and our present understanding of MOC driving mechanisms is summarized. Evidence for the changing MOC is reviewed, covering recent developments in observing and modeling the MOC, and the climatic consequences of MOC variability. On a timescale of the next 5‐10 years, further developments in MOC monitoring, modeling and prediction are both anticipated and recommended. In the context of what is presently known about the MOC, the evidence for a recent slowing trend is considered. Copyright © 2010 John Wiley & Sons, Inc. This article is categorized under: Paleoclimates and Current Trends > Paleoclimate

Twice daily time series of Florida Straits transport (blue), Ekman transport (black), upper mid‐ocean transport (magenta), and reconstructed MOC transport (red). Transports in Sv (1 Sv = 106 m3 s−1), positive northward. Florida Straits transport is based on electromagnetic cable measurements. Ekman transport is based on QuikScat winds. The upper mid‐ocean transport is the vertical integral of the transport per unit depth down to 1100 m. Overturning transport is the sum of Florida Straits, Ekman, and upper mid‐ocean transport.2 The mean ± standard deviation of Gulf Stream, Ekman, upper‐mid ocean, and overturning transports are 31.7 ± 2.8 Sv, 3.5 ± 3.4 Sv,—16.6 ± 3.2 Sv and 18.5 ± 4.9 Sv, respectively. These data products (and several other relevant quantities) and the gridded files used in their computation are freely available without restriction at http://www.noc.soton.ac.uk/rapidmoc/. All calibrated instrument records may be obtained from http://www.bodc.ac.uk/. We encourage download and analysis of the data.

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Solid lines denote power spectra of the maximum of the overturning stream function (ψmax red), Gulf Stream (T blue), Ekman (T black), and upper‐mid ocean (T magenta) for the period from April 2004 to April 2006.

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Time series of 5‐daily MOC strength at 26°N in the 1/12° OCCAM model, 1988–2006 (black line57, alongside published estimates of the MOC in 1992, 1998 and 2004, with published error bars,12 and RAPID array estimates twice daily from 2 April 2004 to 1 October 2007 (red line,2).

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Idealized meridional section representing a zonally‐averaged picture of the Atlantic sector of the Global Ocean. Straight arrows sketch the MOC. The color shading depicts a zonally‐averaged density profile derived from observational data.28 The thermocline, the region where the temperature gradient is large, separates the light and warm upper waters from the denser and cooler deep waters. The two main upwelling mechanisms, wind‐driven and mixing‐driven, are displayed. Wind‐driven upwelling is a consequence of a northward flow of the surface waters in the Southern Ocean, the Ekman transport, that is driven by strong westerly winds. Because the Ekman transport is divergent, waters upwell from depth. Mixing along the density gradient, called diapycnal mixing, causes mixing‐driven upwelling; this is partly because of internal waves triggered at the ocean's boundaries. Deepwater formation (DWF) occurs in the high northern and southern latitudes, creating North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), respectively. The locations of DWF are tightly linked with the distribution of surface fluxes of heat and fresh water; because these influence the buoyancy of the water, they are subsumed as buoyancy fluxes. Part of the freshly formed NADW has to flow over the shallow sill between Greenland, Iceland, and Scotland. Close to the zone of wind‐driven upwelling in the Southern Ocean is the ‘Deacon Cell’ recirculation, visible in the zonally‐ integrated meridional velocity in ocean models, but largely counteracted by the effects of ocean eddies. Note that, in the real ocean, the ratio of the meridional extent to the typical depth is about 5000 to 1 (Reprinted with permission from Ref 26. Copyright 2007 American Geophysical Union).

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Data‐based meridional overturning stream function (Sv) for the Global Ocean taken from Figure 2 of Ref 14. Also indicated are typical winter‐mixed‐layer depths (white line), and the mean depth of ocean ridge crests (gray). (Reprinted with permission from Ref 14. Copyright 2007 American Meteorological Society)

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