Recent Variability of the North Atlantic Thermohaline Circulation Inferred from Surface Heat and Freshwater Fluxes

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Abstract
An annual-mean surface-forced component of the North Atlantic thermohaline circulation (THC) in density space, ψsurf(ϒ,σ), is diagnosed from observed surface heat and freshwater fluxes. The climatological mean of ψsurf over 1980–97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing: ∼8 Sv of Greenland Sea Deep Water (GSDW), ∼4 Sv of Labrador Sea Water (LSW), and ∼4 Sv of 18° Water (STMW). Obtained for each year over 1980–97, ψsurf reveals interannual-to-decadal variability in the renewal and overturning of these water masses. The total surface-forced overturning rate varies in the range 10–20 Sv, reaching peak values in 1989–90 and generally decreasing over the period 1990–97. More dramatic changes are apparent in the overturning of different water masses. The overturning of STMW exhibits large-amplitude interannual variability. A background positive trend over 1980–97 is dominated by variance of 13.4 Sv. By contrast, the overtur... Abstract An annual-mean surface-forced component of the North Atlantic thermohaline circulation (THC) in density space, ψsurf(ϒ,σ), is diagnosed from observed surface heat and freshwater fluxes. The climatological mean of ψsurf over 1980–97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing: ∼8 Sv of Greenland Sea Deep Water (GSDW), ∼4 Sv of Labrador Sea Water (LSW), and ∼4 Sv of 18° Water (STMW). Obtained for each year over 1980–97, ψsurf reveals interannual-to-decadal variability in the renewal and overturning of these water masses. The total surface-forced overturning rate varies in the range 10–20 Sv, reaching peak values in 1989–90 and generally decreasing over the period 1990–97. More dramatic changes are apparent in the overturning of different water masses. The overturning of STMW exhibits large-amplitude interannual variability. A background positive trend over 1980–97 is dominated by variance of 13.4 Sv. By contrast, the overtur...

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