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

Abstract

An annual-mean surface-forced component of the North Atlantic thermohaline circulation (THC) in density space, ?surf(?,σ), is diagnosed from observed surface heat and freshwater fluxes. The climatological mean of ?surf 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, ?surf 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 overturning rates of LSW and GSDW vary more slowly, on decadal timescales. LSW overturning rates increase from near zero in the early 1980s to a maximum of ?10 Sv in 1990 and return to near zero by 1997. Seemingly in antiphase, GSDW overturning rates decline from a peak rate of ?11 Sv in 1981 to a minimum of ?3.5 Sv in 1991, and then increase somewhat up to 1997. The variability in these overturning rates is related to changes in the North Atlantic oscillation (NAO) and the Arctic oscillation (AO) over 1980?97. Correlation coefficients between total overturning rate and three versions of the NAO index are significantly positive at a 99% confidence level. Also highly significant is a correlation of LSW overturning and the AO index, and an anticorrelation between overturning of LSW and GSDW. The overturning of STMW is more weakly (and not significantly) anticorrelated with NAO indices, although there is highly significant anticorrelation between interannual changes in STMW overturning and one version of the NAO index. Stronger LSW (GSDW) overturning in years when NAO indices are relatively high (low) is consistent with recent hydrographic evidence. The extent to which these relationships support a hypothesis that the NAO drives interannual-to-decadal variability in the THC, through wintertime atmospheric buoyancy forcing of the ocean, is discussed.