Abstract
Decadal-scale climate variability in the North Atlantic thermohaline circulation is simulated using a sigma-coordinate primitive equation model, forced by NCEP–NCAR reanalysis surface forcing fields for the period from 1958 to 1997. Surface heat and freshwater flux are expressed in terms of surface thermal and haline density inputs, diagnosed by the model. Variability in surface density fluxes is closely correlated with the North Atlantic Oscillation and demonstrates differences with the original surface heat and freshwater fluxes. Leading modes of surface water mass transformation are considered in the T–S plane. They identify decadal-scale variability associated with the transformation of the Labrador Sea Waters and Subtropical Mode Waters. Analysis of the model responses to the surface forcing shows an immediate reaction of meridional heat transport to the wind stress curl, resulting in a decrease of meridional heat transport at 48°N and an increase in the subtropics. Delayed baroclinic responses to the surface heat forcing are identified at time lags of 3 and 7 yr. The 3-yr response is represented by an increase in the total meridional heat transport in subpolar latitudes and its simultaneous increase in the Tropics and midlatitudes. The 7-yr delayed response to the surface heat forcing is associated with the strengthening of meridional heat transport at all latitudes. However, 7-yr responses may be influenced by the self-correlation in the meridional heat transport and forcing function. Meridional overturning is largely responsible for the variability observed, demonstrating high correlation with meridional heat transport.
Highlights
The North Atlantic plays an important role in global climate variations, being largely responsible for the intensity of interocean exchanges and meridional overturning
One objective of our study is to investigate the projections of the North Atlantic Oscillation (NAO) buoyancy forcing onto formation rates of the North Atlantic Mode Waters
We have described the results of the numerical simulation of the large-scale interannual variability of the North Atlantic circulation in a numerical simulation carried out with the sigma-coordinate primitive equation model (SPEM) model driven by NCEP–NCAR reanalysis surface fluxes for the period 1958–97
Summary
Fyfe 1996; Hakkinen 2000; Gulev et al 2002). the NAO certainly has signatures in the deep ocean circulation driven by the surface fluxes of heat, freshwater, and momentum. The original density flux (Fig. 4a) indicates large positive values (generation of denser water) over the Gulf Stream (where the maximum is 12 ϫ 10Ϫ6 kg mϪ2 sϪ1), in the North Equatorial Current (the southern limit of the subtropical gyre), and in the Labrador Sea where the convection region at 60ЊN is well identified. It shows fairly continuous, positive transformation rates in the temperature range of 24Њ–12ЊC, associated with the formation of the STMW (17Њ–20ЊC), and with the densification of the central North Atlantic waters flowing to the northeast in the NAC to give birth to the SPMW (10Њ–14ЊC). Both 7-yr responses should be considered with caution because they may be influenced by autocorrelation in the MHT and the forcing function
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