Signal propagation related to the North Atlantic overturning -: art. no. L09602

Changes of the meridional overturning circulation (MOC) due to surface heat flux variability related to the North Atlantic Oscillation (NAO) are analyzed in various ocean models, i.e., eddying and non-eddying cases. A prime signature of the forcing is variability of the winter-time convection in the Labrador Sea. The associated changes in the strength of the MOC near the subpolar front ( 45 degrees N) are closely related to the NAO-index, leading MOC anomalies by about 2 - 3 years in both the eddying and non-eddying simulation. Further south the speed of the meridional signal propagation depends on model resolution. With lower resolution ( non-eddying case, 4/3 degrees resolution) the MOC signal propagates equatorward with a mean speed of about 0.6 cm/s, similar as spreading rates of passive tracer anomalies. Eddy-permitting experiments (1/3 degrees) show a significantly faster propagation, with speeds corresponding to boundary waves, thus leading to an almost in-phase variation of the MOC transport over the subtropical to subpolar North Atlantic.