Variability in sea surface height:: A qualitative measure for the meridional overturning in the North Atlantic

Sea surface height (SSH) from altimeter observations (1992-2000) and modeling results are investigated to determine the modes of variability and the linkages to the state of oceanic circulation in the North Atlantic. Three different model experiments are used with climatological surface forcing appended by wind stress and/or buoyancy flux anomalies, The simulated SSH and gyre circulation and altimeter data are analyzed using the empirical orthogonal function analysis, It is shown that decadal variability in the leading SSH and gyre circulation mode originates from the basin-scale thermal forcing component, not from wind stress driving. This means that low-frequency variations of SSH along the Gulf Stream reflect predominantly overturning changes. The horizontal gyre circulation changes are also related to the overturning due to the topography which couples baroclinic and barotropic flows. SSH variability outside the western boundary current region, on the eastern side of the North Atlantic basin, is determined by local and remote (Rossby waves) wind stress forcing. The model simulations and altimeter data suggest that there was a large SSH change in 1995-1996 over the Gulf Stream which was associated with an abrupt weakening of the overturning.