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 modeli ng results are investigated to determine the modes of variability and the l inkages to the state of oceanic circulation in the North Atlantic. Three di fferent model experiments are used with climatological surface forcing appe nded by wind stress and/or buoyancy flux anomalies, The simulated SSH and g yre circulation and altimeter data are analyzed using the empirical orthogo nal function analysis, It is shown that decadal variability in the leading SSH and gyre circulation mode originates from the basin-scale thermal forci ng component, not from wind stress driving. This means that low-frequency v ariations of SSH along the Gulf Stream reflect predominantly overturning ch anges. The horizontal gyre circulation changes are also related to the over turning due to the topography which couples baroclinic and barotropic flows . SSH variability outside the western boundary current region, on the easte rn side of the North Atlantic basin, is determined by local and remote (Ros sby waves) wind stress forcing. The model simulations and altimeter data su ggest that there was a large SSH change in 1995-1996 over the Gulf Stream w hich was associated with an abrupt weakening of the overturning.