Changes of the geostrophic surface circulation in the North Atlantic related to the North Atlantic Oscillation as measured by satellite altimetry

Interannual variations in the geostrophic circulation of the upper layer of the North Atlantic between 20°N to 70°N and 1°W to 80°W are investigated using Topex/Poseidon, ERS-1, and ERS-2 altimeter data. The aim of this investigation is to study the variations in the surface circulation that have occurred when the North Atlantic Oscillation (NAO) Index switched from a strongly positive to a negative value in winter 1995/1996. The authors have analyzed altimetric data from these 3 satellites for the period of 10/92 to 9/97 with the aim to calculate anomalies of the geostrophic surface velocity in the North Atlantic. A mean geostrophic velocity calculated from mean temperature and salinity profiles on a 1° by 1° grid has been added to the velocity anomalies to derive absolute values. The gridded temperature and salinity data provided by the Special Analysis Center of the World Ocean Circulation Experiment (WOCE) located in Hamburg consists of profiles which have been measured from ships during the last 70 years. Oceanic fronts are well preserved in the gridded data. Their analysis of this limited data set shows a strong southward shift of the Gulf Stream Extension during the period of negative NAO index. It is accompanied with the weakening of the geostrophic component of the North Atlantic Current and most of its branches-an indication for a decrease of the geostrophical driven heat transport to the northeastern North Atlantic in the upper layer