3 Discussion of the dynamics

The observation of an anticyclonic circulation of the AAIW underneath the surface subtropical gyre led to Buscaglia's (1971) conclusion that the AAIW circulation is governed by Sverdrup dynamics. We tested this hypothesis first by comparing the observed transport in a layer reaching from the sea surface down to the lower boundary of the AAIW layer with transports determined from the wind field using a Sverdrup model, then by applying a more sophisticated model of the ventilated thermocline to the problem. A simple transport comparison is the only possible way to infer the vertical extent of the gyre circulation with a Sverdrup model.

To force the models we used seasonal winds from a reanalysis of the European Center for Medium-range Weather Forecasting (ECMWF) winds from March 1991 until November 1993 (Barnier et al., 1995) and winds from the ERS-1 satellite for September 1991 until February 1995 (CERSAT and IFREMER, 1996). Because ECMWF winds are only available for more recent years, we selected a mean for the same season (March to May) as the SAVE 5&6 section and arbitrarily chose the year 1993. This wind field is used in all case studies.