On the role of the Azores Current in the ventilation of the North AtlanticOcean

Three high-resolution ocean circulation models of the North Atlantic, diffe ring chiefly in their description of the vertical coordinate, are used to e xamine the ventilation of the North Atlantic subtropical gyre. All the mode ls produce mode waters of realistic densities in the Sargasso Sea and the E uropean Basin, but have Azores Currents of differing strengths, which are c ategorised as strong (of realistic transport), intermediate, and weak. Thes e differences have a critical impact upon the ventilation of the gyre. Most importantly, the strong Azores Current forms an effective barrier to the s outhward ventilation of Eastern North Atlantic Water from the northern Euro pean Basin, initially driving it southwestwards into the central gyre, befo re turning it back eastwards again in a general cyclonic circulation north of the Current. The intermediate and weak Azores Currents instead allow the southward ventilation of this water mass near the European and African coa sts. The situation in Nature appears to be intermediate between these two c ases, with the Azores Current acting as a partial block. The study also rai ses the possibility of the Azores Current forming an advective connection b etween the Sargasso Sea Mode Waters in the western basin and modes of simil ar densities found in the eastern basin on the southern side of the Current . Although there are high levels of variability in the extent of these ligh ter modes in the eastern basin in Nature, this postulate is supported by a number of observational studies. In addition, the present study also provid es some support for the local production of Madeira Mode Water in the easte rn basin, associated with retroflecting current branches on the southern si de of the Azores Current. Overall, the Azores Current is, therefore, likely to have a critical impact upon the ventilation of the subtropical gyre ove r a large area, rather than just locally, affecting the potential vorticity and density structure of the upper ocean between subtropical latitudes and the northern European Basin. The study also contributes to an ongoing comm unity effort to assess the realism of our current generation of ocean model s. (C) 2001 Elsevier Science Ltd. All rights reserved.