Circulation characteristics in three eddy-permitting models of the North Atlantic

A systematic intercomparison of three realistic eddy-permitting models of t he North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopoten tial levels; isopycnal layers, terrain-following (sigma) coordinates, respe ctively. Although these models were integrated under nearly identical condi tions, the resulting large-scale model circulations show substantial differ ences. The results demonstrate that the large-scale thermohaline circulatio n is very sensitive to the model representation of certain localised proces ses, in particular to the amount and water mass properties of the overflow across the Greenland-Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at sm all or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different model represe ntation of these processes. (C) 2001 Elsevier Science Ltd. All rights reser ved.