EFFECT OF THE OVERFLOWS ON THE CIRCULATION IN THE SUBPOLAR NORTH-ATLANTIC - A REGIONAL MODEL STUDY

An ocean circulation model for process studies of the Subpolar North A tlantic is developed based on the Geophysical Fluid Dynamics Laborator y (GFDL) Modular Ocean Model (MOM) code. The basic model configuration is identical with that of the high-resolution model (with a grid size of 1/3 degrees x 2/5 degrees) of the World Ocean Circulation Experime nt (WOCE) Community Modeling Effort (CME), except that the domain of i ntegration is confined to the area from 43 degrees to 65 degrees N. Op en boundary conditions are used for the inflows and outflows across th e northern and southern boundaries. A comparison with the CME model co vering the whole North Atlantic (from 15 degrees S to 65 degrees N) sh ows that the regional model, with inflow conditions at 43 degrees N fr om a CME solution, is able to reproduce the CME results for the subpol ar area. Thus the potential of a regional model lies in its use as an efficient tool for numerical experiments aiming at an identification o f the key physical processes that determine the circulation and water mass transformations in the subpolar gyre. This study deals primarily with the representation and role of the overflow waters that enter the domain at the northern boundary. Sensitivity experiments show the eff ect of closed versus open boundaries, of different hydrographic condit ions at inflow points, and of the representation of the narrow Faeroe Bank Channel. The representation of overflow processes in the Denmark Strait is the main controlling mechanism for the net transport of the deep boundary current along the Greenland continental slope and furthe r downstream. Changes in the Faeroe Bank Channel throughflow condition s have a comparatively smaller effect on the deep transport in the wes tern basin but strongly affect the water mass characteristics in the e astern North Atlantic, The deep water transport at Cape Farewell and f urther downstream is enhanced compared to the combined Denmark Strait and Iceland-Scotland overflows. This enhancement can be attributed to a barotropic recirculation in the Irminger Basin which is very sensiti ve to the outflow conditions in the Denmark Strait. The representation of both overflow regions determine the upper layer circulation in the Irminger and Iceland Basins, in particular the path of the North Atla ntic Current.