AN INTERCOMPARISON OF A BRYAN-COX-TYPE OCEAN MODEL AND AN ISOPYCNIC OCEAN MODEL .1. THE SUBPOLAR GYRE AND HIGH-LATITUDE PROCESSES

This paper describes a model intercomparison between a Bryan-Cox-type ocean model and an isopycnic-coordinate ocean model. The two models ar e integrated for 30 years on a domain of the North Atlantic stretching from 20 degrees S to 82 degrees N. The main purpose of this work is t o illuminate aspects of the respective models that give a realistic re presentation of the North Atlantic circulation and the physical proces ses that occur therein, and to identify those which need to be improve d. To this end, the same forcing fields and as many of the same parame ter settings as possible were chosen so that differences between the m odels would be due to distinct model features rather than choice of pa rameters. Where we felt that using the same setup between the models w as inappropriate or impossible, we examined the possible difference th is could make to the simulations. In the isopycnic model, the path of the North Atlantic Current after separation is simulated quite realist ically, whereas in the Bryan-Cox model it becomes much too zonal in th e central North Atlantic. This difference has an impact on the simulat ion in the subpolar gyre and in the Greenland-Iceland-Norway basin. Th e representation of dense overflows across the Greenland-Iceland-Scotl and ridge is found to be the main underlying difference in the way the simulations develop in the two models. The isopycnic model has specif ied isopycnic and diapycnic mixing, and its deep, dense hows over the ridge system retain their water properties. This is not the case in th e Bryan-Cox model, in which the quasi-isopycnal mixing tensor includes both explicit background horizontal mixing (which could have a diapyc nic component) and implicit diapycnic mixing arising from a limitation on the allowable slope of the mixing tenser. It is found that in this model the dense overflows mix vigorously with the surrounding warmer, saltier water as they flow over the ridge so that their water propert ies change relatively quickly as they travel downstream. The formation of subpolar mode waters occurs primarily in the Irminger Basin in bot h models, and in the isopycnic model this mode water has reasonable ch aracteristics. In the Bryan-Cox model the processes downstream of the dense overflows are degraded due to the development of a homogeneous w ater mass below the surface, originating from the mixed overflow water . This water mass prevents the mixed layer from deepening substantiall y in the subpolar gyre, and so prevents the formation of realistic amo unts of mode water. The growth of this homogeneous water mass may also be at least partly responsible for the zonality of the North Atlantic Current in the Bryan-Cox model. The results provide guidance on the f uture development of both types of model.