An Intercomparison of a Bryan–Cox-Type Ocean Model and an Isopycnic Ocean Model. Part I: The Subpolar Gyre and High-Latitude Processes

Abstract

This paper describes a model intercomparison between a Bryan-Cox-type ocean model and an isopycnic-coordinate ocean model. The two models are integrated for 30 years on a domain of the North Atlantic stretching from 20°S to 82°N. The main purpose of this work is to illuminate aspects of the respective models that give a realistic representation of the North Atlantic circulation and the physical processes that occur therein, and to identify those which need to be improved. To this end, the same forcing fields and as many of the same parameter settings as possible were chosen so that differences between the models would be due to distinct model features rather than choice of parameters. Where we felt that using the same setup between the models was inappropriate or impossible, we examined the possible difference this could make to the simulators. In the isopycnic model, the path of the North Atlantic Current after separation is simulated quite realistically, whereas in the Bryan-Cox model it becomes much too zonal in the central North Atlantic. This difference has an impact on the simulation in the subpolar gyre and in the Greenland-Iceland-Norway basin. The representation of dense overflows across the Greenland-Iceland-Scotland ridge is found to be the main underlying difference in the way the simulators develop in the two models. The isopycnic model has specified isopycnic and diapycnic mixing, and its deep, dense flows over the ridge system retain their water properties. This is not the case in the Bryan-Cox model, in which the quasi-isopycnal mixing tensor includes both explicit background horizontal mixing (which could have a diapycnic component) and implicit diapycnic mixing arising from a limitation on the allowable slope of the mixing tensor. 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 properties change relatively quickly as they travel downstream. The formation of subpolar mode waters occurs primarily in the Irminger Basin in both models, and in the isopycnic model this mode water has reasonable characteristics. In the Bryan-Cox model the processes down-stream of the dense overflows are degraded due to the development of a homogeneous water mass below the surface, originating from the mixed overflow water. This water mass prevents the mixed layer from deepening substantially in the subpolar gyre, and so prevents the formation of realistic amounts 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 future development of both types of model.