Imperfections of the North Atlantic wind-driven ocean circulation: Continental geometry and windstress shape

Multiple equilibria of the wind-driven gyres have been found in idealized quasi-geostrophic and shallow water models. In this paper we demonstrate that multiple equilibria persist within a reduced gravity shallow water model under quite realistic continental geometry and windstress forcing for the North Atlantic. Multiple mean flow patterns of the Gulf Stream exist and differ with respect to their separation behavior along the North American coast. The origin of these equilibria is investigated by determining the structure of steady solutions within a hierarchy of equivalent barotropic ocean models using continuation techniques. Within each model, the magnitude of lateral friction is used as a control parameter. It is shown that symmetry breaking, found in a quasi-geostrophic model for a rectangular ocean basin with idealized wind forcing is at the origin of two different mean states of the Gulf Stream. The steady states found become unstable only to a small number of oscillatory modes, which either have intermonthly or interannual periods. The modes of variability remain strongly related through the hierarchy of models indicating that their physics is not strongly dependent on the shape of the continents but is controlled by internal ocean dynamics