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 q uasi-geostrophic and shallow water models. In this paper we demonstrate tha t multiple equilibria persist within a reduced gravity shallow water model under quite realistic continental geometry and windstress forcing for the N orth Atlantic. Multiple mean flow patterns of the Gulf Stream exist and dif fer with respect to their separation behavior along the North American coas t. The origin of these equilibria is investigated by determining the struct ure of steady solutions within a hierarchy of equivalent barotropic ocean m odels using continuation techniques. Within each model, the magnitude of la teral friction is used as a control parameter. It is shown that symmetry br eaking, found in a quasi-geostrophic model for a rectangular ocean basin wi th 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 nu mber of oscillatory modes, which either have intermonthly or interannual pe riods. The modes of variability remain strongly related through the hierarc hy of models indicating that their physics is not strongly dependent on the shape of the continents but is controlled by internal ocean dynamics.