Nonlinear Gulf Stream Interaction with Deep Currents: A Numerical Simulation

A two-way-coupled duo grid primitive-equations-based ocean model is used in a 75-year simulation of the North Atlantic Ocean, Caribbean Sea and Gulf of Mexico region between 10◦ N and 73◦ N. All advection and horizontal pressure gradient terms are fourth-order accurate on a semi-Collocated control volume grid. By construction of surface freshwater and heat fluxes, the model multi-year ensemble mean annual cycle surface layer temperature and salinity converge to the climatological cycle. Hellerman annual cycle wind forcing is used. The duo grid resolutions are: 1/2◦ east of 60◦ W; 1/6◦ west of 60◦ W; 30 z-levels. Upwind-based fluxes across 60◦ W give nearly seamless grid coupling. Open boundary conditions derived from a one degree resolution global model are applied at 10◦ N. Model results show: total Gulf Stream (GS) separation at Cape Hatteras (CH) as observed, and a mean separated GS path close to the observed mean path. The model Deep Current System (DCS: the shelf slope and deep western boundary current) strongly affects the GS separation and path. Only after ∼ 10 years of simulation, when the Labrador-Seamodified dense DCS water arrives in the Grand Banks shelfbreak area, does the GS separation and path come into close agreement with observations. The wedge shaped region between the separated Gulf Stream and continental shelf slope is filled with eddies, including intense warm cores that pinch off the northern tips of Gulf Stream meanders. The isopycnal surfaces have a strikingly small slope in this region’s interior in both Yashayaev’s climatology (2002) and time mean model results, indicating eddy dynamics characteristic of baroclinic instability rather than diffusive lateral mixing between the GS and DCS.