ON THE EFFECTS OF HORIZONTAL RESOLUTION IN A LIMITED-AREA MODEL OF THE GULF-STREAM SYSTEM

An adiabatic, primitive equation, eddy-resolving circulation model has been applied to the Gulf Stream System from Cape Hatteras to east of the Grand Banks (30-degrees-48-degrees-N, 78-degrees-45-degrees-W). A two-layer version of the model was driven both by direct wind forcing and by transport prescribed at inflow ports south of Cape Hatteras for the Gulf Stream and near the Grand Banks of Newfoundland for the deep western boundary current. The mean upper-layer thickness was sufficie ntly large for interface outcropping not to occur. Numerical experimen ts previously run at 0.2-degrees horizontal resolution (approximately 20 km) had some realistic features, but a key unresolved deficiency wa s that the highest eddy kinetic energies obtained near the Gulf Stream were too low relative to data by a factor of about 2, with inadequate eastward penetration. A unique set of new numerical experiments has e xtended previous results to higher horizontal resolution, all other co nditions being held fixed. At 0.1-degrees horizontal resolution, eddy kinetic energies in the vicinity of the Gulf Stream realistically incr ease by a factor of roughly 2 relative to 0.2-degrees. The increase in eddy activity is a result of enhanced energy conversion from mean flo w to fluctuations due to barotropic and baroclinic instabilities, with the nature of the instability mixture as well as eddy energy changing with increased resolution. One experiment at 0.05-degrees horizontal resolution (approximately 5 km) yielded kinetic energies and key energ y transfer terms that are within 10% of the equivalent 0.1-degrees cas e, suggesting that convergence of the numerical solutions has nearly b een reached.