Ring Genesis and the Related Heat Transport. Part II: A Model Comparison

Abstract

Various ocean circulation models have been compared with respect to their performance in the genesis of rings and the subsequent heat transport. Emphasis has been placed on the role of the spurious diapycnal fluxes of heat and momentum in Cartesian models, arising when the horizontal dissipation mixes through sloping isopycnals. Quasigeostrophic, isopycnal coordinate, and Cartesian primitive equation models in a two-layer periodic channel domain have been used to simulate the process of eddy detachment from an eastward-flowing jet. This jet is modeled after the Gulf Stream east of Cape Hatteras. On this jet a small sinusoidal disturbance is super-imposed, which, through the release of available potential energy, grows until it ultimately has developed into ringlike eddies. Simulations with the Cartesian primitive equation model appear to suffer from spurious diapycnal mixing of both heat and momentum. This retards the process of Rossby wave breaking and prolongs the growth of the meander, thus causing a doubled heat transport at 10-km resolution, compared to a 5-km resolution experiment. The isopycnic model does not show this degree of overshoot in heat transport. In general, the Cartesian model is much more sensitive to both resolution and closure formulation than the isopycnic model. The quasigeostrophic model does not simulate the small-scale processes of Rossby wave steepening and breaking correctly. However, as a consequence the diapycnal mixing of heat and momentum hardly affects these processes. For this reason, the quasigeostrophic model does not show an overshoot in heat transport.