Comparison of Thermally Driven Circulations from a Depth-Coordinate Model and an Isopycnal-Layer Model. Part II: The Difference and Structure of the Circulations

Abstract

Thermally driven ocean circulations in idealized basins are calculated with two well-known model codes, one based on depth-level coordinates and the other based on isopycnal coordinates. In addition, the two models have very different representations of convection. In the level-coordinate model, convective adjustment is used, while in the isopycnal-coordinate model, convection is simulated by a transformation of the surface layer to the layer below. Both models indicate a three-layer structure in the circulation. The lower and middle layers have a flow structure that corresponds with the classical abyssal circulation models. The upper flow is strongly constrained by the buoyancy flux field at the upper surface and the convective parameterization. The model with convective adjustment and level coordinates is dominated by an eastward flow, which sinks to subsurface level at the eastern boundary. It lacks any indication of a surface cyclonic flow, even in the vicinity of sinking at the northern wall. On the other hand, in the model based on density coordinates the eastward surface flow turns to the north at the eastern boundary and forms a pronounced cyclonic circulation at high latitudes. Due to the cyclonic circulation, the coldest surface water is found near the northwestern corner, while in the level model the coldest water is near the northeastern corner. The isopycnal model appears to be a more realistic representation of the real ocean since both wind and the thermohaline circulation are thought to contribute to the North Atlantic subarctic cyclonic gyre. Although the zonally averaged buoyancy flux produced by the two model codes is the same, the actual patterns of buoyancy flux at the surface are not similar at high latitudes. This suggests that the two types of numerical models would indicate very different air–sea interaction if coupled to atmospheric models and used to simulate climate. The application of the Gent–McWilliams parameterization of mesoscale eddies to the model with z coordinates and convective adjustment reduces the differences between the surface circulation of the two models by a small amount.