Mixed Boundary Conditions versus Coupling with an Energy–Moisture Balance Model for a Zonally Averaged Ocean Climate Model

Abstract

The Wright and Stocker oceanic thermohaline circulation model is coupled to a recently developed zonally averaged energy moisture balance model for the atmosphere. The results obtained with this coupled model are compared with those from an ocean-only model that employs mixed boundary conditions. The ocean model geometry uses either one zonally averaged interhemispheric basin (the ?Atlantic?) or two zonally averaged basins (roughly approximating the Atlantic and the Pacific Oceans) connected by a parameterized Antarctic Circumpolar Current. The differences in the steady states and their linear stability are examined over a wide range of parameters. The presence of additional feedbacks between the ocean circulation and the atmosphere and hydrological cycle in the coupled model produces significant differences between the latter and the ocean-only model, in both the one-basin and two-basin geometries. The two models generally have different (though similar) equilibria and, most importantly for the issue of climate change, the variability in the models near similar steady states is quite different. In the one-basin case, three different steady states were found with both models, an unstable two-cell circulation with equatorial upwelling, and two stable states with a one-cell (pole-to-pole) circulation. In the one-cell states, there is an interhemispheric oceanic heat transport that cannot affect the implicit atmosphere under mixed boundary conditions, but which changes the surface air temperature in the coupled model, and which also leads to several feedbacks on the ocean circulation. Consequently, the corresponding states in the coupled model are different from those in the ocean-only model. In the two-basin case, five basic steady states were found in the ocean-only model: a state with two cells in both basins, a conveyor state, a reverse conveyor state, a state with northern sinking circulation in both basins, and a state with southern sinking in both basins. The state with southern sinking in both basins could not be found in the coupled model. In addition, two more steady states, each with a two-cell circulation in one basin and a one-cell circulation in the other, were found for both models during sensitivity tests. The bifurcation structures for the two models are very different, and also, the two-basin conveyor circulation is shown to be more stable to freshwater perturbations in the coupled model. The authors conclude that due to the effects produced by the feedbacks in the coupled model, they must have serious reservations about the results concerning long-term climate variability obtained from ocean-only models. Thus, to investigate long-term climatic variability a coupled model is necessary.