MIXED BOUNDARY-CONDITIONS VERSUS COUPLING WITH AN ENERGY-MOISTURE BALANCE MODEL FOR A ZONALLY AVERAGED OCEAN CLIMATE MODEL

The Wright and Stocker oceanic thermohaline circulation model is coupl ed to a recently developed zonally averaged energy moisture balance mo del for the atmosphere. The results obtained with this coupled model a re compared with those from an ocean-only model that employs mixed bou ndary conditions. The ocean model geometry uses either one zonally ave raged interhemispheric basin (the ''Atlantic'') or two zonally average d basins (roughly approximating the Atlantic and the Pacific Oceans) c onnected by a parameterized Antarctic Circumpolar Current. The differe nces in the steady states and their linear stability are examined over a wide range of parameters. The presence of additional feedbacks betw een 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 geometr ies. The two models generally have different (though similar) equilibr ia and, most importantly for the issue of climate change, the variabil ity in the models near similar steady states is quite different. In th e one-basin case, three different steady states were found with both m odels, 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 t hat cannot affect the implicit atmosphere under mixed boundary conditi ons, but which changes the surface air temperature in the coupled mode l, and which also leads to several feedbacks on the ocean circulation. Consequently, the corresponding states in the coupled model are diffe rent from those in the ocean-only model. In the two-basin case, five b asic steady states were found in the ocean-only model: a state with tw o cells in both basins, a conveyor state, a reverse conveyor state, a state with northern sinking circulation in both basins, and a state wi th 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 durin g 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. T he authors conclude that due to the effects produced by the feedbacks in the coupled model, they must have serious reservations about the re sults concerning long-term climate variability obtained from ocean-onl y models. Thus, to investigate long-term climatic variability a couple d model is necessary.