| A zonally averaged coupled atmosphere-ocean model for climate studies is developed. The ocean component is the Wright and Stocker two-dimensional thermohaline circulation (THC) model, and the atmospheric component is a zonally averaged energy-moisture balance model for the atmosphere. Both single- and multi-basin configurations of the model are considered.; The results obtained with this coupled model are compared with those from an ocean-only model that employs mixed boundary conditions. The differences in the steady states of the two models and their linear stability are examined over a wide range of parameters, for both one- and two-basin ocean models. 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. The two models generally have different (though similar) equilibria, but, more importantly for the issue of climate change, the variability in the models near similar steady states is quite different. These differences indicate that to perform relevant investigations of long-term climatic variability, a coupled model is necessary.; Next the coupled model with three-ocean basins is applied to last glacial maximum (LGM) conditions. It is found that to achieve realistic results, it is necessary to add a thermodynamic sea ice model into the coupled atmosphere-ocean model. The variability of the LGM conveyor circulation in the coupled ocean-sea ice-atmosphere model is then examined, and the model is subjected to a range of freshwater perturbation experiments. The conveyor state circulation is quite sensitive to the interbasin atmospheric transport of water vapour from the Atlantic to the Pacific. In particular, increasing this transport makes the conveyor state more robust. The LGM model circulation does not exhibit internal century-to-millennial scale variability, nor can the latter be excited by steady freshwater forcing. However, rapid climatic change on a timescale of decades can be generated through transient freshwater forcing of the northern North Atlantic. Perturbations in the ocean circulation are also found to propagate from the Atlantic Ocean to the Pacific Ocean in a few decades. Stochastic, white noise forcing of the model results in a mainly red noise response but also excites a natural mode of THC variability with a timescale of about 150 years. |
