Present-day and last-glacial-maximum ocean thermohaline circulation in a zonally averaged coupled ocean-sea-ice-atmosphere model

A simple ocean-atmosphere model suitable for long-term paleoclimate studies is presented. It consists of a three-basin zonally averaged ocean model coupled to an energy moisture-balance atmospheric model and a thermodynamic sea-ice model. The model is used to simulate the "present-day'' climate conditions and the conditions resembling those of the last glacial maximum (LGM). In the first case, the model has an ocean thermohaline (THC) circulation in which the overturning in the North Atlantic is slightly more than 15 Sv. Two methods are used to spin up the model to the LGM climate. First, a cool state is obtained by running the coupled model with a reduced model greenhouse effect and increased albedo, but without making any explicit changes to the hydrological cycle. In the second approach, the model is directly set up for LGM conditions, in a manner similar to the method used to simulate the present-day climate. In both cases the computed LGM sea surface temperatures are reasonable, but only in the second case is a realistic ocean THC obtained. The LGM THC is steady and no self-sustained millennial-scale oscillations are present for conventional values of the diffusivity coefficients. The absence of these oscillations in the model and their presence in other simple climate models is discussed, and it is suggested how a combination of paleoceanographic data and modeling might be used to determine whether the glacial THC indeed exhibited such internal oscillations.