Low frequency variability in a simulated atmosphere-ocean system

A new coupled ocean atmosphere model was constructed for the purposes of studying the low frequency internal variability of the atmosphere ocean system. Extant models of the atmosphere and ocean designed for execution on massively parallel computing platforms were combined via a new component called the coupler to form the new model. This model is the first to have a closed hydrologic cycle via a river runoff model, a simple sea ice model, and a relatively high resolution ocean model all in one simulation. No flux corrections or other restraints on the model climate are employed. The model climate compares well with other coupled models as seen in means and trends presented here. A simulation of over 600 years was performed.; An assessment of the model's low frequency variability was made with a rotated Empirical Orthogonal Function analysis of sea surface temperature in three frequency bands. The first mode of variability in the highest frequency band of interest (less than five years) was related to the model ENSO. In the midband frequencies (five to thirty years), a decadal mode in the Tropical Pacific is most prominent followed by an Antarctic mode and decadal variability in the gyres of the North Pacific and Atlantic. The multidecadal (thirty years and longer) variability is mostly in the form of a thermohaline oscillation in the North Atlantic with a period of nearly fifty years. Other low frequency variability centers occur near locations of model deep water formation.; Additional experiments were performed with the coupled model to determine which of several new atmosphere parameterizations is most responsible for an improvement in the simulated Tropical Pacific over earlier model versions. The addition of a parameterization for deep convection is found to be most responsible.