A thermal oscillation under a restorative forcing

The authors report an interdecadal oscillation in a windand thermally-driven ocean general circulation model (OGCM). The oscillation is tantalizing in that it occursunder a relatively strong thermal damping(26.3 W m-2K-’). Examinations involving a two-dimensional OGCM, a simple thermal ‘flip-flop’ model, and a three-dimensional OGCM with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so-called convective oscillator, or the advective overshooting oscillator. Instead, the oscillation is associated with the propagation of modelled long Rossby-waves with westward phase velocity. It is found that the north-south basin extent of the OGCM is an important factor; a larger northsouth basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances. The geophysical relevance of these results is discussed.