On the mechanism of decadal oscillations in a coarse resolution ocean model

The physical mechanism that causes an interdecadal oscillation in a coarse resolution sector ocean model forced by mixed boundary conditions is studied. The oscillation is characterized by large fluctuations in convective activity and air/sea heat exchange on a decadal time scale. When the convective activity is large, a strengthening of the southeastward surface flow advects more relatively fresh water from the northwestern part of the bsain into the convective area, which reduces the convective activity. Similarly, when convective activity is small, the flow of relatively fresh water is weak, which enables the expansion of the convective area. The periodic strengthening and weakening of the southeastward surface flow is revealed by a reverse cell in the meridional overturning at the polar boundary of the basin. The existence of a halocline and an inverse thermocline with cool and fresh water above a warm and salty subsurface is essential for the oscillation.
Further, the oscillation depends critically on how the ocean circulation, and in particular the surface circulation, responds to convective activity. Horizontal boundaries turn out to play an important role in the dynamical response of the ocean circulation. Finally, the sensitivity of the oscillation to salt perturbations and the restoring constant for the air/sea heat flux is investigated.