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

The 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 basin into the convective area, which reduces the convective activity. Similarly, when the convective activity is small, the flow of relatively fresh water is weak, which enables the expansion of the convective area. The periodically strengthening and weakening of the southeastward surface flow at the polar boundary of the basin is revealed by a negative polar cell in the meridional overturning. 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 critically depends on how the ocean circulation, and especially the surface circulation, responds to anomalous convective activity. Horizontal boundaries turn out to play an important role in the dynamical response of the ocean circulation. That the dynamical reponse is essential to the oscillation is confirmed with two simple (conceptual) models, and some idealized ocean experiments. Finally, the sensitivity of the oscillation to salt perturbations and the restoring constant for the air/sea heatflux is investigated.