  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 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.
