  The global ocean circulation plays a pivotal role in the regulation of the
Earth's climate. The specific pattern and strength of circulation also
determines how carbon and nutrients are cycled and via the resulting
distribution of dissolved oxygen, where habitats suitable for marine animals
occur. However, evidence from both geological data and models suggests that
state transitions in circulation patterns have occurred in the past.
Understanding the controls on marine environmental conditions and biological
productivity then requires a full appreciation of the nature and drivers of
such transitions. Here we present an analysis of stable millennial oscillations
that occur in the ocean circulation of an Earth System Model of intermediate
complexity in the presence of a circumpolar current. To demonstrate that a
circumpolar current can act as a driver of stable oscillations, we adapt a
simple ocean box model to include a delayed feedback to represent a circumpolar
current and investigate the stable millennial oscillatory solutions that arise
in the box model by bifurcation analysis. Our results provide new insights into
the nature of oscillations that could have occurred under certain continental
configurations in the geological past, and also highlight the potential
influence of changing circumpolar current speed on the stability of the ocean's
meridional overturning circulation.
