Talk by Andreas Born, University of Bern

Titel: Multiple equilibria as a possible mechanism for decadal
variability in the North Atlantic Ocean

The cyclonic circulation of the Atlantic subpolar gyre (SPG) is a key
mechanism for North Atlantic climate variability on a wide range of time
scales. Model results suggest that heat transport and circulation
indices in this region might be predictable several years ahead.
However, the complex dynamics of the region are insufficiently
understood to make predictions in the real world. I propose a simplified
four-box model for the SPG. It formalizes a bistability that is also
found in more complex general circulation models. This model is forced
by observed surface air temperatures and compared with several ocean
reanalysis datasets. A hysteresis shapes the response of the SPG to
forcing by atmospheric temperatures. Large variations in the SPG
correspond to the crossing of a bifurcation point that is predicted from
idealized experiments and an analytical solution of our model. The
performance of our conceptual model is tested against a statistical
stochastic model.