Joel Hirschi, Johannes Sander, Thomas Stocker, Climate and Environmental Physics, Physics Insitute, University of Bern, Bern, Switzerland
Intermittent convection and its consequences on the stability of the thermohaline circulation are investigated with a global circulation model (GCM) and simple box models. A two-box model shows that intermittency is a consequence of the non-linearity of the equation of state and of the ratio of heat and freshwater fluxes at surface versus the fluxes at depth. Intermittency is not necessarily suppressed by long restoring times. Because intermittent convection causes temporal variations of the ocean-atmosphere fluxes, a GCM cannot reach an exact equilibrium. After the switch to mixed boundary conditions, changes of the convective activity occur in areas where intermittency is observed. Intermittent convection becomes either continuous or is stopped depending on the method used for calculating the freshwater fluxes. Advective and diffusive fluxes between these regions and their surroundings change in order to balance the altered convective fluxes. A comparison between the GCM and a six-box model illustrates that this may lead to an important alteration of adjacent deep convection and of the related deep water formation.