Sack Lunch Seminar (SLS)

Ocean Heat Transport, Sea Ice, and Multiple Equilibria of the Climate System




The oceans carry about 2 PW of heat out of the tropics and release most of it to the atmosphere in subtropical to lower mid-latitudes. This pattern is found in observations and is a remarkably robust feature of coupled climate simulations. I study the fundamental role of the ocean heat transport (OHT) in the climate system, and argue that the basic pattern of OHT interacts with sea ice cover in two distinct but important ways. In cold climates, the OHT convergence creates a mid-latitude barrier to sea ice expansion, thus stabilizing large ice cap states to further equatorward expansion. In warm climates, mid-latitude OHT convergence exerts a remote warming effect at the poles by feeding the poleward atmospheric heat transport. These effects, coupled to the ice-albedo feedback, allows for the possibility of multiple equilibria of the climate system with large and small ice caps. I demonstrate these principles through a hierarchy of idealized atmosphere-ocean-ice models, ranging from Energy Balance Models (EBM) to fully coupled aquaplanet GCMs, all of which exhibit multiple equilibria. I will discuss some details of the multiple solutions of the coupled GCM, and rationalize its behavior in terms of a simple EBM. Finally I will discuss the dynamics of abrupt transitions between the warm and cold states of the GCM in response to slowly varying forcing.