Multiple climate and sea ice states on a coupled Aquaplanet

A fully coupled atmosphere-ocean-sea ice GCM is used to explore the climates of Earth-like planets with idealized ocean basin geometries. We find three very different stable equilibria under identical external forcing: an equable ice-free climate, a cold climate with ice caps extending into mid-latitudes, and a completely ice-covered "Snowball" state. These multiple states persist for millennia despite a full seasonal cycle and vigorous internal variability of the system on all time scales (from day-to-day weather perturbations to multi-decadal fluctuations).

We report on two key findings:

1) a complex coupled system with many degrees of freedom can sustain three equilibrium states and

2) the meridional structure of the ocean heat transport (OHT) is essential in the maintenance of three multiple states.

The latter is demonstrated through 1) an extension of the Budyko-Sellers model to include explicit OHT, and the insulation of the ice-covered sea surface and 2) a set of sensitivity tests conducted with a slab ocean GCM and prescribed OHT. The OHT in the coupled system is dominated by wind-driven subtropical overturning cells carrying ~2 PW of energy out of the deep tropics, most of which converges in the subtropics to lower mid-latitudes. This convergence pattern (similar to modern Earth) is robust to changes in the ocean basin geometry, and is directly responsible for the stabilization of the large ice cap.

The similarity between the OHT of our coupled system and that of modern Earth leads us to believe that multiple equilibria could be a feature of the real climate system.

Exploration of the transitions between states and paleoclimate implications will be discussed in a companion paper presented by B. Rose.