Climate variability of coupled atmosphere-ocean aquaplanet simulations

A long-term aquaplanet simulation is conducted with a global spectral atmospheric general circulation model (the Planet Simulator AGCM) coupled to the Hamburg Large Scale Geostrophic ocean circulation model (LSG-OGCM). In an aquaplanet set-up the entire surface of the earth is covered by one ocean. In this idealized environment no continents break the zonal symmetry in the atmosphere and the ocean is also completely zonally symmetric due to a flat bottom topography. Furthermore, all external forcings are symmetric about the equator to eliminate seasonality and to create a symmetry between the northern and southern hemisphere. The mean state and the variability of the coupled climate system on the aquaplanet are analyzed. Different states of aquaplanet climates have been found in previous studies of coupled atmosphere-ocean GCM simulations. While some observed warm greenhouse climates with ice-free poles and shallow equator-to-pole temperature gradients, other simulations resulted in cold aquaplanets with sea ice caps over the poles down to the mid-latitudes and steeper temperature gradients between the tropics and high latitudes. One study analyzed multiple equilibria which emerged in a coupled aquaplanet simulation. Even though multiple states in aquaplanet simulations have been analyzed before, there is no previous study which describes an oscillation between cold and warm climates, as the one presented here. A low-frequency oscillation can be observed in our coupled aquaplanet simulations: warmer climates without polar sea ice cover and weaker oceanic overturning and cold climates with sea ice cover down to 65°N/S and a stronger MOC alternate with a period of approximately 700 years.