Mean climate and variability of the atmosphere and ocean on an aqua-planet

Numerical experiments are described that pertain to the climate of a coupled atmosphereocean-ice system in the absence of land, driven by modern-day orbital and CO2 forcing. Millennial timescale simulations yield a mean state in which ice caps reach down to 55 of latitude and both the atmosphere and ocean comprise eastward and westward-flowing zonal jets, whose structure is set by their respective baroclinic instabilities. Despite the zonality of the ocean, it is remarkably efficient at transporting heat meridionally through the agency of Ekman transport and eddy-driven subduction. Indeed the partition of heat transport between atmosphere and ocean is much the same as the present climate, with the ocean dominating in the tropics and the atmosphere in middle-to-high latitudes. Variability of the system is dominated by the coupling of annular modes in the atmosphere and ocean. Stochastic variability inherent to the atmospheric jets drives variability in the ocean. Zonal flows in the ocean exhibit decadal variability which, remarkably, feeds back to the atmosphere, coloring the spectrum of annular variability. A simple stochastic model can capture the essence of the process. Finally, we briefly review how the aqua-planet can inform us about the processes which set the partition of heat transport and climate of earth.