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

Numerical thought experiments are described that pertain to the climate of a coupled atmosphere-ocean-ice system in the absence of land, driven by modern-day insolation and radiative forcing. Millennial timescale simulations yield a mean state in which ice caps reach down to 60 degrees 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 marked decadal variability which, remarkably, feeds back to the atmosphere, coloring the spectrum of annular variability in the atmosphere. A simple stochastic model can capture the essence of the process.

Finally, we briefly review how the aqua planet can inform us about mechanisms at work (or not at work) in our own climate.