Energy and water transport in climates simulated by a general circulation model that includes dynamic sea ice

Abstract

We analyze energy and water transport in present, doubled CO2, and tripled CO2 climates simulated by the Mark 2 CSIRO nine‐level general circulation model with a mixed layer ocean. The model differs from the Mark 1 version by the inclusion of dynamic sea ice, a semi‐Lagrangian water vapor transport, and an enhanced land‐surface scheme, and it includes prescribed ocean heat transport. We describe a 30‐year climatology of the 1×CO2 simulation, emphasizing the sea ice and the mean meridional energy and water transport. The ice depths, concentrations, and velocities are moderately realistic in both hemispheres. Poleward energy transport is inferred (calculated indirectly from vertical energy fluxes) for both the atmosphere and ocean, although the oceanic flux is much weaker than observational estimates for the southern hemisphere. Atmospheric water transport is also poleward outside the tropics and compares well with observations. Energy transport within the ice layer has been evaluated by both direct and indirect methods. As it is largely due to the latent heat of ice formation, it is closely proportional to the water transport by ice. The meridional transports by ice of both energy and water are relatively important at high latitudes. The divergence of the ice energy transport corresponds to a significant component of the surface energy budget, reaching ±10 W m−2 or more at some polar locations. The equilibrated doubled CO2 global mean surface warming of the Mark 2 mixed layer model is 4.3°C. The reduction from the Mark 1 result (4.8°C) follows largely from a 40% reduction of the warming over high‐latitude oceans. This is attributed to the presence of dynamically induced leads in the ice cover. The equilibrated warming for 3 × CO2 is 6.8°C. The model atmosphere transports less heat poleward in the doubled CO2 climate, largely as a response to increased solar radiation absorbed at high latitudes. This behavior contrasts with the change at CO2 doubling in a transient simulation by the Mark 2 model coupled to a full ocean model, in which heat is taken up in the midlatitudes, particularly by the Southern Ocean, and supplied by a net top‐of‐atmosphere radiative imbalance distributed over all latitudes (global mean, 1.8 W m−2). The atmospheric water transport is enhanced by 10–20% in the warmer climates at most latitudes.