The Role of the ocean in CO₂-Induced Climate Change: Preliminary Results from the Osu Coupled Atmosphere-Ocean general Circulation model

The OSU coupled global atmosphere-ocean general circulation model (GCM) has been used to investigate the role of the ocean in CO₂-induced climate change. Two 16-year simulations have been made, a 1 X CO₂ simulation with a CO₂ concentration of 326 ppmv, and a 2 X CO₂ simulation with a CO₂ concentration of 652 ppmv. The results of the simulations are presented in terms of the 2 X CO₂ -1 X CO₂ differences in temperature. The evolution of the global-mean temperature differences displays a rapid warming of the atmosphere followed by a more gradual warming of the ocean and atmosphere. The annual-mean zonal-mean warming for year 16 shows that the atmospheric warming at the surface increases from the tropics to the subtropics, decreases towards the middle latitudes, and increases towards high latitudes. The warming increases with altitude in the tropics and subtropics, and decreases with altitude elsewhere. The oceanic surface warming increases from the tropics towards the mid-latitudes of both hemispheres. This is similar to the latitudinal distribution of the excess ¹⁴C over the pre-nuclear value that is observed at the sea surface. The oceanic warming penetrates to a greater depth in the subtropics and mid-latitudes than in the equatorial region. This also is similar to what is shown by the observed penetration depths of excess ¹⁴C. The warming decreases with depth everywhere equatorward of 60 degrees latitude in each hemisphere, and extends from the subtropical surface water downward towards high latitudes. Particularly interesting features of the temperature change at depth are the maxima located near 65°N and 60°S in the 250–750 m ocean layer. The geographical distributions show that the latter feature is located in the vicinity of the Ross Ice Shelf. Since the 16-year simulations are not of sufficient duration for the equilibrium change to have been attained, an analysis of the coupled GCM results is performed with an energy balance climate/box ocean model. From this analysis it is estimated that the gain (sensitivity) of the coupled GCM is 0.72°C (Wm⁻²)⁻¹, the global-mean air-sea heat transfer coefficient is 8.0Wm⁻² °C⁻¹, and the effective oceanic thermal diffusivity κ is 3.2 cm² s⁻¹ at 50 m depth, 3.8 cm² s⁻¹ at 250 m, and 1.5cm² s⁻¹ at 750m. The mass-averaged κ = 2.25 cm² s⁻¹ is in agreement with the best estimate based on the observed penetration of bomb-produced tritium and ¹⁴C into the ocean. Furthermore, a box-diffusion climate model with κ = 2.25–2.50 cm² s⁻¹ is successful in reproducing the evolution of the 2 X CO₂ —1 X CO₂ differences in the surface air and ocean surface layer temperatures simulated by the coupled GCM. Consequently, it appears that the coupled GCM transports heat from the surface downward into the ocean at a rate which is commensurate with the rate observed for the downward transport of tritium and ¹⁴C. A projection of the GCM results by the simpler climate model indicates that the time required for the ocean to reach 63% of its equilibrium warming is 75 years. The possible implications of this memory of the climate system are discussed in terms of the detection of a climate change and its attribution to CO₂.