Transient Response of a Global Ocean-Atmosphere Model to a Doubling of Atmospheric Carbon Dioxide

The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insulation. An equilibrium climate of the coupled model is perturbed by an abrupt doubling of the atmospheric carbon dioxide. The evolution of the model climate during the 60-year period after the doubling is compared with the result from a control integration of the model without the doubling. The increase of surface air temperature in middle and high latitudes is slower in the Southern Hemisphere than the Northern Hemisphere The large thermal inertia of the ocean-dominated hemisphere is partly responsible for this difference. The effective thermal inertia of the oceans becomes particularly large in high southern latitudes. Owing to the absence of meridional barriers at the latitudes of the Drake Passage. a wind-driven. deep cell meridional circulation is maintained in the Circumpolar Ocean of the model. In addition, a deep reverse cell develops in the immediate vicinity of the Antarctic Continent. The thermal advection by these cells and associated convective overturning result in a very efficient mixing of heat in the 2-km thick upper layer and increase the effective thermal inertia of the ocean, thereby contributing to the slow down of the CO2-induced warming of the near-surface layer of the Circumpolar Ocean of the model. It is surprising that during the last 15 years of the 60-year experiment, sea surface temperatures in the Circumpolar Ocean actually reduce with time. Because of the increase in precipitation caused by the enhanced penetration of warm, moisture-rich air aloft into high latitudes, the surface halocline of the Circumpolar Ocean intensifies, thereby suppressing the convective mixing between the surface layer and the warmer underlying water. Thus, sea surface temperature is reduced in the Circumpolar Ocean towards the end of the experiment. In the Northern Hemisphere the CO2 induced warming of the lower troposphere increases with increasing latitudes and is at a maximum near the North Pole due partly to the albedo feedback process involving sea ice and snow cover. The warming of the upper ocean layer also increases with increasing latitudes up to about 65°N where the absorption of solar radiation increases markedly due to the poleward of sea ice. Over the Arctic Ocean, the warming is very large in the surface layer of the model atmosphere, whereas it is very earner small in the underlying water. Both sea ice and a stable surface halocline act as thermal insulators and are responsible for the large air-sea contrast of the warming in this region. In short, the CO2 induced warning of the sea surface has a large interhemispheric asymmetry, in qualitative agreement with the results from a previous study conducted by use of a coupled model with a sector computational domain and an idealized geography. This asymmetry induces an atmospheric response which is quite different between the two hemispheres.