Influence of oceanic heat transport upon the sensitivity of a model climate

Abstract

The influence of oceanic heat transport on the sensitivity of climate to an increase of the atmospheric CO2 concentration is studied by comparing the CO2‐induced changes of two mathematical models. The first model is a general circulation model of the coupled ocean‐atmosphere system which includes ocean currents. In the second model the oceanic component of the first model is replaced by a simple mixed layer without ocean currents. Both models have limited computational domain with idealized geography and annual mean insolation. For each model, the sensitivity of climate is evaluated from the difference between the equilibrium climates of the normal CO2 and 4 times the normal CO2 concentrations. The results indicate that the presence of ocean currents reduces the sensitivity of surface air temperature because of the difference in magnitude of the surface albedo feedback effect. The poleward transport of heat by ocean currents raises the surface temperature at high latitudes, shifts poleward the margins of snow and sea ice, decreases the contribution of the albedo feedback effect, and reduces the sensitivity of climate. The equilibrium response of climate is compared with the transient response of climate to a sudden increase of atmospheric CO2 content. According to this comparison, the latitudinal dependence of the equilibrium response of zonally averaged surface temperature is qualitatively similar to the transient response approximately 25 years after the time of the sudden CO2 increase. This result suggests that the distribution of the zonally averaged temperature change in response to a gradual increase of atmospheric carbon dioxide also resembles the distribution of the equilibrium response provided that the characteristic time scale of the CO2 increase is longer than 25 years.