The role of deep sea heat storage in the secular response to climatic forcing

Abstract

The influence of the world oceans on climatic response is considered here with emphasis on the heat transferred to waters beneath the well‐mixed surface layer and to polar bottom water forming zones. An upwelling‐diffusing model is formulated to treat this problem whose effective transport properties are calibrated from the steady state vertical profiles of radiocarbon, potential temperature and other tracers measured by chemical oceanographers. The key issue with regard to the question of atmospheric temperature response to external climatic forcing is whether heat is exchanged between the surface mixed layer and deep sea at rates comparable to heat transfer rates between the planetary radiation field and the atmosphere‐mixed layer system. An important model parameter appearing in the analysis is the polar sea warming coefficient ∏ equal to the rate of change of polar sea temperature relative to changes in areally averaged mixed layer temperature. For ∏ values in the range of 0 to 2 the models predicts response times in the range of 8 to 20 years to attain 63% of the equilibrium temperature change for a step function climatic forcing, and 50 to 1000 years to get 90% of the equilibrium response. These may be compared with the roughly 4 year response time one gets with an oceanic mixed layer only model. To study the carbon dioxide climate problem, a more realistic time‐dependent forcing function is used based on the historical growth of fossil fuel CO 2 and a logarithmic scaling law for the temperature increment which would obtain at any instant if the system were in radiative‐convective equilibrium. Our results suggest the influence of deep sea thermal storage could delay the full value of temperature increment predicted by equilibrium models by 10 to 20 years in 1980 to 2000 A.D. time frame. Also considered is the model response to periodic forcing, the sensitivity of the results, and the implications of the model results with regard to climatic changes on a decadal to millenial timescale.