Heat-Engine and Entropy-Production Analyses of the World Ocean

A heat-engine analysis is performed in order to determine the ocean's Carnot efficiency, the maximum thermal energy (i.e., the Carnot work) available to drive the ocean circulation, and the ocean's irreversible entropy production. The analysis is based on a 36-year monthly climatology of the surface heat flux; estimates of geothermal input, mechanical energy input, and heat storage; and a detailed treatment of freshwater fluxes. The ocean's Carnot efficiency is estimated to be 0.86%. The Carnot work is equal to 110TW, which exceeds the mechanical energy input by the winds and tides. The net entropy flux from the ocean to its surroundings is found to be 0.617TW/K, in good agreement with an independent estimate of the irreversible entropy production from viscous dissipation, thermal mixing, and haline mixing, demonstrating a balanced entropy budget of the ocean. Thermal mixing accounts for 56% of this entropy production, with most of the remaining production due to viscous dissipation and only small contributions from haline mixing and sea ice melting. Two thirds of the thermal entropy production is due to isopycnal mixing. The effect of thermal diffusion is to reduce the Carnot work significantly. The actual reduction is proportional to the amplitude of the (poorly known) eddy thermal diffusivity. Plain Language Summary Surface fluxes of energy and entropy are used to assess the ocean's heat engine and are compared with internal entropy production.