The available energy of the ocean is the excess of the sum of the ocean's internal and gravitational energies with respect to its equilibrium state, which is in thermodynamic equilibrium and has the same total entropy as the ocean. The equilibrium state for the world ocean is rigorously determined to be an isothermal ocean with a temperature of 3.66˚C and a horizontally uniform absolute salinity that increases monotonically from 27.30 g kg–1 at the surface to 47.39 g kg–1 at a depth of 5.5 km. This salinity profile is consistent with a uniform relative chemical potential of 47.44 J g–1 salt. The ocean's available energy is 220 × 1021 J or 630 MJ m–2. Most (72%) of the available energy is due to the internal energy difference between the ocean and its equilibrium state; the remaining 28% is due to the gravitational energy difference. The ocean's available energy is shown to be concentrated vertically in the upper half kilometer and geographically in the tropics and subtropics. This distribution is accurately represented by the temperature variance from the equilibrium temperature. The contributions of sea ice and variable sea surface height to the available energy are estimated to be small.
Bannon, Peter R., and Raymond G. Najjar. 2014. "Available energy of the world ocean." Journal of Marine Research 72, (4). https://elischolar.library.yale.edu/journal_of_marine_research/392