The energy budget is a core question for ocean circulation and climate change. In fact, it is one of the most challenging problems facing in our scientific field. Bannon & Najjar  present a thorough and detailed discussion of the entropy budget in the ocean and the ocean’s Carnot efficiency is estimated to be 0.86%. The energy budget in the ocean is also a very controversial problem. There have been two competing theories. One is based on the entropy budget, which is the basis for the estimate of the Carnot efficiency in this paper. The other is the theory of Available Potential Energy, which provides a different way to define another kind of thermodynamic efficiency. The link between the two approaches is still unclear, and clarifying it is an important remaining area of research.
Citation: Bannon, P. R., & Najjar, R. G. . Heat‐engine and entropy‐production analyses of the world ocean. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2018JC014261
—Lei Zhou, Editor, Journal of Geophysical Research: Oceans