On the thermodynamics of the oceanic general circulation: entropy increase rate of an open dissipative system and its surroundings

The rôle of thermodynamics in the oceanic general circulation is investigated. The ocean is regarded as an open dissipative system that exchanges heat and salt with the surrounding system. A new quantitative method is presented to express the rate of entropy increase for a large‐scale open system and its surroundings by the transports of heat and matter. This method is based on Clausius's definition of thermodynamic entropy, and is independent of explicit expressions of small‐scale dissipation processes. This method is applied to an oceanic general circulation model, and the entropy increase rate is calculated during the spin‐up period of the model. It is found that, in a steady‐state, the entropy increase rate of the ocean system is zero, whereas that of the surroundings shows positive values, for both heat and salt transports. The zero entropy increase rate of the ocean system represents the fact that the system is in a steady‐state, while the positive entropy increase rate in the surroundings is caused by irreversible transports of heat and salt through the steady‐state circulation. The calculated entropy increase rate in the surroundings is 1.9×10¹¹ W K⁻¹, and is primarily due to the heat transport. It is suggested that the existence of a steady‐state dissipative system on the Earth, from a living system to the oceanic circulation, has a certain contribution to the entropy increase in its nonequilibrium surroundings.