10 Thermodynamics of the Ocean Circulation: A Global Perspective on the Ocean System and Living Systems

Abstract

In this chapter, we investigate thermodynamics in a global-scale open ocean circulation and discuss the physical properties of “living systems”, that is, individual organisms, by analogy to the behavior of the ocean system. Despite the fact that the ocean system has long been examined from a dynamic point of view, its thermodynamic aspects remain to be explored. We show a quantitative method that expresses the rate of entropy production in an open dissipative system that exchanges heat and matter with its surrounding system. This method is applied to an ocean circulation model, and the rate of entropy production is examined in relation to the dynamic behavior of the system. Multiple steady states can exist under the same set of boundary conditions, and the state can be shifted by applying perturbations at the surface boundary. The perturbations tend to shift the system to a state of higher entropy production, except when a perturbation destroys the initial circulation completely. This result supports the hypothesis that a nonlinear dynamic system tends to move to a state with higher entropy production by producing an active circulation in the system when triggered by perturbations. When such a system is subject to random perturbations for a certain period of time, the most probable state to result will be the one with the maximum entropy production. The entropy produced in a steady-state dissipative system is discharged into the surrounding system through boundary fluxes of heat and matter, thereby contributing to the entropy in the surrounding system. Finally, an analogy is suggested between the ocean system and a living system, in which a highly organized circulatory structure of fluids has evolved from a less organized primeval one, thereby producing entropy in the surrounding system at an increased rate.