Hisashi Ozawa’s research while affiliated with Hiroshima University and other places

In this study, we investigate a transition process among multiple steady states of oceanic circulation under the same set of boundary conditions, and clarify the relationship between entropy production and the strength and direction of fresh water perturbations. Our results are found to be consistent with “the principle of maximum entropy production (MEP)” in non-equilibrium thermodynamics, and can be understood in a consistent manner by a concept of “dynamic potential” that regards the rate of entropy production as a kind of thermodynamic potential. MEP could be a general thermodynamic principle that determines the behavior of oceanic circulation in response to external perturbations, leading to a better understanding of abrupt climate changes such as the Younger Dryas event and Dansgaard–Oeschger oscillations.

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.

On prediction of natural disaster, it is very important to predict rapid and discontinuous changes of non-linear phenomena (multiple regimes). As examples of multiple regimes related to natural disaster, there are various phenomena; appearance and disappearance of atmospheric blocking, transition among patterns of deep water circulation, transition between large meander and non-meander of Kuroshio path, and quasi-periodic collapse of snow avalanche. Recently, some new trial approaches to predict their behavior are conducted. In this paper, among their approaches, we introduce mainly a thermodynamic approach and its application to deep water circulation. It is a trial to understand the essence of phenomena by detecting macroscopic characteristics not depending on the details of system and is valid to predict various phenomena related to natural disaster involved in complex factors.

The long-term mean properties of the global climate system and those of turbulent fluid systems are reviewed from a thermodynamic viewpoint. Two general expressions are derived for a rate of entropy production due to thermal and viscous dissipation (turbulent dissipation) in a fluid system. It is shown with these expressions that maximum entropy production in the Earth's climate system suggested by Paltridge, as well as maximum transport properties of heat or momentum in a turbulent system suggested by Malkus and Busse, correspond to a state in which the rate of entropy production due to the turbulent dissipation is at a maximum. Entropy production due to absorption of solar radiation in the climate system is found to be irrelevant to the maximized properties associated with turbulence. The hypothesis of maximum entropy production also seems to be applicable to the planetary atmospheres of Mars and Titan and perhaps to mantle convection. Lorenz's conjecture on maximum generation of available potential energy is shown to be akin to this hypothesis with a few minor approximations. A possible mechanism by which turbulent fluid systems adjust themselves to the states of maximum entropy production is presented as a self-feedback mechanism for the generation of available potential energy. These results tend to support the hypothesis of maximum entropy production that underlies a wide variety of nonlinear fluid systems, including our planet as well as other planets and stars.

The mechanism of transitions among multiple steady states of thermohaline circulation is investigated from a thermodynamic viewpoint. An oceanic general-circulation model is used to obtain the multiple steady states under the same set of wind forcing and mixed boundary conditions, and the rate of entropy production is calculated during time integration. Three states with northern sinking and four states with southern sinking are shown to exist in the model by perturbing the high-latitude salinity. It is found that for transitions among southern sinkings the transition tends to occur from a state with a lower rate of entropy production to a state with a higher rate of entropy production, but the transition in the inverse direction does not occur. These transitions can thus be said to be irreversible or directional, in the direction of the increase of the rate of entropy production. For transitions between northern sinking and southern sinking, the rate of entropy production can either increase or decrease depending on the direction of the perturbation. The decrease is found to be associated with a collapse of the northern sinking circulation by a certain amount of negative salt (positive fresh water) perturbation to the northern hemisphere. After this collapse, a new southern sinking circulation develops, and the corresponding rate of entropy production increases. All these results tend to support the hypothesis that a nonlinear system is likely to move to a state with maximum entropy production by perturbation. Copyright © 2002 Royal Meteorological Society

Turbulence is ubiquitous in nature, yet remains an enigma in many respects. Here we investigate dissipative properties of turbulence so as to find out a statistical "law" of turbulence. Two general expressions are derived for a rate of entropy increase due to thermal and viscous dissipation (turbulent dissipation) in a fluid system. It is found with these equations that phenomenological properties of turbulence such as Malkus's suggestion on maximum heat transport in thermal convection as well as Busse's sug- gestion on maximum momentum transport in shear turbulence can rigorously be ex- plained by a unique state in which the rate of entropy increase due to the turbulent dissipation is at a maximum (dS/dt = Max.). It is also shown that the same state cor- responds to the maximum entropy climate suggested by Paltridge. The tendency to increase the rate of entropy increase has also been confirmed by our recent GCM ex- periments. These results suggest the existence of a universal law that manifests itself in the long-term statistics of turbulent fluid systems from laboratory-scale turbulence to planetary-scale circulations. Ref.) Ozawa, H., Shimokawa, S., and Sakuma, H., Phys. Rev. E 64, 026303, 2001.

Dissipative properties of various kinds of turbulent phenomena are investigated. Two expressions are derived for the rate of entropy increase due to thermal and viscous dissipation by turbulence, and for the rate of entropy increase in the surrounding system; both rates must be equal when the fluid system is in a steady state. Possibility is shown with these expressions that the steady-state properties of several different types of turbulent phenomena (Bénard-type thermal convection, turbulent shear flow, and the general circulation of the atmosphere and ocean) exhibit a unique state in which the rate of entropy increase in the surrounding system by the turbulent dissipation is at a maximum. The result suggests that the turbulent fluid system tends to be in a steady state with a distribution of eddies that produce the maximum rate of entropy increase in the nonequilibrium surroundings.

The dissipative properties of turbulence, including those of the global climate system, have been investigated from a thermodynamical viewpoint. It is found that the steady-state properties of the present climate [1], transport properties of turbulence [2], and variations of a regime of thermal convection [3,4] can be explained to a certain extent by the same condition that the rate of entropy increase by the turbulent dissipation (thermal and viscous dissipation) is at a maximum (dS/dt = Max.). The result suggests the existence of a universal law that seems to govern the evolution of the climate system of the earth, as well as that of other planets and stars. Previous works on thermodynamics of the climate system have been reviewed. Paltridge's work on the maximum entropy production [5] and Essex's work on radiation entropy [6] are found to be consistent with the above mentioned hypothesis (dS/dt = Max.) so far as the rate of entropy increase by the turbulent dissipation is concerned to be a maximum, regardless of the entropy increase by the absorption of solar radiation. [1] H. Ozawa and A. Ohmura, J. Climate 10, 441 (1997). [2] H. Ozawa et al., Phys. Rev. (submitted). [3] S. Minobe et al., Nagare Multimedia, Nagare 16 (2000). [4] S. Shimokawa and H. Ozawa, Tellus A 53, 266 (2001). [5] G. W. Paltridge, Quart. J. Roy. Meteor. Soc. 101, 475 (1975); 104, 927 (1978). [6] C. Essex, J. Atmos. Sci. 41, 1985 (1984).

The role of thermodynamics in the oceanic general circulation is investigated. The ocean isregarded as an open dissipative system that exchanges heat and salt with the surroundingsystem. A new quantitative method is presented to express the rate of entropy increase for alarge-scale open system and its surroundings by the transports of heat and matter. This methodis based on Clausius’s definition of thermodynamic entropy, and is independent of explicitexpressions of small-scale dissipation processes. This method is applied to an oceanic generalcirculation model, and the entropy increase rate is calculated during the spin-up period of themodel. 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. Thezero entropy increase rate of the ocean system represents the fact that the system is in a steadystate,while the positive entropy increase rate in the surroundings is caused by irreversibletransports of heat and salt through the steady-state circulation. The calculated entropy increaserate in the surroundings is 1.9× 10 11 WK −1 , and is primarily due to the heat transport. It issuggested that the existence of a steady-state dissipative system on the Earth, from a livingsystem to the oceanic circulation, has a certain contribution to the entropy increase in itsnonequilibrium surroundings. DOI: 10.1034/j.1600-0870.2001.00122.x

Vertical heat transport through thermal convection of the earth's atmosphere is investigated from a thermodynamic viewpoint. The postulate for convection considered here is that the global-mean state of the atmosphere is stabilized at a state of maximum entropy increase in a whole system through convective transport of sensible and latent heat from the earth's surface into outer space. Results of an investigation using a simple vertical gray atmosphere show the existence of a unique set of vertical distributions of air temperature and of convective and radiative heat fluxes that represents a state of maximum entropy increase and that resembles the present earth. It is suggested that the global-mean state of the atmospheric convection of the earth, and that of other planets, is stabilized so as to increase entropy in the universe at a possible maximum rate.