Shinya Shimokawa

On the thermodynamics of the oceanic general circulation: Irreversible transition to a state with higher rate of entropy production

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

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

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 1.9×1011WK-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.

A new typhoon bogussing scheme to obtain the possible maximum typhoon and its application for assessment of impacts of the possible maximum storm surges in Ise and Tokyo Bays in Japan

We developed the new typhoon bogussing scheme to obtain the possible maximum typhoon approaching any region under any climatic conditions by using a potential vorticity inversion method. Numerical simulations with the new typhoon bogussing scheme are conducted for assessment of storm surges by possible maximum typhoons under the present-day and global warming climatic conditions in Ise and Tokyo Bays in Japan. The results suggest that the storm tide higher than the maximum storm tide in recorded history can occur in Ise and Tokyo Bays even for the present-day climate and the storm tide higher than the design sea level can cause severe damage to Nagoya and Tokyo megacities, in particular, airport facilities in Ise Bay for the global warming climate. These results suggest that the new typhoon bogussing scheme we developed is useful for assessment of impacts of storm surge by the possible maximum typhoons.

Relationship between coral distributions and physical variables in Amitori Bay, Iriomote Island, Japan

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. The main results of this study indicate that wave height and the number of soil particles have a significant correlation with coral distribution. Higher wave heights result in greater coverage of tabular coral and lower coverage of branching coral. A greater number of soil particles relates to lesser coverage of tabular coral. On the contrary, the number of soil particles is not correlated with branching coral coverage. The potential habitats for tabular corals largely depend on the species. Acropora hyacinthus is distributed in conditions with low numbers of soil particles, A. digitifera is distributed in conditions with low numbers of soil particles and high wave heights, and A. millepora is distributed in conditions with high numbers of soil particles and low wave heights. Averages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermediate part of the bay with the intermediate physical disturbances is higher than it, which seems to support the intermediate disturbance hypothesis.

A new index for evaluation of risk of complex disaster due to typhoons

This study examines a new index to evaluate complex disaster risk in coastal zones involving typhoons. Typhoons generate not only strong winds but also storm surges and high waves. Therefore, complex disasters attributable to typhoon forces can be expected to occur in coastal zones. The durations for which the wind speed, storm tide, and wave height simultaneously exceed their respective design values were calculated as simultaneous excess duration (SED) to evaluate the risk of a complex disaster. To verify the utility of SED, numerical simulations were conducted for intensified typhoons under both present-day and global warming climates in Ise Bay, Japan, using an atmosphere–ocean–wave coupled model with a typhoon bogussing scheme. Results showed that the middle part of Ise Bay is more dangerous from the standpoint of SED than the inner part of Ise Bay, which has been regarded as the most dangerous area from the standpoint of extreme values of storm tide. These results suggest that SED is important as an index of risk of complex disaster, and the risk of typhoon disaster should be evaluated not only from extreme values of storm tide but also from SED.

Chaotic Behaviors in the Response of a Quasigeostrophic Oceanic Double Gyre to Seasonal External Forcing

Abstract In an oceanic double-gyre system, nonlinear oscillations of the ocean under seasonally changing external forcing are investigated using a 1.5-layer quasigeostrophic model and a simple model related to energy balance of the oceanic double gyre. In the experiments, the variable parameter is the amplitude of external seasonal forcing and the Reynolds number is fixed as 39, at which periodic shedding of inertial subgyres occurs. The authors found that entrainment (at 2 times the period of the forcing) and intermittency (on–off type), phenomena that are often seen in nonlinear systems, emerge with increasing amplitude of the forcing. They seem to be related to the generation mechanism and characteristics of long-term (from interannual to decadal) variations in the strong current region of subtropical gyres such as the Kuroshio and its extension region.

The asymmetry of recirculation of a double gyre in a two layer ocean

Using an eddy-resolving two layer primitive-equation model forced by symmetric wind stress, we investigate the asymmetry of the recirculation of a double gyre (subpolar gyre and subtropical gyre). In the case where the layer thickness change is large, cyclonic recirculation becomes unstable and splits into meso-scale vortices more easily than anti-cyclonic recirculation in their developing stage. The subpolar gyre is, therefore, filled with more vortices than the subtropical gyre. Moreover, the effect of the upper layer potential vorticity on the lower layer potential vorticity in the subpolar gyre is stronger than that in the subtropical gyre. The characteristic of turbulence in the subpolar gyre is different from that found in the subtropical gyre and, therefore, the asymmetry of the recirculation of the double gyre is maintained by this difference. The asymmetry can not be produced in a quasi-geostrophic model because it ignores the nonlinearity associated with layer thickness change. Moreover, we investigate the effects of layer thickness and lateral viscosity on the asymmetry of the recirculation of the double gyre. In the case of realistic physical parameters, the asymmetry of the recirculation of the double gyre is noticeable from the view of the activities of the eddies. In the case with the shallowest upper layer, the position of separation of the western boundary current moves further southward.

The Time Evolution of Entropy Production in Nonlinear Dynamic Systems

General characteristics of entropy production in a fluid system are investigated from a thermodynamic viewpoint. A basic expression for entropy production due to irreversible transport of heat or momentum is formulated together with balance equations of energy and momentum in a fluid system. It is shown that entropy production always decreases with time when the system is of a pure diffusion type without advection of heat or momentum. The minimum entropy production (MinEP) property is thus intrinsic to a pure diffusion-type system. However, this MinEP property disappears when the system is subject to advection of heat or momentum due to dynamic motion. When the rate of advection exceeds the rate of diffusion of heat or momentum, entropy production tends to increase over time. The maximum entropy production (MaxEP), suggested as a selection principle for steady states of nonlinear non-equilibrium systems, can therefore be understood as a characteristic feature of systems with dynamic instability. The observed mean state of vertical convection of the atmosphere is consistent with the condition for MaxEP presented in this study.

STORM SURGE INUNDATION PREDICTION DUE TO HUGE TYPHOON AT HEAD OF ISE BAY

伊勢湾沿岸域は世界有数の工業集積地帯であり,スー パー中枢港湾の名古屋港,四日市港を有し,我が国の経 済における重要度は極めて高い.一方,同沿岸域は, 1959年の伊勢湾台風などで甚大な高潮・高波被害を被っ てきたため,伊勢湾台風級の台風による高潮・高波に耐 えられる堤防・防波堤等が整備されてきた.しかし,昨 今,地球温暖化による台風の強大化が懸念されており, 将来的に伊勢湾台風を超える巨大台風の襲来について考 慮しておく必要がある.その場合,襲来する可能性があ る台風の規模,および台風襲来によって起こりうる高潮 災害の規模を予測することが必須となる. 村上ら1)は,現在気候の下で大気・海洋力学的に最大 級となる50通りの台風を取り上げ,大気-海洋-波浪結 合モデルを用いて伊勢湾の高潮計算を行った.その結果, 伊勢湾台風時における名古屋港での潮位偏差 3.5 mを超 える5.5 mの高潮が現在気候において発生する可能性を 示した.また,村上ら2)は,地球温暖化がIPCCのA1Bシ ナリオどおりに今世紀末まで進んだ場合の伊勢湾全域で 予想される可能最大級の高潮・高波について,大気・海 洋力学的に基づき,数値解析を実施した.そして,既往 の伊勢湾台風による高潮・高波の実測値との比較検討を 行い,温暖化時における可能最大級高潮・高波の実態と その危険度について示した.このように,過去最大級の 伊勢湾台風を超える巨大台風によって発生する高潮・高 波の特性を把握することは,沿岸防災の観点から非常に 重要である.しかしながら,村上ら1), 2)の研究では,現 在/将来気候下での高潮による浸水計算を実施していな いため,伊勢湾の沿岸陸域に及ぼす浸水影響について議 論することができない. そこで,本研究では,室戸台風級の超巨大台風時,お よび村上ら1), 2)が検討した現在/将来気候から予想され る最大高潮条件を対象に伊勢湾湾奥部の高潮浸水計算を 行い,超巨大台風による浸水特性について検討すること を主たる目的とする.

Thermodynamics of Coral Diversity — Diversity Index of CoralDistributions in Amitori Bay, Iriomote Island, Japan and Intermediate Disturbance Hypothesis

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Amitori Bay is located in the northeast region of Irio‐ mote Island, Japan. Broad areas of coral have developed in the bay, and their life forms, coverages, sizes, and species vary depending on their locations. In addition, Amitori Bay has no access roads, and the bay perimeter is uninhabited. Thus, this small bay, with its variety of environments and lack of human impact, is considered to be one of the most suitable areas for studying the relationship between coral distribution and physical varia‐ bles. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. Our results showed that the life forms, sizes, and species of corals significantly varied de‐ pending on their locations in the bay, because the physical variables differed significantly among these locations. From the results of the above observations and simulations, we calculated diversity index of coral distributions and its relation to physical variables. The diversity index, DI is de‐ fined as DI=-Σ ci log2 ci, where ci is the ratio of i-th group coverage to total coverage. DI is a quantitative measure for the degree in which a dataset includes different types and is related closely to entropy concept in Thermodynamics. The value of DI increases when both the number of types and the evenness increase. For a given number of types, the val‐ ue of DI is maximized when all types are equally abundant. The results show that Aver‐ ages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermedi‐