Ocean Modeling

The ocean, which covers 70% of the earth's surface, absorbs solar energy and releases heat and vapor into the atmosphere and is a source of dynamical energy in the atmosphere. Greenhouse gases such as carbon dioxide are also dissolved in the ocean, and concentrations of such gases are determined by exchange between the atmosphere and the ocean. The heat and dissolved gases bring about important interactions with the living organisms in the ocean. Understanding of this massive ocean is indispensable for understanding the climate system. This is the goal of the Ocean Modeling Group.

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Research subjects

• Interaction between mesoscale eddies and the general circulation

• The role of the Antarctic Circumpolar Ocean in the general circulation

• Formation processes of deep water and bottom water

• Parameterization of mixing

• The carbon cycle in the ocean

Present Status of Understanding of the General Circulation and Prospects

The general circulation can be divided into two parts in terms of the forcing agents, the wind-driven circulation brought about by surface wind, and the thermohaline circulation caused by regional differences of heat and water budget through the sea surface. For convenience, we call the thermocline where the water temperature suddenly drops, the intermediate layer; above it, the surface layer; and below it, the deep layer. It is believed that in the surface layer, the wind-driven circulation is predominant; and in the deep layer, the thermohaline circulation dominates, and in the intermediate layer, these circulations compete. Compared to the surface layer, the circulation in the intermediate and deep layers is unknown due to the difficulties in observation. For example, in the deep water, the average speed of flow is less than 1cm/sec, too small to be directly measured, and mesoscale eddies with diameters of 100~400km exist everywhere in the oceans from the surface to the bottom.

Figure 1 shows schematically the general circulation deduced from the observed distributions of tracers such as salinity, dissolved oxygen, freon and various isotopes, and through theoretical study of the wind-driven circulation and thermohaline circulation. As shown in the figure, the ocean is divided into four layers, each of which has a different circulation. Our aim is to obtain an accurate picture of the general circulation and to construct a model of it. In order to verify the model, we are now undertaking comparative studies of data acquired from TOGA and WOCE. Obtaining a better picture of ocean circulation through such comparisons is one of the most important goals of the group (Fig. 2).

General Circulation Model

กก The mathematical equation describing the general circulation is simple compared to that of the atmosphere since it doesn't include cloud or radiation processes. The same can be said for sea ice, whose thermodynamic process is comparatively simple. On the other hand, phenomena at various time and space scales co-exist in the ocean and interaction among them is very strong, and the most important issue is how to parameterize phenomena smaller than the grid resolution. Since the geographical features of coasts and ocean floors are so complex and there exists stratification closely related to the circulation, it is necessary to utilize a variety of means in solving these equations. Furthermore, in order to study global climate variability, we must treat together all the oceans connected by Antarctic Circumpolar Ocean. In such a large-scale system, various time-scale phenomena occur in the complex manner described above, and enormous computer time is necessary. The horizontal grid resolution may only reach one degree by the end of this century due to present computing ability . We must therefore continue basic studies of parameterization of phenomena which cannot be described by such grid spacing. Parameterizing small-scale mixing processes of various origins is necessary, and understanding of the interaction between mesoscale eddies and the general circulation is indispensable.