27 June 2004
Japan Agency for Marine-Earth Science and Technology
Successful Simulation of the Pathways of Antarctic Bottom Water
using the Earth Simulator
Yoshikazu Sasai, Akio Ishida, and Yasuhiro Yamanaka of Ecosystem Change Research Program of Frontier Research Center for Global Change, and Hideharu Sasaki of Atmosphere and Ocean Simulation Research Group of the Earth Simulator Center, projects run by the Japan Agency for Marine-Earth Science and Technology (an independent administrative institution) conducted a numerical experiment to simulate chlorofluorocarbon (CFC) using OFES (note 1), the super-high resolution ocean general circulation model for the Earth Simulator, and successfully reproduced the pathways of Antarctic Bottom Water (AABW).
This research was published in volume 31 of the Geophysical Research Letters by the American Geophysical Union.
Oceanic uptake of anthropogenic carbon dioxide (CO2) is one of the important processes when considering global warming issues. The Southern Ocean is thought to be a key area for the exchange of CO2 with the atmosphere because newly formed AABW (note 2) effectively carries CO2 into the deep layer. It is therefore important to study the pathways of the deep water by reproducing the global ocean circulation using a numerical model. As it is difficult to observe velocities in deep oceans, the distributions of the observed concentrations of the dissolved gasses such as CFCs (note 3) are used to investigate the pathways and spreading of the deep and bottom waters and to validate the results of numerical models. Though many numerical studies have been conducted to simulate the CFC distributions, there have seen quite a large variations among the model results and there have been no satisfying simulation of observational results so far. It is cited as one of the causes of the deficiencies that the models used in the past studies have coarse grid spacing and do not have a capability to reproduce the fine scale structures of the currents. Thus, there has been a demand for the simulation using a model with small grid interval.
A numerical experiment has been carried out to simulate the distribution of CFC and to investigate the pathways of the deep water formed in the Weddell Sea by using a super-high resolution OGCM called OFES (Ocean General Circulation Model for the Earth Simulator) which has the horizontal resolution of about 10 km. The model results well matched with the observation (Fig. 1). The model reveals two pathways by which deep water spreads from the Weddell Sea to the AJAX observation section at the Greenwich Meridian: an upper pathway through the Scotia Sea, and a lower pathway south along the South Scotia Ridge (Fig. 2). This successful simulation is owing to that the high resolution model can represent the fine structures of the currents by resolving the complicated geographical distribution of the sea bottom such as ridge and trough, which affect the pathways of the deep and bottom waters.(Ref.1)
This research is expected to make contribution in the study of the ocean general circulation, and further for studies on climate change including global warming, affected by the variation in the ocean general circulation.
Japan Agency for Marine-Earth Science and Technology
Frontier Research Center for Global Change
Contact: Ms. Ota
Tel: +81 (0)45-778-5687 (direct)
Earth Simulator Administrative Office
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Public Relations Office, General Affairs Department
OFES: It stands for Ocean General Circulation Model for the Earth Simulator, the ocean model developed to study ocean currents and eddies as well as ocean circulation at global scale. OFES was jointly developed by the Earth Simulator Center, and the Frontier Research System for Global Change (the present Frontier Research Center for Global Change) based on the Modular Ocean Model (MOM 3), the ocean circulation model program developed by Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration in the United States. In the ocean, typical size of eddies is horizontal 100 km order, which plays significant role on momentum and heat transportation. In order to reproduce eddies, the horizontal resolution of OFES is as small as 10 km. Meanwhile, in order to simulate the entire ocean, the whole globe except the two poles was set to be the calculation area. In the ocean simulation by OFES, various currents such as the Kuroshio south of Japan as well as fairly small eddies of 100 km order are all well reproduced.
Antarctic Bottom Water (AABW): The densest water in the world oceans which has temperature of under zero degree Celsius, formed mainly in the Weddle Sea. The circulation of this water is vitally important because it transport large amount of heat and materials. Along with the North Atlantic Deep Water (NADW) formed in the northern North Atlantic, AABW is considered to be one of the two major sinks of carbon dioxide.
chlorofluorocarbon (CFC): Chlorofluorocarbon has received a great deal of attention, and notoriety, as a result of being implicated in the destruction of the ozone layer and global warming. On the other hand, CFC is known as an ideal tracer for the study of material transportation in the ocean because it is stable and biologically inert. CFC is emitted in the atmosphere, small amount of which will be absorbed in the ocean. Study of the CFC in the ocean is one of the very important research themes to know the absorption and transportation of anthropogenic carbon dioxide into the ocean.