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  • Author or Editor: Yasuhiro Yamanaka x
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Akio Ishida
,
Yoshikazu Sasai
, and
Yasuhiro Yamanaka

Abstract

In this study the global distribution and transport of chlorofluorocarbons (CFCs) in the oceans are investigated using a high-resolution numerical model. The authors focus on the effects of wind-driven processes such as subduction and ventilation on the distribution of CFC-11 within surface and thermocline layers. Local maxima of tracer inventory are identified in oceanic regions. Two major absorption regions in the South Pacific Ocean are located west of South America and northeast of New Zealand in boundary latitudes between subtropical and subantarctic areas. In the North Pacific and south Indian Oceans, the positions of the local maxima of tracers migrate west–east with increasing density. The relationship between water formation and tracer distribution is discussed by comparing the wintertime mixed layer depth and mean circulation. The contributions of mean and eddy transport of the meridional overturning cells to tracer transport are analyzed. Stationary eddies have a large impact in the Southern Ocean, while transient eddies are crucial for the compensation of mean transport in the equatorial oceans. Mean and seasonal variability components of meridional tracer transport are dominant in the subtropics; however, eddy transport also plays a significant role.

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Yasuhiro Yamanaka
,
Ryo Furue
,
Hiroyasu Hasumi
, and
Nobuo Suginohara

Abstract

The authors compare two classical advection schemes, the centered difference and weighted upcurrent, for coarse-resolution OGCMs, using an idealized ocean basin and a realistic World Ocean topography. For the idealized basin, three experiments are run, one with 12 vertical levels and the centered difference scheme, one with 12 levels and the weighted upcurrent scheme, and the other with 800 levels and the centered scheme. The last experiment perfectly satisfies the grid Péclet number stability criterion and is regarded as the “true solution.” Comparison of the coarse vertical resolution experiments with the true solution indicates 1) that with the centered scheme, when strong vertical motion crosses a strong stratification, false density values are created in the coarse resolution model and this leads to false convective adjustment, which transports those false density values downward; and 2) that because of computational diffusion, the weighted upcurrent scheme leads to a less dense deep water with a stronger stratification than those of the true solution. These characteristics also apply even to the World Ocean model with relatively small grid Péclet numbers (moderately high vertical resolution and relatively large vertical diffusivity): the centered scheme leads to artificial convective adjustment near the surface in the equatorial Pacific, creating an artificial circulation, and the weighted upcurrent scheme leads to a warmer deep water and more diffuse thermocline. Deep equatorial “stacked jets” are found in all idealized-basin experiments, in particular, in the super-high vertical resolution case. Horizontal diffusion is found to dominate the density balance at the bottom jet in the super-high-resolution model, as previously found in an OGCM with a moderately high vertical resolution. This is consistent with the hypothesis that the jets exist owing to diapycnal mixing.

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