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Three-Dimensional Structure of Thermohaline Circulation Steered by Bottom Topography

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  • 1 Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, Japan
  • | 2 Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, Japan
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Abstract

An investigation based on the expansion in terms of diffusive vertical modes is made on the effect of an isolated seamount on the three-dimensional structure of thermohaline circulation driven by an inflow of deep water. While the circulation has a relatively simple structure dominated by the first baroclinic mode in a flat ocean, it becomes rather complicated in an ocean with a seamount. Nevertheless its three-dimensional structure is reconstructed and understood well by analyzing the dynamics of the vertical modes. It is shown that the vertical modes respond to the topography-induced stretching in a similar way irrespective of the mode number. When synthesized in the deep layer, therefore, the vertical modes interfere constructively to yield the same pattern of circulation as a single vertical mode or the diffusive reduced-gravity model. It is thus confirmed that the abyssal circulation certainly has the features predicted by the diffusive reduced-gravity model such as cyclonic circulation over the mountain or downwelling in the geostrophic island, in which geostrophic contours close on themselves. Close to the surface, however, different modes of different vertical structures interfere destructively so that the horizontal distribution of topography-induced circulation in the deep layer decays, upward shifting its center westward.

* Current affiliation: Division of Ocean and Atmospheric Sciences, Graduate School of Environmental Earth Science, Hokkaido University, Hokkaido, Japan.

Corresponding author address: Dr. Genta Mizuta, Division of Ocean and Atmospheric Sciences, Graduate School of Environmental Earth Science, Hokkaido University, Kita-10, Nishi-5, Kita-ku, Sapporo-si, Hokkaido 060, Japan.

Email: mizuta@ees.hokudai.ac.jp

Abstract

An investigation based on the expansion in terms of diffusive vertical modes is made on the effect of an isolated seamount on the three-dimensional structure of thermohaline circulation driven by an inflow of deep water. While the circulation has a relatively simple structure dominated by the first baroclinic mode in a flat ocean, it becomes rather complicated in an ocean with a seamount. Nevertheless its three-dimensional structure is reconstructed and understood well by analyzing the dynamics of the vertical modes. It is shown that the vertical modes respond to the topography-induced stretching in a similar way irrespective of the mode number. When synthesized in the deep layer, therefore, the vertical modes interfere constructively to yield the same pattern of circulation as a single vertical mode or the diffusive reduced-gravity model. It is thus confirmed that the abyssal circulation certainly has the features predicted by the diffusive reduced-gravity model such as cyclonic circulation over the mountain or downwelling in the geostrophic island, in which geostrophic contours close on themselves. Close to the surface, however, different modes of different vertical structures interfere destructively so that the horizontal distribution of topography-induced circulation in the deep layer decays, upward shifting its center westward.

* Current affiliation: Division of Ocean and Atmospheric Sciences, Graduate School of Environmental Earth Science, Hokkaido University, Hokkaido, Japan.

Corresponding author address: Dr. Genta Mizuta, Division of Ocean and Atmospheric Sciences, Graduate School of Environmental Earth Science, Hokkaido University, Kita-10, Nishi-5, Kita-ku, Sapporo-si, Hokkaido 060, Japan.

Email: mizuta@ees.hokudai.ac.jp

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