Dynamics of Cross-Isobath Dense Water Transport Induced by Slope Topography

Yoshimasa Matsumura Atmosphere and Ocean Research Institute, University of Tokyo, Chiba, Japan

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Hiroyasu Hasumi Atmosphere and Ocean Research Institute, University of Tokyo, Chiba, Japan

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Abstract

Dynamics of cross-isobath downslope transport of a dense water mass induced by small-scale topographic variation is investigated based on a high-resolution numerical experiment with realistic settings, a simplified analytical model for water particle advection, and idealized sensitivity experiments. The existence of a submarine ridge induces two different processes for cross-isobath downslope transport of dense water: a strong but narrow and thin downslope current at the east side of the ridge and cyclonic eddies with dense water cores to the west of the ridge. The former downslope current is produced in response to the rapid increase of slope angle near the ridge. The latter eddies are formed by stretching of the dense water layer near the crest, where isobath curvature is so high that offshore centrifugal force overcomes the coastward Coriolis force. From a simple analysis on the equation of motion for a fluid particle placed on a slope with curved isobaths, a general criterion that describes whether a density current follows or crosses isobaths is derived, which is supported by idealized sensitivity experiments. The location where cross-isobath transport of dense water takes place is determined by relative magnitude between spatial derivatives of isobath curvature, planetary vorticity, and slope angle. Based on these arguments, a parameterization is proposed to represent the effect of unresolved small-scale topography in coarse-resolution models.

Current affiliation: Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.

Corresponding author address: Yoshimasa Matsumura, Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan. E-mail: ymatsu@lowtem.hokudai.ac.jp

Abstract

Dynamics of cross-isobath downslope transport of a dense water mass induced by small-scale topographic variation is investigated based on a high-resolution numerical experiment with realistic settings, a simplified analytical model for water particle advection, and idealized sensitivity experiments. The existence of a submarine ridge induces two different processes for cross-isobath downslope transport of dense water: a strong but narrow and thin downslope current at the east side of the ridge and cyclonic eddies with dense water cores to the west of the ridge. The former downslope current is produced in response to the rapid increase of slope angle near the ridge. The latter eddies are formed by stretching of the dense water layer near the crest, where isobath curvature is so high that offshore centrifugal force overcomes the coastward Coriolis force. From a simple analysis on the equation of motion for a fluid particle placed on a slope with curved isobaths, a general criterion that describes whether a density current follows or crosses isobaths is derived, which is supported by idealized sensitivity experiments. The location where cross-isobath transport of dense water takes place is determined by relative magnitude between spatial derivatives of isobath curvature, planetary vorticity, and slope angle. Based on these arguments, a parameterization is proposed to represent the effect of unresolved small-scale topography in coarse-resolution models.

Current affiliation: Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.

Corresponding author address: Yoshimasa Matsumura, Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan. E-mail: ymatsu@lowtem.hokudai.ac.jp
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