The eddy-induced mass transport along the mean current by the fluctuation of the stream axis

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  • Masuda Akira
    Research Institute for Applied Mechanics, Kyushu University

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  • 流軸の揺れで生じる流軸に平行な向きの渦成海水輸送
  • リュウジク ノ ユレ デ ショウジル リュウジク ニ ヘイコウナ ムキ ノ ウズセイ カイスイ ユソウ

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Abstract

Numerical simulation with an eddy-permitting OGCM (Ocean General Circulation Model) revealed that western boundary currents in the North Pacific are accompanied by eddy-induced mass transport along the mean stream axis which is much larger than that across it, where "eddy-induced mass (thickness) transport" means the mass flux by fluctuating velocity along with the fluctuating thickness of a layer of the same density. The eddy-induced mass transport is directed downstream and upstream in the left-hand and right-hand sides of the mean stream axis, respectively. The property holds not only for the subtropical gyre (Kurohsio), but also for the subarctic gyre (Oyashio and Alaska Current). A simple mechanism is proposed for this eddy-induced mass transport along the mean current. The model is based on the fluctuation of the stream axis with its cross-stream structure invariant. The model qualitatively explains well the downstream and upstream mass transport in the left-hand and right-hand sides of the current. The magnitude and the cross-stream profile of the along-current mass transport depend on factors such as the mean distribution of the current, the characteristic amplitude of the migration of the current axis. Analytical formulas are obtained for the distribution of the eddy-induced mass transport for suitable assumptions of the Gaussian fluctuation of the stream axis and the specific profile of the thickness associated with the current. Roughly speaking, the eddy-induced mass transport has a cross-stream profile like the derivative of the probability density distribution of the Gaussian distribution. Also it is to be noted that the instantaneous current profile is recovered from the mean current profile in the model of the fluctuating stream axis.

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