A note on some aspects of surface currents and tides in relation to the measurement with ocean radar

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  • Masuda Akira
    Research Institute for Applied Mechanics, Kyushu University

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  • 表層流・潮汐に関する覚書 : 海洋レーダー観測に関連して
  • ヒョウソウリュウ チョウセキ ニ カンスル オボエガキ カイヨウ レーダー カンソク ニ カンレンシテ

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Abstract

A concise and mathematical description is given of several aspects of surface currents and tides that are encountered in the measurement and analysis of surface currents with ocean HF radar. The subjects in concern are not only for practical data analysis but also for theoretical investigation: the representation of tidal ellipses in terms of complex harmonic constants, the veering of tidal ellipses with depth, Ekman currents in response to stochastic wind stress, and nearby fields around amphidromic or anti-amphidromic points. Complex representation turns out quite useful in discussing those subjects. It is shown that, even when the vertical eddy viscosity is uniform, the unsteadiness of wind stress (and shallow depth of the sea) lowers the angle between wind stress and surface current below 45 degrees on an average with a weight proportional to the power of wind stress. The vertical veering of tidal ellipse is argued to occur when the vector of the surface tidal current rotates in the same direction as the inertial oscillation, in certain circumstances. Under the same conditions, analytical expression is obtained for the depth dependence of the major axis of the tidal ellipse as well. The co-range and co-tidal lines around an amphidromic point (the point of vanishing tidal amplitude) are shown to be robust. On the other hand, the co-tidal lines around the anti-amphidromic point do not necessarily assume such typical hyperbolic curves, if the anti-amphidromic point is defined merely as a saddle point with respect to the amplitude of the component tide.

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