Kenji Marubayashi

A surface velocity spiral observed with ADCP and HF radar in the Tsushima Strait

[1] The structure of a wind-driven flow in the Tsushima Strait is investigated with moored acoustic Doppler current profiler (ADCP) and HF radar. Two ADCPs of high and low acoustic frequencies are simultaneously used to measure velocities in both the surface boundary layer and the interior with high resolutions. The velocity relative to an interior flow in the surface boundary layer is estimated by subtracting the reference velocity (estimated from velocities at greater depths) from a velocity in the surface layer, and complex principal component analysis (PCA) of the lagged wind stress and the relative velocity is performed. Despite a short (2 weeks) observation period of relatively calm and variable wind, a clockwise velocity spiral similar to a theoretical Ekman spiral is detected as the first mode of PCA. Ekman transport estimated from the relative velocities of the first mode agrees best with Ekman transport expected from wind stress of the first mode with 11–13 hours time lag, for which the explained variance of the first mode is also largest. This indicates that a wind-driven flow is balanced with wind stress after 11–13 hours, half of the inertial period at this latitude. Eddy viscosity is also inferred from wind stress and the relative velocities of the first mode. It is found to increase from O(10 � 3 )m 2 s � 1 at greater depth to O(10 � 2 )m 2 s � 1 near the sea surface.

A laboratory experiment and numerical simulation of an isolated barotropic eddy in a basin with topographic β

An initial localized eddy was generated in a rotating tank by a source–sink method to study the behaviour of an isolated barotropic eddy on a β-plane. The evolution of the eddy was compared with the laboratory experiments by Firing & Beardsley (1976) and by Takematsu & Kita (1985, 1988), confirming the northwestward (southwestward) translation of a cyclonic (anticyclonic) isolated eddy due to nonlinear effects. Anticyclonic eddies were contrasted with cyclonic eddies in the tank experiment, showing a cyclonic–anticyclonic asymmetry due to the topographic β as a substitute for the planetary β. The fluid experiment was simulated well by numerical simulation based on the quasi-geostrophic vorticity equation. Numerical experiments verified the northwestward (southwestward) translation both for an initially Gaussian and initially Rankine-type isolated cyclonic (anticyclonic) eddy.

Bachelor-modon type eddies and isolated eddies near the coast on an f-plane

The behavior of isolated meso-scale eddies near the coastal boundary is studied by numerical experiments based on the quasi-geostrophic equation in a basin on an f-plane. First, Bachelor-modon type eddies are investigated as an idealized model of isolated eddies close to the wall. The first-mode Bachelor-modon type eddy is found to be robust enough to recover its original form even after it turns a corner of the basin. In contrast, the second mode is unstable; it tends to move away from the wall and finally splits into two eddies proceeding in opposite directions along the wall. An initially Gaussian eddy a little distant from the boundary interacts with a Bachelor-modon type eddy translating along the boundary, sometimes resulting in vortex merging and pairing just as in the head-on collision of two modons on a beta-plane. It is found that an initially Gaussian eddy located moderately close to the coast rapidly settles down to a steadily translating eddy, which can be approximated remarkably well by a first-mode Bachelor-modon type eddy not only in appearance but also in translation velocity within an error of about 20%.

Statistics of Wind and Waves off Tsuyazaki, Fukuoka, in the Eastern Tsushima Strait

The variability of the sea surface wind and wind waves in the coastal area of the Eastern Tsushima Strait was investigated based on the hourly data from 1990 to 1997 obtained at a station 2 km off Tsuyazaki, Fukuoka. The annual mean wind speed was 4.84 m s−1, with strong northwesterly monsoon in winter and weak southwesterly wind in summer. Significant wave heights and wave periods showed similar sinusoidal seasonal cycles around their annual means of 0.608 m and 4.77 s, respectively. The seasonal variability relative to the annual mean is maximum for wave heights, medium for wind speeds, and minimum for wave periods. Significant wave heights off Tsuyazaki turned out to be bounded by a criterion, which is proportional to the square of the significant wave period corresponding to a constant steepness, irrespective of the season or the wind speed. For terms shorter than a month, the significant wave height and the wave period were found to have the same spectral form as the inshore wind velocity: white for frequencies less than 0.2 day−1 and proportional to the frequency to the −5/3 power for higher frequencies, where the latter corresponds to the inertial subrange of turbulence. The spectral levels of wave heights and wave periods in that inertial range were also correlated with those of the inshore wind velocity, though the scatter was large.