Spatio-Temporal Variation of the M-2 Tidal Current from Field Observations in the Jeju Strait

Abstract

The variability in the tidal current in the Jeju Strait was investigated based on sea surface current fields observed by a high-frequency (HF) radar and vertical current profiles observed by three bottom-moored acoustic Doppler current profilers (ADCPs). The monthly M-2 tidal current, the most dominant constituent in the study area, had significant temporal variability, and there were spatial changes in the temporal variation trend. While the semi-major axis of the M-2 tidal current in winter was stronger than that in summer in some parts of the HF radar observation range, the summertime semi-major axis was stronger than that in wintertime in other parts of the observation range. Both opposite trend areas coexisted within the HF radar coverage area of several tens of kilometers. The vertical structure of the tidal currents in winter was similar to the theoretical structure in which Ekman dynamics was applied under depth-independent vertical eddy viscosity, and the year-to-year change was weak. In contrast, the vertical structure of the tidal currents in summer significantly varies over depth and years. Furthermore, the seasonal variation patterns at the three ADCP stations differed spatially. The temporal and spatial variability of tidal currents could be explained as a result of the strengthening stratification and, consequently, semidiurnal internal tides in summer. In summer, an internal tide with a wavelength of tens of kilometers could be generated in the vicinity of and propagate into the study area. The overlapping and offsetting of the barotropic and internal tidal components within the HF radar observation range caused complex spatial variations.