Semidiurnal internal tide observed over Ohmuro-Dashi, the northern part of the Izu Ridge

Repeated measurements by acoustic Doppler current profiler and expendable bathythermograph were carried out across Ohmuro-Dashi, a rise at the northern part of the Izu Ridge, to investigate the generation and propagation of internal tides in October 1993. The barotropic current with semidiurnal period was perpendicular to the bottom contours on the ridge, indicating an effective generation of semidiurnal internal tide. The predominance of semidiurnal internal tide was confirmed from analysis of the temperature and current distributions. The vertical displacement of thermocline due to semidiurnal internal tide reached 35 m at the western side of the rise. The vertical shear of the horizontal current had a beam-like structure and was the strongest at the western side of the rise. From ray-trace of semidiurnal internal tide, the structure of vertical shear at the western side of Ohmuro-Dashi was found to be due to superposing of the rays emanating from two peaks of the ridge and the near critical slope. The distribution of gradient Richardson number calculated from the vertical shears for M2 constituent and temperature inversions observed in the thermocline indicated that the vertical mixing was possibly occurring over Ohmuro-Dashi and was related to the internal tides.