Internal waves with high vertical wavenumber structure generated by diurnal tidal flow over the eastern ridge of Luzon Strait

• Published in: Journal of oceanography 2021-10, Vol.77 (5), p.703-718
• Main Authors: Sakai, Akie, Senjyu, Tomoharu, Matsuno, Takeshi, Tsutsumi, Eisuke, Endoh, Takahiro
• Format: Article
• Language: English
• Subjects: Computational fluid dynamics; Diurnal; Diurnal tides; Doppler sonar; Earth and Environmental Science; Earth Sciences; Flow profiles; Freshwater & Marine Ecology; Internal waves; Inversions; Oceanography; Special Section: Original Article; Structures; Submarine ridges; Temperature inversion; Temperature inversions; Tidal currents; Tidal flow; Turbulent mixing; Wave motion; Wave propagation; Wavelengths; Waves; Zonal flow (meteorology)
• ISSN: 0916-8370; 1573-868X
• DOI: 10.1007/s10872-021-00615-4

Internal waves with high vertical wavenumber structures on the northern part of the eastern ridge in Luzon Strait were investigated using shipboard observations and a wave propagation model. Detailed repeat observations across the ridge revealed complex undulations of isopycnals near the ridge that were characterized by vertically alternating concave and convex structures. In addition, strong eastward and westward flow areas with horizontal and vertical scales of 3–5 km and 150–250 m, respectively, occurred alternately on the downstream side of the ridge. These high-wavenumber structures in flow and density fields were associated with internal waves generated by interactions between diurnal tidal flow and the meridional submarine ridge in the strait; an acoustic Doppler current profiler recorded no signals of the Kuroshio Current and semi-diurnal tides in the zonal flow on the western slope of the eastern ridge. Many temperature inversions around the ridge exhibited vertical scales of predominantly ~ 30 m. Simple calculations of internal wave motion reveal that each internal wave mode tends to stagnate near the summits of the ridge depending on the direction and strength of the background tidal flow. This suggests that the observed high-wavenumber structure in the flow profiles near the ridge is ascribed to the superposition of transient internal wave modes, although this is not the main cause of the temperature inversions. As a possible mechanism to explain the temperature inversions, we propose that higher-mode internal waves emanating from the unresolved topographic undulations are trapped by the observed high-wavenumber structures, resulting in turbulent mixing.