Direct evidence of standing internal tide west of the Luzon Strait observed by a large-scale observation array

Using a large-scale observation array of 27 simultaneous pressure-recording inverted echo sounders (PIESs), the standing wave features of the mode-1 M2 internal tide west of the Luzon Strait (LS) were identified. These features exhibited non-monotonic spatial phase shifts and half-wavelength amplitude modulation, resulting in spatially varying amplitudes under PIES observations, which have not been previously observed in field observations west of the LS. Satellite altimeter measurements also identified standing wave patterns consistent with the PIES observations. These patterns emanated from interference between the northwestward and southeastward beams from the LS and the slope of the southern Taiwan Strait, respectively. Near the LS, the two beams superimposed into partial standing waves, whereas the superimposed waves tended to become perfect standing waves near the slope of the southern Taiwan Strait. The nodes and antinodes of the wave shifted under the influence of an anticyclonic eddy. The eddy-induced background current modified the phase speed of the internal tides, and the superimposed standing wave nodes and antinodes deflected clockwise. The node shifted during three anticyclonic eddy events, and two stations on two sides of the wave node showed opposite variations in amplitude.