Semidiurnal internal tides southeast of Miyakojima observed by the PIES array

Abstract

The Ryukyu Island chain is one of the major generation regions for internal tides in the western North Pacific, notably generating strong semidiurnal internal tides. However, due to limited in situ observational data, the characteristics of internal tides east of Ryukyu Island chain remain unclear. In this study, we used pressure-sensor-equipped inverted echo sounders (PIES) deployed southeast of Miyakojima to assess the spatial structure and temporal variability of the internal tides from June 2015 to June 2017. Observations indicated stronger semidiurnal internal tides with larger fluctuations onshore than offshore. Both PIES observations and the satellite-derived product indicated that the M₂ internal tides propagated from the onshore to the offshore side; however, the amplitudes were substantially underestimated in the satellite product. The first mode of the Complex Empirical Orthogonal Function analysis revealed that the temporal amplitude exhibited clear seasonal variability, with larger amplitudes in spring (mean, 26.4 m) and autumn (mean, 22.2 m), and smaller amplitudes in summer (mean, 18.3 m) and winter (mean, 21.3 m). The pycnocline depth derived from the PIES observations also exhibited distinct seasonal variability, showing a strong correlation with sea level anomaly. These findings demonstrate that the temporal variability of the semidiurnal internal tides is likely attributable to mesoscale eddies, which modulate the pycnocline depth. Anticyclonic eddies deepen the pycnocline and suppress the amplitude of the semidiurnal internal tides. In nearly 70 % of the cases affected by mesoscale eddies during the observation period, decreased (increased) semidiurnal internal tide amplitudes occurred concurrently with a deepened (shoaled) pycnocline.