Tidal asymmetry and transition in the Singapore Strait revealed by GNSS interferometric reflectometry

The Singapore Strait is located at the transition between the dominantly semidiurnal Indian Ocean and the mixed-to-diurnal South China Sea, resulting in complex tidal dynamics. In this work, we use sea-level estimates from two coastal Global Navigation Satellite Systems (GNSS) stations and one tide gauge to study tides and tidal asymmetry in the Strait. We first generate sea-level measurements from GNSS signal-to-noise ratio (SNR) data using the GNSS Interferometric Reflectometry technique, which can estimate sea-surface heights from a coastal GNSS station. Second, we perform tidal harmonic analysis and quantify tidal asymmetry based on the skewness method. Finally, we examine seasonal sea-level changes in the Strait from GNSS SNR data, tide-gauge records and satellite altimetry. Our results reveal an increase in M2 and S2 amplitudes toward the west of the Strait and a decrease in the K1 and O1 amplitudes. Our results also show that tides at the two sites in the east are ebb dominant with asymmetry originating from the O1–K1–M2 triad by astronomical forcing, whereas tidal asymmetry at the site in the west is flood dominant and mainly caused by non-linear interaction of the major tidal constituents. Analysis of seasonal sea-level changes shows that annual amplitudes in the east are around 13.6 cm, and 6.7 cm in the west. A possible explanation for the discrepancy in the amplitudes is the effect of seasonal monsoon winds flowing from the South China Sea.