Wintertime Variability of Currents in the Southwestern Taiwan Strait

Abstract

Abstract A 57‐days record of surface currents observed from high‐frequency radars on the Chinese coast, along with wind records, are analyzed to investigate wintertime variability of surface currents in the southwestern Taiwan Strait. The M 2 tide dominates tidal variability, and the semimajor axes of semidiurnal tidal current ellipses coincide well with bathymetry contours suggesting interactions with a propagating tidal wave. However, phase progressions suggest that this wave undergoes an angular reflection near the Taiwan Bank (TWB). Diurnal currents are less correlated to bathymetry perhaps because many different diurnal tide waves interact here. Surface currents are strongly related to winds at subtidal frequencies. Subtidal along‐strait currents have a correlation of 0.8 with along‐strait winds. However, three different spatial current patterns appear depending on the strength of along‐strait winter monsoon. Downwind currents occur when northeasterly wind is stronger than 10 m s −1 . For medium winds, a gradual change in the pattern of currents is observed, including an offshore flow north of the TWB. In low winds conditions, when winds can be either northeasterly or (sometimes) southwesterly, the eastward flow along the northern edge of the TWB increases to 20 cm s −1 ; elsewhere flow is to the northeast. This forms a counter‐wind current when winds are from the northeast. By investigating the momentum balance, we find that in along‐strait direction wind stress and pressure gradient are important over the TWB, and a geostrophic balance dominates north of bank. In the cross‐strait direction, momentum is dominated by a geostrophic balance over the entire region.

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