Importance of tides and winds in influencing the nonstationary behaviour of coastal currents in offshore Singapore

Abstract. Coastal currents significantly impact port activities, coastal landform morphodynamics and ecosystem functioning. It is therefore necessary to understand the physical characteristics and natural variability of these currents within coastal settings. Traditional methods such as harmonic analysis assume stationarity of tide-driven currents and thus may not be applicable to systems modulated by variable nontidal inputs and processes. Here we deployed eight tilt current meters at shallow (< 5 m) coral reef environments in southern Singapore. Tilt current meters were positioned around the reefs at the main compass bearings to analyse the spatiotemporal variability of coastal currents in the frequency domain for one year (March 2018 to March 2019). Tidal motions were the primary mechanism of current flow on reefs and account for between 14–45 % of total variance across all sites, with diurnal currents having either similar or greater proportion of energy than semidiurnal currents. In Singapore, the diurnal wind stress, characteristic of the land–sea breeze, strengthens during the monsoons, and its effect on currents was investigated using wavelet coherence. Findings suggest that currents and wind stress were highly correlated at the diurnal and subtidal frequencies during the monsoons with a varying time lag of up to 6 hours with respect to both the phase and the antiphase. We find that wind forcing was responsible for the observed seasonal variations in the diurnal K₁ tidal constituent, its amplitude derived from short-term harmonic analysis. Given the importance of wind, we thus require longer time-series datasets to examine how atmospheric phenomena affect currents at greater time scales to improve predictions.