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 may thus not be applicable to systems modulated by variable nontidal inputs and processes. Here we deployed eight tilt current meters in 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 1 year (March 2018 to March 2019). Tidal motions were the primary mechanism of current flow on reefs and account for between 14 % and 45 % of total variance across all sites, with diurnal currents having either a similar or greater proportion of energy compared to semidiurnal currents. The relationship between currents and wind stress was then investigated across various frequencies. There is high correlation at low frequencies during the northeast monsoon, when the Madden–Julian Oscillation (MJO) is more active, thus generating currents that propagate either in phase or ahead of the MJO. At diurnal frequencies, the interaction between P1 and K1 results in a semi-annual cycle where currents are stronger during the monsoon seasons. This interaction could help to explain the seasonal variation in correlation as well as the K1 amplitude, the latter of which could be further enhanced by the diurnal wind stress. The phase relationship between currents and wind stress is highly complex due to the variable bathymetry and could only be partially accounted for by the orientation of the coastlines relative to that of the wind. Given the importance of wind, we thus require longer time-series datasets to examine the role of atmospheric phenomena at greater timescales to improve our understanding of the variability of coastal currents.