The effect of water currents on wind drag – a case study of tidal currents and sea breeze in a semi-enclosed embayment

Abstract

The details of how energy and momentum are exchanged at the interface between ocean surface and the atmosphere is complex and the subject of new and more complete models. The need to improve models of how wind interacts with oceans is driven in part by the growth of offshore wind farms, and the need to predict their likely performance. The geographic features of Shark Bay allow several of the factors affecting the influence of currents on wind speed to be separated and analysed. Shark Bay is the largest semi-enclosed embayment on the Australian coast. It is tidal and aligned north-south in the direction of the sea breeze. The prevailing southerly wind, and the absence of openings to the ocean in the south of the bay, limits the fetch of waves, providing waves of predictable age in the bay with an absence of longer wavelength swell. The sea breeze in this region is characterised among the strongest and most reliable anywhere in the world. Although the tide heights are not large, the geography of the bay ensures strong tidal currents. Hence Shark Bay provides an excellent opportunity to study the effects of currents on winds. This study demonstrates that the effects of the tidal current are apparent in the wind speed record. It shows that simply subtracting a 29-day running average of the particular time of day from the wind speed reveals the effect of an incoming or outgoing tide. Time-series analysis of this outcome shows the periodicity and modulation of the tides. The analysis is further improved through using the Weather Research and Forecasting (WRF) code and subtracting its predictions from the raw data. Time-series analysis of the outcome demonstrates that the resultant difference has two diurnal and two semi-diurnal components with the correct periods and amplitudes of the known tidal variations in that region of Shark Bay. Hence the neglect of the interaction between water currents and wind stress is demonstrated to produce a systematic deviation in the predictions of the WRF from the measured wind values for Shark Bay.