Wavelet transform analyses of non-stationary tidal currents

The focus of this paper is on the use of wavelet transform analyses to extract time-varying tidal signals from current meter data. The properties of wavelet analyses are contrasted with those of traditional techniques of tidal analysis and other methods that may be used to study non-stationary signals. Wavelets are more flexible than other techniques in the vital tradeoff between time resolution and bandwidth, the only practical method for tidal analysis that allows resolution of events, and incorporate non-tidal processes gracefully. However, they do not and are not intended to achieve the frequency resolution of traditional tidal analysis methods. Two examples of the use of wavelets illustrate these points. Wavelets show clearly the response of estuarine internal asymmetry to tidal monthly variations in stratification. They also allow definition of a rapid, nonlinear response of shelf internal tides near the mouth of the Columbia river to large pulses of river flow.