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Abstract Numerous tidal phenomena, including river tides, internal tides, and tides in ice‐covered bay, are nonstationary, which pose a great challenge for traditional tidal analysis methods. Based on the independent point scheme and cubic spline interpolation, a new approach, namely the enhanced harmonic analysis, is developed to deal with nonstationary tides. A MATLAB toolbox, STIDE, developed from the widely used TTIDE, is used to realize the approach. The efficiency of STIDE is validated by analyzing a set of hourly water level observations from stations on the lower Columbia River. In all stations, the hindcast of STIDE is more accurate than NSTIDE that is a powerful nonstationary tidal analysis tool adapted to river tides. The changing mean water level and tidal constituent properties obtained by STIDE are similar to those obtained by NSTIDE, continuous wavelet transform, and empirical mode decomposition and highly consistent with theory on river tides. Moreover, different from NSTIDE that only can be applied to river tides, enhanced harmonic analysis is free of dynamic content, assuming only known tidal frequencies. Therefore, STIDE can be applied to all kinds of nonstationary tides theoretically. Though powerful, STIDE also has some limitations: STIDE cannot be used for prediction and too many independent points in STIDE may induce computational memory overflow and unrealistic results.
Pan et al. (Thu,) studied this question.