Two-step harmonic analysis for capturing seasonally-varying amplitudes and phase lags of the predominant tidal constituents

Recent studies have revealed that the predominant tidal constituents have seasonal variations at some locations. However, how to accurately obtain these variations remains a problem for the traditional harmonic analysis (HA) due to the tradeoff between length of time window and resolution of constituents. Therefore, a method named as “two-step HA” is developed in this study, which consists of both long- and short-time-window HA. Through a series of ideal experiments, practical application at two tidal gauges and comparison with the traditional HA, the feasibility and accuracy of the two-step HA are verified: The two-step HA performs better than the traditional HA in estimating monthly amplitudes and phases for the predominant constituents, whether they have seasonal variability or not. In addition to capturing variations of the predominant constituents at tidal gauges, the two-step HA would be useful in investigation of the coherence and incoherence of internal tides.