Seasonal variability of the M2 tide in the Bohai Bay: development and application of modified enhanced harmonic analysis

The seasonal variability of the M₂ tide exerts a major influence on the coastal ocean environment. In this study, a novel method, namely modified enhanced harmonic analysis (MEHA), is developed to synchronously extract the temporally varying amplitudes and phase lags of significant tides and the constant values of the other constituents. The seasonal variability of the M₂ tide in the Bohai Bay, China, is investigated by analyzing one-year sea level observations at two stations with MEHA.

In ideal twin experiments, the artificial sea level observations were analyzed. Both the estimated temporally varying amplitude and phase lag of the M₂ tide and constant values of the S₂, K₁ and O₁ tides using MEHA were much closer to the prescribed values than those obtained using the other methods, indicating the capability and efficacy of MEHA. When the real sea level observations were analyzed using MEHA, the estimated M₂ tidal amplitude has significant seasonal variability, with large values in summer and small values in winter, which is robust and not affected by experimental settings. The results of numerical experiments indicate that the seasonality of vertical eddy viscosity induces seasonal variations of the M₂ tide in the Bohai Bay.