Seasonal variations of tidal currents in the deep Timor Passage

Exact knowledge on the seasonal variations of main tidal constituents is beneficial for improving tidal prediction. The semi-annual cycles in K-1 and S-2 tides are abnormally exaggerated by astronomical P-1 and K-2 tides, which interferes with our understanding on tidal seasonality. The widely-used tidal inference method in previous studies cannot fully separate astronomical P-1 and K-2 tides from seasonal P-1 and K-2 tides due to inaccurate inference relationship. In this study, on the basis of the 'credo of smoothness' which indicates that tidal admittances are smooth functions of tidal frequencies, we develop a novel but simple method to address this intractable issue and applied this method to explore the seasonality of tidal currents observed in the deep Timor Passage at the depth of 1800m. We find that the timing and range of seasonal modulations of M-2, S-2, K-1, and O-1 tides are distinct. Annual variations in tidal currents are much stronger than semi-annual variations in tidal currents. The annual and semi-annual ranges of M-2 tide can reach 2.69 cm/s and 1.51 cm/s, which are largest among main constituents. Although the annual range of K-1 tide is only 1.85 cm/s, considering the relatively small amplitude of time-averaged K-1 tide (2.87cm/s), K-1 the most affected tide by the annual cycle. The seasonal cycles of semi-diurnal tides (M-2 and S-2) are basically synchronous while those of diurnal tides (K-1 and O-1) are generally out-of-phase. As a general method, the proposed method can be widely applied to other sea areas to explore local tidal seasonality.

Automated summary

Accurate understanding of seasonal variations in tidal constituents is crucial for better tidal prediction. However, the commonly used method for tidal inference in previous studies fails to separate astronomical and seasonal tides due to inaccurate inference relationship. In this study, we develop a novel method based on the concept of smoothness to address this issue and apply it to investigate the seasonality of tidal currents in the Timor Passage. Results show distinct timing and range of seasonal modulations for different tidal constituents, with annual variations being stronger than semi-annual variations. The proposed method can be widely applied to explore tidal seasonality in other sea areas.