Anomalously large seasonal modulations of shallow water tides at Lamu, Kenya

Understanding seasonal tidal changes is vital for accurate tidal prediction which is essential for numerous practical purposes, such as flood warning and navigation. It is common knowledge that main constituents show significant seasonal modulations in the global ocean due to non-astronomical factors such as river discharge, sea ice and ocean stratification. Less known, and less studied, is the seasonal variability of shallow water constitu-ents. In this paper, we revisit the abnormally large semi-annual modulation of shallow water constituents at Lamu indicated by a previous study. We find that large semi-annual modulations of shallow water tides at Lamu are mostly not physical, but artifacts induced by unresolved nearby nonlinear shallow water tides due to short length of records. We also propose a novel method based on the 'credo of smoothness' to extract real semi-annual modulations of shallow water tides. Tidal harmonic analysis and other widely-used time series methods are generally frequency-depend, which means that they can only extract the amplitude and phase of one specific tidal frequency, but cannot distinguish different origins of a tidal constituent. It is suggested that care must be taken to ensure the obtained tidal amplitude/phase series are really reflecting tidal changes and are not artifacts of frequency-depend methods.

Automated summary

Understanding seasonal tidal changes is crucial for accurate tidal prediction and has practical applications such as flood warning and navigation. While the seasonal modulations of main constituents in the global ocean are well known, the seasonal variability of shallow water constituents is less studied. This paper revisits the large semi-annual modulation of shallow water constituents at Lamu, finding that they are mostly artifacts due to unresolved nearby nonlinear shallow water tides. A novel method based on smoothness is proposed to extract real semi-annual modulations.