Anomalously large seasonal modulations of shallow water tides at Lamu, Kenya. (5th February 2023)

• Record Type: Journal Article
• Authors: Pan, Haidong; Xu, Tengfei; Wei, Zexun
• Is Part Of: Estuarine, coastal and shelf science. Volume 281(2023)
• Publication Date: 2023-02-05
• Subjects: Ocean tides -- Harmonic analysis -- Seasonal modulation -- Shallow water tides; Estuarine oceanography -- Periodicals; Coasts -- Periodicals; Estuarine biology -- Periodicals; Seashore biology -- Periodicals; Coasts; Estuarine biology; Estuarine oceanography; Seashore biology; Periodicals; 551.461805
• DOI: 10.1016/j.ecss.2022.108203
• Languages: English
• ISSNs: 0272-7714

Abstract

Abstract: 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 constituents. 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. Highlights: The seasonal variations of shallow water constituents at Lamu are revisited. A novel method is proposed to extract real semi-annual modulations of shallow water tides. The real semi-annual cycles in shallow water tides are very weak.