Exploring tidal constituent trends: unveiling the impact of the 18.6-year lunar nodal cycle through harmonic analysis and long-term tide gauge records

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Understanding tidal constituent trends is becoming increasingly important in a world where climate change puts pressure on the tidal regime across the globe. Tidal constituents change constantly, but there is strong evidence that nodal modulation interferes with constituent amplitude values, thus hindering efforts to accurately measure their trends. Therefore, this paper proposes a practical approach to remove the influence of nodal modulation in constituent trend analysis. We collected multiple 18.6-year series of sea level data from tide gauges in Brest (France), Cananeia (Brazil), and Eastport (USA) and performed a harmonic analysis. Our main focus is to assess the interference of the nodal cycle on M2 tidal constituents. Although 19-year series are optimal to minimize this interference, they drastically reduce the number of data sets analyzed. To mitigate this problem, we employed a sliding window approach where each 19-year series starts one year after the previous one. The results of all three surveyed sites show that by employing this approach, the trends of the tidal constituents change significantly compared to what was previously seen with nodal interference. For instance, in Eastport, the analysis indicates that nodal modulation is partially responsible for the apparent reduction of the M2 amplitude tendency slope after 1980, a change that is softened when the effects of this modulation are removed. The reliability of the trends identified in this study suggests that this practical approach can also help future research predict the slope tendency of main tidal constituents.