Nodal Cycle Sea Level Modulation

Nodal-cycle sea-level modulation is the 18.61-year variation in tidal amplitude caused by the precession of the Moon’s orbital plane, which cycles the lunar declination and so modulates the principal tidal constituents. The dominant effect is on the M2 amplitude, with a peak-to-peak swing of a few percent, plus a direct nodal tide. Over the cycle, mean high water and the height of tidal extremes rise and fall, shifting the timing of nuisance and storm-tide flooding by years. Long-term sea-level trend estimation and extreme-value analysis must account for the 18.61-year signal, or short records alias it into a spurious acceleration or trend.