The effect of the 18.6-year lunar nodal cycle on steric sea level changes

We show that steric sea-level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6-year cycle do not always correspond to the equilibrium nodal tide. This finding allows to better filter natural variability when estimating the current rate of sea level rise along the European coast. Practical applications include the detection of an acceleration of sea level rise and the comparison between tide gauge and satellite observations with sea level projections.