The semidiurnal and diurnal lunar tidal forces are modulated by a few percent over the 18.61-year nodal cycle. A simple model of vertical mixing in coastal seas indicates that a fractional variation &egr; in the strength of tidal currents should lead to a variation in sea surface temperature (SST) of at least 4/3&egr; times the average temperature difference from surface to bottom. Thus we hypothesize that long-term records of SST might show a significant nodal cycle, and we find evidence for this at some North American locations.