There is no six-year periodicity in tidal forcing

The El Nino–Southern Oscillation (ENSO) is one of the most important interannual modulations of Earth’s climate, and efforts to improve ENSO forecasting are most welcome. Lin and Qian1 (hereafter LQ19) offer an intriguing tidal connection, which, if correct, brings in a highly deterministic component that could simplify and improve forecasts. The LQ19 argument rests on the existence of a 6-year variability in the tidal forces acting on the ocean. There are several aspects to their argument that merit comment, but here we confine ourselves to the simple question of the tidal force itself. Accurate quantitative knowledge of this force has existed for over a century2,3, and it would be surprising to learn that an important 6-y component has only now been discovered. In fact, there is no 6-y component in the tidal force nor in the Earth’s ocean tide.As LQ19 note, there is a 6-y periodicity in the Moon’s orbital motion which is most simply described as follows: The Moon’s orbit plane (or its ascending node) precesses with a period of 18.6 y, and its orbital ellipse (or perigee) precesses with a period of 8.85 y, both motions taken relative to the equinox. If we consider the motion of perigee relative to the moving node, its period is 6 years, simply because (1/8.85 + 1/18.6) = 1.600 (the two frequencies are added, not subtracted, because the node and perigee move in opposite directions). But does this relative motion induce a 6-y tide on Earth? It does not. We begin by reviewing the two ways a 6-y variability in tidal forcing could possibly occur—these mechanisms do occur at other periods such as 18.6 y—and we then explain why both can be ruled out. We then examine three physical explanations that LQ19 offer in support of a 6-y tide, and we explain why each of those can also be ruled out.