Yasuda (2018) reported a statistically significant phase-locking of El Niño and La Niña events to the 18.6-year lunar nodal tidal cycle over 1867–2015 (extended back to 1706 with tree-ring proxy data), but explicitly deferred the physical mechanism to future work. The open problem is to identify and demonstrate how the astronomical 18.6-year modulation of ocean tides—acting through tide-induced vertical mixing in candidate hot spots such as the Kuril Straits and the Indonesian seas—is converted into ENSO-band interannual variability: where the forcing enters and in what proportion between remote midlatitude and direct tropical pathways, how bi-decadal forcing generates interannual variance (e.g., whether a 5×f18.65\times f_{18.6} component of period 3.7 years arises through nonlinear dynamics and resonates with equatorial basin modes), and why Indonesian-seas sea surface temperature lags the cycle by about five years—or, alternatively, to demonstrate that the reported ENSO–tidal relation is not a physical linkage at all. As of 2026 the mechanism remains undemonstrated, and even the existence of the linkage is actively contested.

Background

El Niño–Southern Oscillation (ENSO) is the dominant year-to-year climate mode of the tropical Pacific, arising from coupled ocean–atmosphere feedbacks—principally the Bjerknes feedback, by which a warm sea-surface-temperature anomaly in the eastern Pacific weakens the trade winds, which in turn deepens the eastern thermocline and reinforces the warming—and oscillating irregularly on 2–7-year timescales; standard theory treats it as largely internal to the ocean–atmosphere system. The 18.6-year lunar nodal cycle is the oscillation of the Moon's orbital-plane inclination relative to Earth's equator (±5.2° about a mean of 23.4°, period 18.61 years); it modulates the amplitudes of the major diurnal tidal constituents (K1K_1, O1O_1) by roughly 11–19% and the principal semidiurnal constituent M2M_2 by about 4%, with the diurnal and semidiurnal modulations out of phase. Tide-induced vertical mixing is the turbulent mixing produced when tidal currents dissipate energy over rough topography, sills, and narrow straits; it is concentrated in hot spots such as the Kuril Straits (between the North Pacific and the Sea of Okhotsk) and the Indonesian seas, and is represented in climate models as enhanced vertical diffusivity. Loder and Garrett (1978) first proposed that this nodal modulation of tidal mixing could imprint an 18.6-year cycle on sea surface temperature in shallow seas. Subsequent work found 18.6-year signals in North Pacific ocean and climate records (Yasuda, Osafune and Tatebe, 2006) and in a tree-ring reconstruction of the Pacific Decadal Oscillation (Yasuda, 2009)—the Pacific Decadal Oscillation being the leading decadal-scale pattern of North Pacific sea-surface-temperature variability—and Hasumi, Yasuda, Tatebe and Kimoto (2008) showed in a single coupled climate model that imposing an 18.6-year modulation on mixing near the Kuril Islands excites 18.6-year-period ENSO-like Pacific variability, with the signal propagating along the western boundary as coastally trapped waves (ocean waves guided along the continental margin, which carry midlatitude signals toward the equator) and along the equatorial thermocline. Yasuda (2018) then documented the observational relation directly: defining a tide-year counter that numbers each year by its phase within the 18.6-year cycle, tide year 1 being the year of maximum diurnal tide, El Niño events over 1867–2015 (and in a 310-year proxy of the Southern Oscillation Index—the standardized sea-level-pressure difference across the tropical Pacific—reconstructed from tree rings back to 1706) tend to occur in the 1st, 10th, and 13th tide years and La Niña events in the 3rd, 12th, and 16th, at statistically significant rates assessed by Monte Carlo simulation against red-noise surrogate series, with corresponding sea-surface-temperature and sea-level-pressure patterns resembling the Pacific Decadal Oscillation. The paper's Discussion, however, explicitly leaves the mechanism open: it is unclear how a bi-decadal forcing is converted into ENSO-scale variability (the suggestion that a 5×f18.65\times f_{18.6} component with period 3.7 years, visible in the proxy index, is generated by nonlinear dynamics and resonates with equatorial waves—Kelvin and Rossby waves trapped near the equator, the adjustment modes that carry ENSO signals across the basin—bounded by the basin coasts is conjecture requiring future study); the relative roles of remote forcing from midlatitude mixing regions such as the Kuril Straits and direct forcing in the tropical Pacific, especially the Indonesian seas, are undetermined; and the roughly five-year (quarter-phase) lag of Indonesian-seas August sea surface temperature behind the 18.6-year cycle is unexplained. The author calls for direct mixing observations and climate models with refined tide-induced vertical mixing in mixing hot spots to test the relationship.

Problem Statement

Identify and demonstrate the physical mechanism by which the 18.6-year lunar nodal modulation of ocean tides could produce the tide-year phase-locking of El Niño–Southern Oscillation events reported by Yasuda (2018)—the tide-year counter numbering each year by its phase within the 18.6-year cycle, tide year 1 being the year of maximum diurnal tide—or demonstrate that this reported relation is not a physical linkage. A mechanistic answer must address the linked sub-questions the source explicitly left open: (i) the location and relative contribution of the forcing, i.e., remote forcing from midlatitude tidal-mixing regions such as the Kuril Straits (propagating to the equator as coastally trapped waves) versus direct tropical forcing in the equatorial Pacific and the Indonesian seas; (ii) the dynamical process converting bi-decadal (18.6-year-period) forcing into ENSO-band interannual variability, including whether a 5×f18.65\times f_{18.6} component with period 3.7 years is generated by nonlinear dynamics and resonates with equatorial waves bounded by the basin coasts; and (iii) the cause of the approximately five-year (quarter-phase) lag of Indonesian-seas sea surface temperature behind the 18.6-year tidal cycle. A mechanistic demonstration must be consistent with the documented statistics (El Niño preference in the 1st, 10th, and 13th tide years; La Niña in the 3rd, 12th, and 16th; the 310-year proxy record) and distinguishable from ENSO's internal variability. An answer that instead demonstrates the reported relation is not a physical linkage is an equally valid resolution of the task; the sub-questions (i)–(iii) and the consistency requirement just stated bind only a mechanistic answer.

References

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