Moderate to strong El Nino events are triggered by the inter-annual variability in the lunar tides.
Moderate to strong El Nino events are triggered by long-term (i.e. inter-annual) variability in the lunar tides. Specifically, the timing of these events is directly related to 31/62 year Perigee/Syzygy lunar tidal cycle.
I do not have all the answers as to how this actually happens but the best answer that I can come up with is that slow forcings applied to the Earth by the lunar tides influences the formation and subsequent propagation of Madden-Julian Oscillations (MJO) along the Equatorial Indian Ocean and Pacific Oceans.
A MJO consists of a large-scale coupling between the atmospheric circulation and atmospheric deep convection. When a MJO is at its strongest, between the western Indian and western Pacific Oceans, it exhibits characteristics that approximate those of a hybrid-cross between a convectively-coupled Kelvin wave and an Equatorial Rossby wave. When a MJO moves from the western Indian Ocean into the western Pacific Ocean, it generally accelerates, becomes less strongly coupled to convection, and transitions into a convectively de-coupled (i.e. dry) Kelvin wave.
Periodically (i.e. roughly once every 4.5 years), the precise alignments of the lunar tidal forcings produce the right conditions that result an upsurge in the number and magnitude of what I call Pacific Penetrating MJO. These are MJO events that travel from the Eastern equatorial Indian Ocean, along the Equator, all the way into the Western Pacific Ocean, where they initiate Westerly Wind Bursts (WWB’s).
The spawning of these WWB’s takes place as the MJO event is transitioning from a hybrid-cross between, a convectively-coupled Kelvin wave and an Equatorial Rossby wave, and a convectively de-coupled (i.e. dry) Kelvin wave. The spawning of the WWB’s occurs in the Western Equatorial Pacific Ocean, somewhere between 60 deg E and 150 deg W longitude. The actual process involves the formation of a typhoon/cyclone pair straddling the equator which produces an intense WWB between the two intense low pressure cells.
The onset of El Nino event are marked by the weakening of the easterly trade winds associated with the Walker circulation. The actual drop off in easterly trade wind strength is always preceded by a marked increase in WWB’s in the western equatorial Pacific Ocean. The WWB’s help initiate an El Nino event by creating downwelling Kelvin waves in the western Pacific that propagate towards the eastern Pacific, where they produce intense localized warming, as well as by generating easterly moving equatorial surface currents which transport warm water from the warm pool region into the central Pacific.
The net result of the Moon’s involvement in the initiation of El Nino events means that:
El Niño events in New Moon epochs preferentially occur near times when the lunar line-of-apse aligns with the Sun at the times of the Solstices.
El Niño events in the Full Moon epochs preferentially occur near times when the lunar line-of-apse aligns with the Sun at the times of the Equinoxes.