Regional climates around the world display cycles corresponding to the 18.61-year maximum lunar declination (MLD) periodicity. We suggest that these cycles are created by a relationship between MLD and El Ni¿o/Southern Oscillation (ENSO). Both equatorial Pacific sea-surface temperature and South Pacific atmospheric pressure significantly correlate with maximum lunar declination. Low MLDs are associated with warmer equatorial Pacific sea-surface temperatures and negative values of the Southern Oscillation Index. A lunar-influenced change in the Pacific gyre circulation presents a viable physical mechanism for explaining these relationships. We suggest that the gyre is enhanced by tidal forces under high MLDs, inducing cold-water advection into the equatorial region but is restricted by the weak tidal forcing of low MLDs thereby favoring El Ni¿o episodes. An astronomical model utilizing this relationship produces a forecast of increased non-El Ni¿o (either La Ni¿a or neutral) activity for the early part of this decade. ¿ 2001 American Geophysical Union