Equinoctial Tide

An equinoctial tide is the enhanced spring tide that occurs near the spring and autumn equinoxes, in late March and late September, when the Sun crosses the equator and solar declination is near zero. At those times the solar tide-generating force concentrates on the equatorial plane and aligns most directly with the lunar semidiurnal tide, so the combined M2 plus S2 range peaks; an equinoctial spring near perigee gives some of the year’s largest ranges. The effect is strongest at low-latitude semidiurnal ports and underlies the seasonal high waters that flood low-lying coasts. It reflects the geometry of declination, not a separate harmonic constituent.