MN4

MN4 is a shallow-water compound tidal constituent formed by the nonlinear interaction of M2 and N2, with a frequency equal to the sum of their speeds, about 57.4238 degrees per hour and a period near 6.269 hours. Unlike M4, which is an overtide of a single constituent, MN4 is a compound tide whose frequency is the sum of two different parent frequencies. It appears alongside M4 and MS4 in the quarter-diurnal band of estuarine records and contributes to the spring-neap modulation of tidal asymmetry. Harmonic analysis of shallow-water stations includes MN4 to reproduce the distortion of the tidal curve that the deep-water constituents alone cannot capture.