Long period tide

Long-Period tides are gravitational tides, typically with amplitudes of a few centimeters or less and periods longer than one day, generated by changes in the Earth's orientation relative to the Sun, Moon, and Jupiter. The distance between a reference point on the surface of the Earth relative to these objects can be expressed as an infinite combination of periods and, as the distance changes, so does the tidal forcing. An analysis of the changing distance by Pierre-Simon de Laplace in the 18th century shows that these periods at which gravity varies cluster into three species, the semi-diurnal and diurnal tide constituents which have periods of a day or less, and the long period tidal constituents (see also tide). Long period tidal constituents with relatively strong forcing include the lunar fortnightly (Mf) and monthly (Ms) as well as the solar semiannual (Ssa) and annual (Sa) constituents. In addition to having periods longer than a day-long period tidal forcing is distinguished from that of the first and second species by being zonally symmetric. The long period tides are also distinguished by the way in which the oceans respond. In contrast to the first and second species, the long period tidal forcings occur sufficiently slowly that they do not excite surface gravity waves. This property of exciting surface gravity waves is responsible for the high amplitude semi-diurnal tides in the Bay of Fundy, for example. In contrast, the ocean responds to long period tidal forcing with a combination of an equilibrium tidal response along with a possible excitation of barotropic Rossby wave normal modes