The long‐period tides

Among the periodic terms expressing the disturbance of the geoid by the sun and moon are those that have no longitudinal dependence and are characterized by periods exceeding two weeks. These long‐period tides have amplitudes of less than 1 cm and have generally been thought, on theoretical grounds, to approximate to static equilibrium in the ocean. A static equilibrium long‐period tide is of great interest to the geophysicist, permitting determination of the elastic Love numbers, and to the oceanographer for determining low frequency oceanic dynamics. The equilibrium hypothesis was tested by computing peridograms for the fortnightly and monthly tides on islands of the Pacific. Despite high noise levels, substantial deviations from equilibrium were found, with fluctuations over distances of about 3000 km. An approximate analytic solution to the Laplace tidal equations shows the measurements to be consistent with the hypothesis that the tides excite the Rossby wave modes of the ocean. Dissipation requires the lower frequency tides to cause ‘Gulf‐Stream‐like’ westward intensification of motion. Previous measurements of the 14‐month pole tide are also consistent with the Rossby wave hypothesis.