A time-varying tidal form factor considering the 18.61-year nodal cycle.

A constant tidal form factor, defined by the ratio of the sum of K 1 and O 1 tidal amplitudes to the sum of M 2 and S 2 tidal amplitudes, has been widely used to represent tidal feature for nearly 120 years. Tides can be classified as semi-diurnal, mixed semi-diurnal, mixed diurnal and diurnal regimes based on tidal form factors. It is well-known that K 1 and O 1 tidal amplitudes have significant 18.61-year nodal variations. Therefore, the relative strength of semi-diurnal and diurnal tides changes during the 18.61-year nodal cycle. Consequently, local tidal regimes in some sea areas are not fixed, but periodically change during the 18.61-year nodal cycle. Such periodical tidal regime shift cannot be reflected by constant tidal form factor, which motivates present research. The purpose of this study is to propose a time-varying tidal form factor which considers the 18.61-year nodal modulations of main constituents based on the equilibrium tidal theory. 1249 tide gauges and a global tide model (EOT20) derived from multi-satellite altimeters are used to detect potential tidal regime shift related to the 18.61-year nodal cycle in the global ocean. It is found that there are 154 tide gauges and 17.32% ocean grid points in EOT20 model which exist tidal regime shift caused by the 18.61-year nodal cycle. Tidal form factor can be modified by ∼17% during the 18.61-year cycle in the global ocean. Furthermore, the deviations between theoretical and actual nodal modulations in some coastal resonant areas do not significantly influence the calculation of temporal tidal form factors. • Constant tidal from factors cannot reflect periodically tidal regime shift. • A temporal tidal form factor is proposed. • Tidal regime shift related to the nodal cycle widely exists in the global ocean. [ABSTRACT FROM AUTHOR]