Tidal fluctuations in sea level and currents are represented by a special set of harmonical functions. In a linear approximation, averaging of tidal fluctuations gives a result close to zero. However, in reality, due to nonlinear effects, such averaging will give a different result. This mechanism will lead to the appearance of a static sea level rise and the generation of a constant current. The change in the average sea level due to nonlinear effects is known as the residual tidal level. Permanent currents resulting from the transfer of energy from semi-diurnal and diurnal tidal waves are known as residual tidal currents, and the system they constitute is called residual tidal circulation.
The first description and explanation of residual tidal currents was given in the work of Timonov, 1960. Since then, numerous works have appeared describing residual tidal currents in various water areas. As a rule, residual tidal circulation was described based on the results of numerical hydrodynamic models. Specific conditions are required for reliable identification of residual tidal circulation in measurement data: strong tidal currents and limited influence of currents caused by other factors. Such conditions are observed in the semi-enclosed waters of the Keret Bay, where the Department of Oceanology of St. Petersburg State University has been conducting long-term research.
The investigation focused on Keret Bay in the Chupa Estuary and Kandalaksha Bay in the White Sea. There are small straits and bays, including the semi-enclosed Lebyazya Bay. The waters in a semi-enclosed basin have special features due to tidal cycles and topography. Numerical simulations and field observations revealed the unique characteristics of tide-induced circulation.
We employed a combination of numerous measurements and results of hydrodynamic simulations. The residual level and residual tidal circulation were calculated based on 30-day numerical experiment results. Filtering was performed twice using a moving average with a step of one day to eliminate harmonic components whose period is not a multiple of the length of the series. The values of the residual current and residual sea level were then determined from the filtered data.
The residual tidal sea level throughout the whole Keret Bay is practically absent. The deviation in the residual tidal level from the average sea level does not exceed a few millimeters. This is less than the accuracy of the methods used for analysis. The freshwater runoff from the Keret River will change the average sea level. It will also cause additional nonlinear effects associated with the interaction of river runoff and tidal waves.
The residual tidal currents obtained from model calculations are of most interest. Lebyazya Bay's permanent circulation, caused by tidal dynamics, is one of the most interesting special features. The characteristic flow velocities are 1-3 cm/s, reaching 5 cm/s. This circulation is a tide-induced permanent residual circulation, which is the result of the complex interaction of tides and topography.
