Purpose. The purpose of the study is to define the possibility of synchronizing long-wave fluctuations in the neighboring bays of the Kholmsk city based on the Van der Pol oscillator model, as well as the possibility for long waves to penetrate to the coastal waters adjacent to Kholmsk from the Moneron Island region and from the water areas near the cities of Gornozavodsk, Nevelsk, and Chekhov using in-situ observation data.

Methods and Results. The autonomous wave recorders ARW-14K installed in the Kholmsk bays, and the ARW-10 recorder located in the water areas nearby the cities of Gornozavodsk, Nevelsk, and Chekhov were used to obtain in situ data on sea level fluctuations. The measurement resolution of all the instruments is 1 s. The time series were researched through the spectral analysis of sea level fluctuations using the Kyma software. Calculation of the dispersion relation for the Stokes edge waves in the flat-sloping-bottom approximation has demonstrated the possibility of generating edge waves with the 8–9 min period in the water areas nearby the populated regions at the southwestern coast of Sakhalin Island. The reasons for unlimited growth of the phase difference of sea level fluctuations shown in the phase diagram for the Kholmsk-Severny and Trade Port bays were analyzed using a numerical solution of the Van der Pol equation.

Conclusions. It is established that a well pronounced wave process with the 8.27 min oscillation period is observed only in the water area nearby Gornozavodsk, and these waves do not reach the Kholmsk outer water area. The sea level fluctuations (period is approximately 8 min.) recorded by the tide gauge at Trade Port are the result of fluctuation interactions in the bays Trade Port and Kholmsk-Severny. A numerical solution of the problem on forced synchronization of a dynamic system subjected to a weak periodic stimulus using the Van der Pol equation has shown the possibility of unlimited growth of the phase differences between the phase of external force, i. e. the waves incoming from the neighboring Kholmsk-Severny Bay (period is 8.65 min), and the phase of natural oscillations in the Trade Port Bay (period is 4.7 min), which are obtained from in situ data. At a small value of the nonlinearity parameter, the oscillations of the Van der Pol oscillator are close to the harmonic ones, and in this case the phase difference of fluctuations changes abruptly and increases continuously.