Purpose. The paper is purposed at studying long-wave processes in Kholmsk bays and on the adjacent shelf (including the interaction of bays) based on the theoretical concepts and the data of sea level field observations obtained in September 2022 – May 2023.
Methods and Results. Three autonomous wave meters ARW-14 K installed in the bays of Trade Port and Kholmsk-Severny, as well as on the shelf at an insignificant distance from the bays were used for observations. The measurement discreteness was 1 second. The time series both including the tides and without them were studied based on the spectral analysis using the Kyma program. Within the range of wave periods 1–30 h, the wave processes of a non-tidal origin and with the periods 1.6–6.7 h were found. They can be attributed to the shelf seiches, the Poincaré waves or the Tatar Strait seiches. Spectral analysis in the period range 1–10 min has shown the presence of seiches with the periods 1.83–8.17 min in Trade Port Bay and those with the periods 1.32–8.65 min in Kholmsk-Severny Bay.
Conclusions. It is established that in course of the whole series of field observations, the coupled oscillations at the periods ~ 8 min took place in the above-mentioned bays. These oscillations correspond to the Helmholtz mode of Kholmsk-Severny Bay. They are induced in this bay and then transmitted to Trade Port Bay due to interaction. At different times they had both in-phase and anti-phase spatial structures. During the periods of high eigen modes the interaction between the bays was not detected. Besides, the spectral analysis of the sea level oscillations under study made it possible to reveal the beats with a period 4.82 h (289.2 min), resulting from the interaction of modes with the close periods equal to 8.17 and 8.65 min. The stated facts, as well as correspondence of the distance between the bays' inlets to the proposed earlier interaction condition criterion allow us to assert that the coupled oscillations are present in two adjacent bays – Kholmsk-Severny and Trade Port.