Based on a multi-month observation time series with one-second temporal resolution of seawater temperature and sea level oscillations obtained in 2015–2025 off the southeastern coast of Sakhalin Island in Mordvinova Bay, low-frequency wave processes in the coastal zone were analyzed. A pronounced seasonal contrast in the amplitude of temperature oscillations was identified: during the autumn–winter period, temperature variations generally do not exceed 2◦C, whereas in the spring–summer season, beginning in May, large-amplitude oscillations develop, increasing with progressive water warming and reaching ∼ 7◦C. Interannual comparison of the time series reveals a high degree of reproducibility in the temporal structure of the oscillations (correlation coefficients between the 2025 and 2020 and 2025 and 2022 records range from 0.76 to 0.82), indicating a common underlying generation mechanism. Spectral analysis identified a set of persistent peaks in the period range of 3–43 days, recurring in different years and partially coinciding with characteristic periods of atmospheric pressure and wind variability. The combination of the observed periods, their seasonal occurrence under stratified conditions, and the consistency of the basin spatial scales with the first baroclinic Rossby radius of deformation supports the interpretation of the temperature oscillations as manifestations of coastal-trapped baroclinic Kelvin waves. Analysis of the 2025 data shows that southerly winds prevail from mid-April to late August, coinciding with the period of intensified baroclinic temperature variability. Calculation of the absolute value of the transfer function for the wind-sea level and wind-temperature time series pairs confirms more efficient energy transfer from wind to baroclinic oscillations during the spring-summer season, indicating the leading role of southerly winds in generating the observed regime. Additional analysis of satellite-derived sea ice concentration shows that the decay of seasonal ice cover occurs by late March–early April, several weeks before the onset of high-amplitude oscillations. This indicates that sea ice processes play a secondary role and cannot explain the observed low-frequency variability.
Based on a multi-month observation time series with one-second temporal resolution of seawater temperature and sea level oscillations obtained in 2015–2025 off the southeastern coast of Sakhalin Island in Mordvinova Bay, low-frequency wave processes in the coastal zone were analyzed. A pronounced seasonal contrast in the amplitude of temperature oscillations was identified: during the autumn–winter period, temperature variations generally do not exceed 2 ◦C, whereas in the spring–summer season, beginning in May, large-amplitude oscillations develop, increasing with progressive water warming and reaching ∼ 7◦C. Interannual comparison of the time series reveals a high degree of reproducibility in the temporal structure of the oscillations (correlation coefficients between the 2025 and 2020 and 2025 and 2022 records range from 0.76 to 0.82), indicating a common underlying generation mechanism. Spectral analysis identified a set of persistent peaks in the period range of 3–43 days, recurring in different years and partially coinciding with characteristic periods of atmospheric pressure and wind variability. The combination of the observed periods, their seasonal occurrence under stratified conditions, and the consistency of the basin spatial scales with the first baroclinic Rossby radius of deformation supports the interpretation of the temperature oscillations as manifestations of coastal-trapped baroclinic Kelvin waves. Analysis of the 2025 data shows that southerly winds prevail from mid-April to late August, coinciding with the period of intensified baroclinic temperature variability. Calculation of the absolute value of the transfer function for the wind-sea level and wind-temperature time series pairs confirms more efficient energy transfer from wind to baroclinic oscillations during the spring-summer season, indicating the leading role of southerly winds in generating the observed regime. Additional analysis of satellite-derived sea ice concentration shows that the decay of seasonal ice cover occurs by late March–early April, several weeks before the onset of high-amplitude oscillations. This indicates that sea ice processes play a secondary role and cannot explain the observed low-frequency variability.