40-year changes in temperature and salinity in the southern Sea of Okhotsk: warming of the Soya Warm Current and offshore freshening

Abstract This study investigates interannual variations in temperature and salinity in the southern Okhotsk Sea, including the Soya Warm Current (SWC) region, based on observational data over the past 40 years (1984—2023). A key challenge is resolving the strong temperature and salinity front between the SWC and offshore areas to accurately assess interannual variability. To address this, we applied a mapping method (Mensah and Ohshima, J Atmos Oceanic Technol, 2023) and updated climatological fields of temperature and salinity. Analysis of anomalies from the climatologies reveal distinct warming trend confined to the SWC region. The strongest warming occurs at depth of 30‒50 m with a rise of 1.36°C over 40 years. These results suggest that the observed coastal warming is driven primarily by inflow from the Japan Sea, rather than by local atmospheric forcing. This is consistent with warming trends in sea surface temperature and 2 m air temperature over the southern Japan Sea. In contrast, the absence of a warming signal offshore of the SWC can be explained by the annual thermal reset, whereby the surface is cooled to the freezing point each winter during sea-ice cover, erasing accumulated thermal anomalies. Regarding salinity, a pronounced freshening trend is observed in autumn in the offshore region of the SWC, mainly attributed to increased Amur River discharge after 2010. Time series of the anomalies indicate that temperature, salinity, and volume transport in the SWC vary synchronously, despite considerable interannual variability.