Marine heatwaves in the Gulf of Bothnia from historical measurements to future projections

Marine heatwaves (MHWs) impact aquatic respiration and contribute to oxygen depletion in marine ecosystems. Understanding the frequency, intensity, and spatial extent of MHWs is critical for predicting ecosystem health. We argue that understanding the spatio-temporal variability and long-term trends of MHWs is essential for marine conservation planning, as managing cumulative impacts would require reducing other environmental stressors from regions where higher impacts of MHWs are to be expected. 

We analyze past MHW events in the Gulf of Bothnia, a subbasin of the Baltic Sea, using historical measurements starting from the beginning of the 1900s. The characteristics of the past heatwave events are compared to MHWs up to year 2100, identified from modeled future projections. The future projections are based on NEMO ocean circulation model, forced with dynamically downscaled atmospheric conditions under RCP4.5 and RCP8.5 emission scenarios. During the observational period, we see decadal variability dominate over the increasing long-term trend in mean temperature. The projections indicate that by the end of this century, the increasing temperature signal exceeds the decadal variability, leading to longer and more frequent MHW events.