Characteristics of Marine Heatwaves of the Baltic Sea for 1993−2022 and Their Driving Factors

Marine heatwaves (MHWs) are extreme ocean events with prolonged discrete periods of anomalously warm water, that have significant impacts on fisheries, tourism, and marine ecosystems. We identify MHWs as discrete periods (≥ 5 days) when the sea surface temperature exceeds the threshold (90th percentile) of the sea surface temperature distribution for specific calendar days and analyze their main properties in the Baltic Sea for the period 1993−2022. Also, we investigate the main mechanisms of evolution one of the most intense and continuous MHW, observed from October 2000 to March 2001. We use temperature, salinity, mixed layer depth, and current velocity daily data from regional reanalysis of the Baltic Sea (2 nautical mile horizontal resolution, vertical step from 1 m on the surface to 24 m on the bottom). We also use monthly data from global climate reanalysis ECMWF ERA5 (0.25° × 0.25°) and meteorological stations of the Swedish Meteorological and Hydrological Institute. From 40 to 90 MHWs with an average duration and intensity (8−24 days and 1.75−3.25 °C) were detected in various parts of the Baltic Sea during the period 1993−2022. The maximum cumulative values (> 2400 °C days) were observed in the Gotland Basins, the Gulf of Finland, and the Gulf of Riga. The mean intensity and cumulative values of MHWs are stronger in summer (3.6 °C and 740 °C days). A long existence of MHW in the autumn–winter period 2000–2001 was associated with positive air temperature anomalies (> 4 °C) and a sharp weakening of wind speed in the Baltic region.