Decadal average sea surface temperature anomaly in different European seas

Time series (1859-2024) of decadal average observed sea surface temperature anomalies (°C), with respect to the period 1991- 2020, for each of the European basins, for the European seas as a whole, and for the global ocean. Data sources: HadSST4.1.0.0 (1850-2024), ERSST v5 (1854-2024), COBE2 (1850-2024), DMI SST v1 (1982-2024) and ERA5 (1979-2022).

Figure 1 presents the post processed data as a range between the maximum and minimum value among the ensemble of datasets at each time step. This range is not an uncertainty estimate but rather the spread among the considerable datasets.

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Methodology

Anomalies are calculated relative to a 1991-2020 average. For the satellite data (DMI SST v1), the anomalies are calculated daily based on a daily climatology computed from a five-day mean centered on each day. For the in situ datasets, anomalies are calculated on a monthly basis by subtracting the 1991–2020 mean anomaly (relative to the original baseline used by the dataset) for each month. Daily anomalies were aggregated to monthly anomalies, and the monthly anomalies were aggregated to annual anomalies giving each month an equal weight.

Area-averaged anomalies were calculated using an area-weighted average of non-missing grid cells within the chosen region. A grid cell was assumed to be within a region if its center was within the region. Ocean area in the in situ products was estimated based on the high-resolution C3S satellite product, assigning 100% ocean area to grid cells populated in the satellite product and 100% land area to grid cells that are missing in that product. A 10-year rolling mean centered on right edge of the window is applied to the annual times series.

The areas used for the regional seas are the following:
• Europe: 35°–70°N, 25°W–40°E
• Baltic Sea: 52.5°–67.5°N, 8.5°–30.5°E
• Black Sea: 39.5°–48.5°N, 27.5°W–42.5°E
• Mediterranean Sea: 30.5°–46.5°N, 6.5°W–38.5°E
• North Sea: 50.5°–60.5°N, 5.5°W–9.5°E
• North Atlantic Ocean: 29.5º–79.5ºN, -40.5ºW–0.5ºE

Decadal average sea surface temperature anomaly in different European seas

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References and footnotes