A monthly reporting on mean sea surface temperatures and marine heatwave conditions.
Key takeaways
May 2025 was :
- the 3rd warmest May on record (after 2023 and 2024) in the global ocean and in the North Atlantic Ocean.
- the 2nd warmest May (after 2018) in the Mediterranean Sea.
- in the top 10 most extreme May months in terms of marine heatwave events in the Mediterranean Sea and in the North Atlantic ocean region.
SEA SURFACE TEMPERATURE
Figure 1: Daily sea surface temperatures (SST) averaged for the global ocean (top), the Mediterranean Sea (left) and the North Atlantic Ocean (right) between 1991-2020 using ESA’s Climate Change Initiative (grey shades), and between 2021-2025 (colored shades) using Mercator Ocean International’s GLO12 analysis and forecast.
- May 2025, at global level (between 60°S and 60°N), was the 3rd warmest May on record (after 2023 and 2024), with a mean Sea Surface Temperature (SST) of 20.84 ± 0.07 °C.
- Regionally, for the Mediterranean Sea, May 2025 was the 2nd warmest May month with a mean SST of 19.64 ± 0.28°C (after 2018).
- For North Atlantic, May 2025 was the 3rd warmest May month (after 2023 and 2024) with a mean SST of 21.62 ± 0.15 °C.
Sea Surface Temperature Anomalies
- May 2025 mean SSTs are above average (see figure 2) for 74% of the global ocean (between 60°S and 60°N), with more than 10% exceeding the average by at least 1°C.
- The monthly mean SSTs are above average for 83% of the Mediterranean Sea.
- The Pacific Ocean shows strong SSTs at midlatitude in both hemispheres, reaching 2°C above average. In the North Atlantic, SSTs are predominantly above average, with higher values in the European region (reaching 2°C above average Southwest of the UK).
- The same is observed for SSTs in the Indian Ocean with largest values between Madagascar and Australia.
Note: In the central Pacific, along the equator, we still observe a large area of below-average SSTs (reaching -1°C).
MARINE HEATWAVES
Ongoing marine heatwaves: 31 May 2025
Long warming events may induce important stress on marine ecosystems *. Here is represented the ongoing marine heatwave (MHW) events at 31 May and their onset time.
- 18% of the global ocean (between 60°S and 60°N) was affected by MHWs on the 31st of May.
- The majority of these MHWs developed recently, with 13.5% of the global Ocean (between 60°S and 60°N) affected by MHWs less than 1 month old and 3% by events between 1- and 2-month-old.
*Marine heatwaves (MHW) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, and human activities. According to the latest report released by the Intergovernmental Panel on Climate Change (IPCC AR6 SYR), it is found with high confidence that in the near-term at 1.5°C global warming, the increasing frequency of marine heatwaves will increase risks of biodiversity loss in the oceans, including from mass mortality events (IPCC AR6 SYR).
Total surface exposed to marine heatwaves: June 2024 – May 2025
We monitor daily the surface occupied by MHWs and show for the past year (since June 2024) the proportion of the global Ocean (between 60°S and 60°N) exposed to MHWs. We focus on the evolution over the past May 2025 (grey bar).
Global Ocean
- In May the total surface of MHW (black line) decreased from 21% to 18% of the global ocean (between 60°S and 60°N). These MHWs consist mainly of moderate and strong events (16% and 2%, respectively, by the end of the month). For comparison, in May 2024, MHW total extent represented above 35% of the global ocean (between 60°S and 60°N), with almost 10% of strong category (not shown).
Mediterranean Sea
In the Mediterranean Sea (see figure 3) :
- The total MHW surface shows an important decrease from ~35% in the first half of May to ~9% by the end of the month (after a drop to 2%).
- These events consist mostly of moderate categories, except for a peak of strong categories of almost 10% at the very beginning of the month.
- For comparison, in May 2024, the total MHW extent varied between 23% and 1% (~12% at the end of the month), with mostly moderate category (not shown).
North Atlantic
Figure 6: Surface occupied by MHWs in the North Atlantic (between 0°N and 60°N) between June 2024 and May 2025 based on daily monitoring. The black solid line represents the total surface of MHW, while the colored lines represent the surface of each MHW category. The gray background highlights the month of May 2025.
For the North Atlantic (between 0°N and 60°N):
- The total MHW surface increases from 15% to 21% during the month of May 2025.
- These events consist mostly of moderate and strong categories (15% and 5% by the end of the month, respectively).
- The MHW in the European region led to a peak of severe and extreme categories in mid-May (~2.5% and 0.5% respectively).
- For comparison, in May 2024, the total MHW extent increased from 33% to more than 45%, with moderate, strong and severe categories (25%, 18% and 2.5% at the end of the month, respectively) (not shown).
Total number of marine heatwave days in May 2025
10% of the surface of the ocean is impacted by strong, or higher, MHW conditions for at least 1 day. The southeast Asia region and the European region Western Europe are the most affected.
Figure 7: May 2025 MHW days of strong and higher category. Number of days during which a MHW of category strong, severe or extreme occurred in May 2025. Blank areas mean that no MHW of strong or higher category happened during the month.
Marine heatwave May statistics
Figure 7: MHW characteristics for all May months between 1993 and 2025. Duration, intensity, extent and activity averaged for each April month between 1993 and 2025 for the global ocean (left), for the Mediterranean Sea (centre) and the North Atlantic Ocean (right). May 2025 is highlighted with a blue edge.
The MHW statistics (duration, intensity, surface occupied and activity) for May 2025 are compared to those for May months from previous years since 1993.
- On a global scale, May 2025 is the 4th most extreme May month in terms of MHW (after 1998, 2023 and 2024) with characteristics similar to May 2022.
- In the Mediterranean Sea and in the North Atlantic, May 2025 is in the top 10 most extreme May in terms of MHW.
About the bulletin
Mercator Ocean International (MOi) publishes a monthly sea surface temperature bulletin reporting on mean temperature and marine heatwave conditions for the month just passed. It includes:
- Regions where MHWs are present and the time these events started,
- Timeseries showing the total surface occupied by MHWs globally and for the European region,
- The total number of days of exposure to MHW during the month (including only strong and higher intensity categories).
Image citation: European Union, Copernicus Marine Service Data 2025 I © Mercator Ocean
All images in this article can be used freely and should be cited with the information above.
Datasets and Products
Sea Surface Temperature
The results are obtained using the Copernicus Marine Service global analysis and forecasting system together with observation-based products.
- GLOBAL_ANALYSISFORECAST_PHY_001_024 Global Ocean Physics Analysis and Forecast sea surface temperature. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
- ESA SST CCI reprocessed sea surface temperature analyses. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00169
- NOAA OI SST V2 High Resolution Dataset data provided by the NOAA PSL, Boulder, Colorado, USA, from their website at https://psl.noaa.gov, Huang, B., C. Liu, V. Banzon, E. Freeman, G. Graham, B. Hankins, T. Smith, and H.-M. Zhang, 2021:
- Improvements of the Daily Optimum Interpolation Sea Surface Temperature (DOISST) Version 2.1, Journal of Climate, 34, 2923-2939. doi: 10.1175/JCLI-D-20-0166.1
- Global Ocean OSTIA Sea Surface Temperature and Sea Ice Analysis. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00165
- ODYSSEA Global Sea Surface Temperature Gridded Level 4 Daily Multi-Sensor Observations. E.U. Copernicus Marine Service Information. Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
Notes: The global analysis system was used to study the ocean surface temperature from 2021 and 2025. To study surface temperature for previous years (1991 to 2020), it was used the ESA CCI data. The other observation-based sea surface temperature products (OSTIA, OI SST and Odyssea) were used for calculating monthly mean and uncertainty values for February 2025.
Marine Heatwaves
The results are obtained using the Copernicus Marine Service global systems including the analysis and forecasting system together with the reanalysis product.
- GLOBAL_ANALYSISFORECAST_PHY_001_024 Global Ocean Physics Analysis and Forecast sea surface temperature. E.U. Copernicus Marine Service Information (CMEMS). Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00016
- GLOBAL_MULTIYEAR_PHY_001_030 Global Ocean Physics Reanalysis sea surface temperature. E.U. Copernicus Marine Service Information (CMEMS). Marine Data Store (MDS). DOI: https://doi.org/10.48670/moi-00021
Notes: The global analysis system was used to study MHWs for 2024 and 2025. To study MHWs for previous Februarys (1993 to 2023), it was used the reanalysis system. The reanalysis product was used for generating a daily climatology using the 30-year period from 1993 to 2022.
