Mercator Ocean International
Ocean Temperature Bulletin

Ocean Temperature Bulletin
July 2025

Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean. They analyse a variety of datasets from observation analyses
(satellite sea surface temperature maps) to model analyses
(assimilating satellite and in situ observations) and model forecasts.¹

All images in this article can be used freely and should be credited with the following information :
Image source: European Union, Copernicus Marine Service Data 2025 © Mercator Ocean .

Key takeaway

Global Ocean: July 2025 is the third-warmest July on record
  • At the global scale (between 60°S and 60°N), July 2025 ranked as the third warmest July on record, with a mean SST of 20.80 ± 0.07°C, behind only 2024 and 2023. 
  • Approximately 71% of the global ocean experienced above-average temperatures, with substantial regional anomalies in the North Pacific, North Atlantic, and Indian Oceans.
The Mediterranean Sea experienced the highest average marine heatwave intensity for a July month 
  • The mean sea surface temperature (SST) reached an unprecedented 26.68 ± 0.15°C, surpassing the previous July record set in 2023.  
  • The warming was widespread, with 95% of the Mediterranean displaying above-average temperatures and 63% of the basin exceeding the normal by at least 1°C, while 40% warmed by over 2°C. The western Mediterranean was most severely affected by these extreme anomalies.

 

sea Surface Temperature

In the global Ocean, July 2025 was the 3rd warmest July on record, with a mean Sea Surface Temperature (SST) of 20.80 ± 0.07 °C — behind only 2024 (20.91 ± 0.06 °C) and 2023 (20.93 ± 0.06 °C).

For the Mediterranean Sea, July 2025 was the warmest July on records with a mean SST of 26.68 ± 0.15°C, surpassing the 2023 record of 26.65 ± 0.21°C.

For North Atlantic, July 2025 was the 4th warmest July month with a mean SST of 24.11 ± 0.12 °C (ranking after 2020, 2024 and 2023).

 

 

July 2025 mean SSTs are above average for 71% of the global Ocean, with 13% exceeding the average by at least 1°C
  • 95% of the Mediterranean Sea showed monthly mean SSTs that were above average for a typical July, with more than half of the basin (63%) exceeding the average by at least 1°C, and 40% by at least 2°C. The western part is the most impacted by these high anomalies. Note that 58% of the basin experienced SSTs ranked amongst the 4 highest monthly SST recorded for July since 1993.
  • The North Pacific Ocean shows high SSTs at midlatitude, with a large region to the west where monthly SSTs reached 3°C above average.
  • The North Atlantic also shows widespread areas with above average SSTs, covering 75% of the area (between 0°N and 60°N). The highest anomalies are observed in the European region and on the western side off the North American coast (reaching more than 1.5°C above average). Further north, the Norwegian and Greenland Sea also experience widespread above average SSTs.
  • In the Indian Ocean, SSTs are predominantly between 0.5°C to 1°C above average.
  • Note: In the central Pacific, along the equator, we observe a large area of below-average SSTs (reaching -1°C).
July 2025 Mean SST Anomaly relative to a 30-year climatology (1993-2022), calculated using daily data from Mercator Ocean International’s GLO12 analysis and forecasting system for 2025 and from the GLORYS12 reanalysis for the climatological mean.

 

Ongoing Marine Heatwaves

The duration of ongoing MHWs is monitored because prolonged events may induce important stress on marine ecosystems.

Ongoing Marine Heatwaves on July 31, 2025 :

  • 21% of the global ocean (between 60°S and 60°N) was affected by MHWs on the 31st of July.
  • The majority of these MHWs developed recently, with 17% of the global Ocean (between 60°S and 60°N) affected by MHWs less than 1 month old and more than 3% by events between 1- and 2-month-old.
Month of onset for the ongoing MHWs on the 31th July 2025. The colors indicate the month during which the MHW started. For instance, ocean pixels in red correspond to an MHW that developed during July 2025 and are less than one month old, in orange MHW between 1 and 2 months old, etc.

Total Surface Marine Heatwaves

MHWs are classified into four categories: Moderate, Strong, Severe, and Extreme. We monitor daily the surface occupied by MHWs and show for the past year (since August 2024) the proportion of the global Ocean (between 60°S and 60°N) exposed to MHWs. We focus on the evolution over the past July 2025 (grey bar).

Global Ocean
  • The total surface of MHW (black line) ranges between 19% and 21% of the global ocean, increasing from the June extents that were the lowest for 2025.
  • These MHWs consist mainly of moderate and strong events (18% and 3%, respectively, by the end of the month).
  • For comparison, in July 2024, MHW total extent varied between 29% and 33% of the global ocean
Surface occupied by MHWs in the global ocean (between 60°S and 60°N) between August 2024 and July 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 July 2025.
Mediterranean Sea
  • For the Mediterranean Sea, the total MHW surface was highest at the beginning of July 2025 with 65% of the basin exposed, then decreases to 15% by the month.
  • This early peak is marked by intense MHW conditions: the proportion of strong MHWs exceeds that of moderate ones (33% against 23%), while the severe and extreme categories peak at 12% and 1%, respectively.
  • By the end of the month, MHW consists mostly of moderate categories (13%). For comparison, in July 2024, the total MHW extent increased from 20% to 56%, with mostly moderate and strong categories (not shown).
Surface occupied by MHWs in the Mediterranean Sea between August 2024 and July 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 July 2025.

 

North Atlantic (between 0°N and 60°N)
  • For the North Atlantic (between 0°N and 60°N), the total MHW surface varied around 19% during the month of July 2025.
  • These events consist mostly of moderate and strong categories (18% and 2%, respectively, by the end of the month).
  • For comparison, in July 2024, the total MHW extent increased from 37% to 47%, with mostly moderate and strong categories (29% and 16%, respectively, by the end of the month) (not shown).
Surface occupied by MHWs in the North Atlantic (between 0°N and 60°N) between August 2024 and July 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 July 2025.

Total Number Marine Heatwaves

We monitor the number of MHW days to highlight the region often impacted or impacted by long MHW. Longer and more regular MHWs increase heat stress on marine ecosystems, especially intense MHW (i.e. MHW of strong or higher category). Here we look at the total number of days where a MHW of strong or higher category occurred during July 2025. For instance, ocean regions in light green correspond to areas where a total of 10 to 15 days of strong or higher MHW were observed during the month of July of 2025.

  • 14% of the surface of the ocean was impacted by strong, or higher, MHW conditions for at least 1 day during July 2025. This is the third widest extent after 2023 and 2024 (20% and 22%, respectively). The northern Pacific Ocean, the European region and the Atlantic Ocean at 30°N are the most affected.
  • Regionally, 68% of the Mediterranean Sea was impacted by strong, or higher, MHW conditions for at least 1 day, which is the 2nd widest extent after 2023 (69%). 18% of the North Atlantic was impacted by such MHW, the 4th widest extent ever recording.
July 2025 MHW days of strong and higher category. Number of days during which a MHW of category strong, severe or extreme occurred in July 2025. Blank areas mean that no MHW of strong or higher category happened during the month. Blue shaded areas in the polar regions represent the sea ice maximum from the climatological period (1993-2022). Monitoring SST extremes with the MHW categorization approach as used here is not best suited for such regions.

Statistics

July 2025 Bubble plots

The MHW statistics (MHW duration, intensity, surface occupied and activity) for July 2025 are compared to those for July months from previous years (since 1993).

  • On a global scale, July 2025 is the 2nd most extreme July month in terms of MHW intensity (after 2023) and the 4th in terms of MHW duration (after 2015, 2024 and 2023). Average MHW extent did not reach levels observed in 2023 or 2024.
  • In the Mediterranean Sea, July 2025 is the most extreme July month on records in terms of MHW intensity and the 2nd and 3rd in terms of MHW extent and duration.
  • In the North Atlantic, MHW extent remained local to regional with July 2025 ranking as the 3rd most extreme July in terms of MHW intensity (after 2024 and 2023).

About the Bulletin

Mercator Ocean International 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).

 

Data sets 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. U. Copernicus Marine Service Information (CMEMS). 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 (CMEMS). 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 (CMEMS). 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 (CMEMS). 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 March 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. 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. 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 March (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