Mercator Ocean International
Ocean Temperature Bulletin

Ocean Temperature Bulletin
September 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

  • September 2025 was the 3rd warmest September on record, with a mean sea surface semperature of 20.76 ± 0.08 °C.
  • 9% of the surface of the ocean was impacted by strong-or-higher MHW conditions for at least 1 day during September 2025 – the 4th widest extent after 2015, 2024 and 2023.
  • September 2025 was the 5th most extreme September month in terms of MHW events, after 2015, 2024, 1997 and 2023.

European region

  • Mediterranean Sea – September 2025 is amongst the 5th warmest September with a mean SST of 26.27 ± 0.14°C — the 26.43 ± 0.28°C record was reached in 2024.
  • In the last month, intense marine heatwaves in the Mediterranean sea occupied 5% of the basin  –  September 2024 registered 61%.
  • North Atlantic region – September 2025 was the 8th warmest September month with a mean SST of 24.79 ± 0.02 °C. September 2025 was  characterized by regional MHW events of intensity and duration higher than the majority of Septembers in the historical period.

sea Surface Temperature

  • September 2025, at global level (between 60°S and 60°N), was the 3rd warmest September on record, with a mean sea surface temperature (SST) of 20.76 ± 0.08 °C — behind only 2024 (20.87 ± 0.06 °C) and 2023 (20.95 ± 0.07 °C).
  • Regionally, for the Mediterranean Sea, September 2025 is amongst the 5 warmest September with a mean SST of 26.27 ± 0.14°C — the record of 26.43 ± 0.28°C was reached in 2024.
  • For the North Atlantic, September 2025 was the 8th warmest September month with a mean SST of 24.79 ± 0.02 °C.

 

 

Daily sea surface temperatures averaged for the global ocean (top), the Mediterranean Sea (middle) and the North Atlantic Ocean (bottom) 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.

 

Sea surface temperature anomalies

September 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.

September 2025 mean SSTs are above average for 68% of the global ocean (between 60°S and 60°N), with 11% exceeding the average by at least 1°C (Figure 2).

  • 87% of the Mediterranean Sea showed above-average SSTs, with the highest SST above average on the western part. Noteworthy, 33% of the basin showed monthly means of more than 1 °C above average.
  • The North Pacific Ocean showed widespread high SSTs at midlatitude spanning the entire width of the basin, with marginal seas on the western boundary showing the largest departure from the long term average with monthly SSTs of 3°C above average. The equatorial region is characterized by a large below average SST continuing the transition from ENSO neutral to La Niña conditions observed in previous months.
  • The North Atlantic showed above average SSTs for 68% of the area (between 0°N and 60°N) mostly mild except towards the Arctic region where large above-average temperature (reaching more than 1.5°C above average) are observed.
  • In the Indian Ocean, SSTs were predominantly between 0.5°C to 1°C above average, with higher values to the east along the Indonesian coast and below-average SSTs to the west.

Ongoing Marine Heatwaves

30 September 2025

  • 17% of the global ocean (between 60°S and 60°N) was affected by MHWs on the 30th of September.
  • The majority of those MHWs developed recently, with 13% of the global Ocean (between 60°S and 60°N) affected by MHWs less than 1 month old and more than 2% by events between 1- and 2-month-old.
Month of onset for the ongoing MHWs on the 3Oth September 2025. The colors indicate the month during which the MHW started. For instance, ocean pixels in red correspond to an MHW that developed during September2025 and are less than one month old, in orange MHW between 1 and 2 months old, etc.

Total Surface Marine Heatwaves

Surface occupied by MHWs in the n the global ocean (between 60°S and 60°N), in the Mediterranean Sea and in the North Atlantic (between 0°N and 60°N) between October 2024 and September 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 September 2025

Global Ocean

  • The total surface of MHW (black line) decreased from 20% to 17% of the global ocean (between 60°S and 60°N). These MHWs consist mainly of moderate events (~15-17%) and to a lesser extent strong events.
  • For comparison, in September 2024, MHW total extent fluctuated between 25% and 29% of the global ocean (between 60°S and 60°N).

Mediterranean Sea 

  •  The total MHW surface fluctuated between 10% to 36% of the basin, with a peak at 29% mid-month. These MHW events consisted of moderate categories only, except for some localized strong event during the peak (<2%).
  • For comparison, in September 2024, the total MHW extent varied between 74% and 20%, with the surface occupied by strong and extreme categories peaking at 34% and 10%, respectively (not shown).

North Atlantic

  • For the North Atlantic (between 0°N and 60°N), the total MHW surface fluctuated between 13% and 17% during the month of September 2025. These events consist mostly of moderate and strong categories.
  • For comparison, in September 2024, the total MHW extent fluctuated between 41% to 33%, with the noteworthy presence of strong (~13%) severe (~5%) and extreme (0.8%) categories at the peak of the event (not shown).

Total Number Marine Heatwaves

Total number of intense MHWs in September 2025

Marine heatwave days are monitored globally to highlight the region often impacted or impacted by long MHWs. Longer and more regular MHWs increase heat stress on marine ecosystems, especially intense MHW (i.e. MHW of strong or higher category). Analysis focuses on MHWs of strong or higher category which occurred in September 2025. 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 September of 2025.

 

 

  • 9% of the surface of the ocean was impacted by strong-or-higher MHW conditions for at least 1 day during September 2025. This is the 4th widest extent after 2015, 2024 and 2023. The marginal seas in the western North Pacific Ocean, the northern European region, the Gulf of Mexico and Caribbean sea in the North Atlantic are the most affected (Figure 7).
  • Regionally, 10% of the North Atlantic (between 0°N and 60°N) was impacted by strong, or higher, MHW conditions for at least 1 day, which makes it the 7th most intense September month considerably lower than the previous two years (41% for 2024 and 55% for 2025).
  • In the Mediterranean Sea, 5% of the region was affected by strong-or-higher MHWs (61% in 2024).
Year by year evolution of the surface impacted by strong and higher MHW during months of September. The size of the bar represents the proportion of the global ocean (60°S to 60°N) exposed to a strong or higher MHW during the month. As indicated by the colours, the proportion exposed to such MHWs is categorized according to the number of days impacted during the month.

Statistics

  • On a global scale, September 2025 is the 5th most extreme September month in terms of MHW after 2015, 2024, 1997 and 2023. These events remained however regional.
  • In the Mediterranean Sea, September 2025 was affected by long and intense MHWs on average but not as extreme as September 2022 and 2024 .
  • In the North Atlantic, September 2025 was characterized by regional MHW events of intensity and duration higher than the majority of Septembers in the historical period.

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