Gulf of Maine Warming Update: Summer 2025

Reports | Oct 14, 2025

Over the past decade, scientists have led a body of research that highlights the rapid pace of warming in the Gulf of Maine. To help keep you informed, we share seasonal and annual updates about conditions in the Gulf of Maine.

Read on for an inside look at what we've learned in our summer 2025 Gulf of Maine warming update.

2025 summer seasonal warming update feature image

For this seasonal report we present an analysis of sea surface temperature (SST) for summer 2025, defined as the period extending from June 1, 2025 through August 31, 2025.

Summertime in the Gulf of Maine is a dynamic time of year for marine life and ocean enthusiasts alike. During the summer months, Gulf of Maine SST approaches its annual high, and the foraging opportunities for marine life hit their yearly peaks. The abundance of zooplankton surges in the summer in response to spring phytoplankton blooms they feed upon. Many species migrate into the Gulf of Maine to capitalize on this bounty of food. For example, Atlantic bluefin tuna and various whales migrate hundreds of miles for a once-a-year feeding opportunity. For year-round resident species, summer can be a period of stress, as this is when SST is most likely to approach or exceed suitable temperature ranges. For other species, these same warmer summer temperatures open the door to seasonal travel into the area.

This year, the average summer SST for the Gulf of Maine was 59.94°F. This seasonal average temperature was 0.83°F above the 1991 – 2020 climatological reference period (CRP) summer average of 59.11°F, ranking as the 15th warmest (30th coldest) summer on record (extending 44 years since satellite observations began in the fall of 1981).

Weekly Temperatures

Table 1 highlights how the SST for each week this summer compares to the 1991 – 2020 CRP for the area shown in Figure 1.

The observed SST, long-term average SST during the CRP, and SST anomalies (i.e., departures from the long-term average SST) are shown.

Weekly SST anomalies this summer ranged from as high as +2.49°F during mid-July, to as low as -0.30°F in mid-August.

Table 1. Observed weekly average SST, climatological average SST for that week, and SST anomaly (i.e., deviation from the climatological average) in the Gulf of Maine during summer 2025. The first row is just one day, as a result of how the weeks are split up.
Table 1. Observed weekly average SST, climatological average SST for that week, and SST anomaly (i.e., deviation from the climatological average) in the Gulf of Maine during summer 2025. The first row is just one day, as a result of how the weeks are split up.

Monthly Statistics

Table 2 shows the average monthly SSTs for the summer of 2025. The SST in June was above the 1991 – 2020 CRP. The SST anomalies became even warmer during July and then cooled, but remained positive, for August. July SST deviated most from the long-term climatological average, with an average SST anomaly of 1.41°F.

Table 2. Monthly ranking, observed average, climatological average, and deviation from the CRP average (i.e., temperature anomaly) for SST at a monthly resolution in the Gulf of Maine during spring 2025.
Table 2. Monthly ranking, observed average, climatological average, and deviation from the CRP average (i.e., temperature anomaly) for SST at a monthly resolution in the Gulf of Maine during spring 2025.

How Does this Summer Compare to the Past?

2025 was the 15th warmest (30th coldest) summer season observed in the Gulf of Maine during the 44 years we have satellite data to analyze.

Figure 1. A ranking of the 15 warmest summer seasons for the Gulf of Maine in the satellite record (1982 – 2025). 2025 was the 15th warmest (30th coldest) summer on record.
Figure 1. A ranking of the 15 warmest summer seasons for the Gulf of Maine in the satellite record (1982 – 2025). 2025 was the 15th warmest (30th coldest) summer on record.

Seasonal Trends and Long-Term Global Context

The Gulf of Maine is an area of particular interest to the scientific community due to the remarkable rate of warming it has experienced in recent years.

Figure 3 illustrates the average summer temperature anomaly for each year in the Gulf of Maine, overlaid with long-term SST anomaly trends for the Gulf of Maine (orange) and global oceans (blue).

The summer rate of warming for the Gulf of Maine (0.99°F per decade) is three times faster than the summer warming rate across the oceans globally (0.33°F per decade).

Figure 2. Average annual summer SST anomalies in the Gulf of Maine from 1982 through 2025 (black dots). The orange line indicates the trend for the full time series for the Gulf of Maine. The blue line indicates the trend for the full time series for the global oceans.
Figure 2. Average annual summer SST anomalies in the Gulf of Maine from 1982 through 2025 (black dots). The orange line indicates the trend for the full time series for the Gulf of Maine. The blue line indicates the trend for the full time series for the global oceans.

Extreme Temperature Conditions

A “marine heatwave” (MHW) is defined by daily average SSTs that exceed the 90th percentile of a climatological (i.e., 30-year) average for at least five consecutive days. A “marine coldspell” (MCS) is the cold-temperature counterpart to a MHW, and is identified using the same approach, but for temperatures below the 10th percentile. Gaps of two days or less in exceeding the threshold do not constitute a break in a single MHW/MCS event.

Using this broadly accepted definition, the Gulf of Maine has experienced no MHW or MCS events this summer (Figure 3). There was a brief period of temperatures above the 90th percentile threshold during May, but conditions did not persist for five days (only four days).

Figure 3. Time series of SSTs in the Gulf of Maine extending from January 1, 2025 through August 31, 2025. Black lines represent the long-term (i.e., 1991 – 2020) average SST, the 10th percentile, and 90th percentile for a given day in the Gulf of Maine. A solid line (red for marine heatwave, light blue for marine coldspells, or blue for non-event SSTs) indicates the observed SST this year. Red (above 90th percentile) and light blue (below 10th percentile) shading illustrates how far the observed SST is from the climatological average.
Figure 3. Time series of SSTs in the Gulf of Maine extending from January 1, 2025 through August 31, 2025. Black lines represent the long-term (i.e., 1991 – 2020) average SST, the 10th percentile, and 90th percentile for a given day in the Gulf of Maine. A solid line (red for marine heatwave, light blue for marine coldspells, or blue for non-event SSTs) indicates the observed SST this year. Red (above 90th percentile) and light blue (below 10th percentile) shading illustrates how far the observed SST is from the climatological average.

Heatmap of Temperature Anomalies and Extreme Events

Using our standard definition for extreme events, summer 2025 was mild with respect to MHW/MCS conditions, with no extreme hot or cold SST events occurring since the brief winter coldspell (Figure 4).

Elevated SST conditions (Figure 4, darker red shading) have been more frequent and persistent after 2010 and throughout the early 2020s in the Gulf of Maine. During this period, the frequency, duration, and intensity of marine heatwaves (Figure 4, red lines) have been more pronounced. Marine coldspells (Figure 4, blue lines) were common in earlier decades but have become rare, with only two short coldspells occurring in the last 15 years.

Figure 4. Heat map of daily SST anomalies from January 1982 through August 2025. Periods where SSTs meet or exceed the criteria for extreme event conditions (heatwaves and coldspells) have been overlaid with a horizontal bar (red for heatwaves, blue for coldspells) for the event’s duration.
Figure 4. Heat map of daily SST anomalies from January 1982 through August 2025. Periods where SSTs meet or exceed the criteria for extreme event conditions (heatwaves and coldspells) have been overlaid with a horizontal bar (red for heatwaves, blue for coldspells) for the event’s duration.

Spatial Distribution of Seasonal Anomalies

When viewed spatially, much of the Gulf of Maine region and the adjacent on-shelf areas experienced warm SST anomalies during summer of 2025 (Figure 5). Cool SST anomalies were found off the southern tip of Nova Scotia, the Nantucket shoals, and along the coast of downeast Maine into the mouth of the Bay of Fundy. Above-average SSTs occurred more within the interior of the Gulf of Maine, along the Western Gulf of Maine as well as the deeper waters just-inside the Northeast channel.

Figure 5. Map of average SST anomalies for each grid cell in the satellite record for summer 2025. The box outlined by the black dashed line denotes the region of study for the analysis (see Figure 7). Darker red regions indicate warmer anomalies, and darker blue regions indicate cooler anomalies. Black contours have been added at 100m and 200m depths.
Figure 5. Map of average SST anomalies for each grid cell in the satellite record for summer 2025. The box outlined by the black dashed line denotes the region of study for the analysis (see Figure 7). Darker red regions indicate warmer anomalies, and darker blue regions indicate cooler anomalies. Black contours have been added at 100m and 200m depths.

Spatial Distribution of Monthly Temperature Anomalies

Monthly average SST anomalies for Summer 2025 are shown spatially in Figure 6. During June, mild above-average SSTs occupied the interior of the Gulf of Maine, with below-average anomalies positioned over the Nantucket Shoals and in the mouth of the Bay of Fundy. During July, the above-average conditions were widely distributed up the coast and in the interior of the Gulf of Maine, with cooler anomalies remaining over Nantucket Shoals, along the coast of downeast Maine into the Bay of Fundy, and off the tip of Nova Scotia. By August, conditions had cooled (relative to the long-term average), with above-average conditions more concentrated within the interior Gulf of Maine over Jordan Basin and below-average SSTs expanding over Georges Bank and remaining off the tip of Nova Scotia and in the Bay of Fundy.

Figure 6. This series of maps shows the monthly average SST anomaly for June, July, and August 2025. The box outlined by the black dashed line denotes the region of study for the analysis. Darker red regions indicate warmer anomalies, and darker blue regions indicate cooler anomalies.
Figure 6. This series of maps shows the monthly average SST anomaly for June, July, and August 2025. The box outlined by the black dashed line denotes the region of study for the analysis. Darker red regions indicate warmer anomalies, and darker blue regions indicate cooler anomalies.

The Gulf of Maine Region

For analyses like these, it is important to be clear about the spatial extent that “defines” the Gulf of Maine (Figure 7), as different borders could produce different results. The spatial domain we use as the “Gulf of Maine” is outlined with the dashed line below. For this body of work we use a rectangular bounding box with longitudinal bounds from -70.875 W to -65.375 W, and latitudinal bounds of: 40.375 N to 45.125 N. This area is consistent with previous seasonal and annual SST reports that scientists at GMRI have produced.

Figure 7. Spatial domain used for Gulf of Maine SST analyses. For this body of work we use longitudinal bounds of: -70.875 W to -65.375 W, and latitudinal bounds of: 40.375 N to 45.125 N. Depth contours are colored at 100 m intervals up to 600 m; deeper blues indicate deeper off-shelf water depths.
Figure 7. Spatial domain used for Gulf of Maine SST analyses. For this body of work we use longitudinal bounds of: -70.875 W to -65.375 W, and latitudinal bounds of: 40.375 N to 45.125 N. Depth contours are colored at 100 m intervals up to 600 m; deeper blues indicate deeper off-shelf water depths.

Citing This Work

If you would like to cite this report, please use:

Gulf of Maine Research Institute. 2025. Gulf of Maine Warming Update: Summer 2025 https://gmri.org/stories/gulf-of-maine-warming-update-summer-2025

Note About the Data: The figures in this report are created using remotely-sensed satellite data as part of publicly funded research efforts. Satellite SST data was obtained from NOAA’s National Center for Environmental Information (NCEI), with all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

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