Gulf of Maine Warming Update: 2023
A Top-Five Hottest Year Despite Relief from Major Storm Systems
Over the past decade, scientists at the Gulf of Maine Research Institute have led a body of research that highlights the rapid pace of warming in the Gulf of Maine. To keep you informed, we share seasonal and annual updates about conditions in the Gulf of Maine.
Highlights from the Year
With an annual average sea surface temperature (SST) of 52.6°F — more than 1.94°F above normal — the Gulf of Maine experienced its 5th warmest year on record in 2023. This year fell short of the previous warmest year on record — 2021 — 1.49°F, but was record-breaking in a number of regards.
The first four months this year average monthly SST was within the top 3 warmest among all years on record. With February, March, and April all the warmest on record.
The most extreme temperatures in 2023 occurred in February and March, each setting new records for highest monthly average SST in the Gulf of Maine.
The “coolest” monthly average temperature anomalies for 2023 occurred during September, November, and December. Each of last five months this year had SSTs below the top ten warmest.
SSTs in 2023 were consistent with the long-term trend of increasingly warm conditions driven primarily by anthropogenic climate change.
Annual Warming Trends in the Gulf of Maine
Since the early 1980s, the rate of warming in the Gulf of Maine 0.87°F / Decade has been more than triple that of the world’s oceans (0.29°F / Decade Figure 1). The precise causes of the pronounced jump in annual average SSTs in the region from 2009 to 2012 remains an area of active research, but the sustained warm conditions suggest a regime shift in the influence of major ocean currents (i.e., Gulf Stream vs Labrador Current) in the Gulf of Maine. While a long-term increase in SSTs is expected as a result of human-caused climate change, we also expect each year to be influenced by large-scale patterns of natural variability, particularly on sub-global (i.e., regional) scales. SST conditions in the Gulf of Maine for 2023 retain the region’s distinction as being one of the fastest warming ocean regions on the planet; including the 2023 data shows that the Gulf of Maine is warming faster than 97% of the world’s ocean surface.
2023: SSTs Across the North Atlantic
In a departure from the now-familiar pattern of regionally concentrated warming, much of the attention this year was on large areas in the eastern Atlantic experiencing a large-scale MHW event. This summer scientists sounded the alarm over record setting temperatures for much of the Eastern Atlantic impacting population centers along the European coast and contributing to sweltering temperatures on land.
Beginning in March, the North Atlantic as a whole began experience day-after-day of record warm sea surface temperatures, particularly in the east near Europe. By June and July, the separation between the region’s SSTs and the previous records had widened by over a degree, creating a dramatic and alarming figure that was shared widely on social media. Figure 2 captures how profoundly and consistently hot this year’s temperatures were across the North Atlantic. SST in the North Atlantic set daily record highs for a total of 301 days this year (82%!). Every single day beginning on March 5th has set a new record daily SST values, with margins exceeding 1F. These facts have drawn increased attention, elevated concerns, and a renewed sense of urgency for understanding the causes behind the rapidly changing ocean environment.
2023: SST in the Gulf of Maine
As these broad regional patterns transpired, the Gulf of Maine was experiencing a range of conditions that stood in contrast to recent years and the timing of the North Atlantic’s record temperatures.
What began as one of the warmest years on record for the Gulf of Maine ended on the cooler side, with seasonal conditions not seen in the last decade.
Air-Sea Interactions: Extratropical Storm Lee
Observed SSTs may both influence and be influenced by atmospheric weather patterns. These interactions can act to amplify warming or cooling of surface waters. Sustained periods of low cloud cover and the consequent direct sunlight can rapidly warm the ocean at the surface. Steady winds can act as a fan, advecting heat away from the ocean and cooling the surface. Strong winds and storm systems may also lower SST through mixing, which can bring cooler water from depth closer to the surface.
During September, extratropical storm Lee traversed the Gulf of Maine, which weakened from hurricane strength as it crossed over Georges Bank on September 16 Figure 4. The impacts of this storm on the region’s SSTs can be seen in Figure 3 when temperature fell over 5F over a period of 12 days.
Similar storm crossings have occurred in the recent past, including in 2019 when Hurricane Dorian passed through the Gulf of Maine after working its way up the US East Coast. Both years had some of the fewest autumn heatwave days seen in the past two decades (2019: 0 days, 2023: 6 days).
Comparing Annual SST to Historical Conditions
When we compare the annual average SST in the Gulf of Maine for 2023 (52.6°F) to other years, we can see it narrowly beat out 2012 as the second warmest year on record. When we look at the deviation from the long-term average SST (i.e., the annual SST anomaly), the last decade stands out for its exceptional warmth (Figure 5). 2023 extends a pattern that began in 2010 of sustained above-average temperatures. With the exception of 2019 (ranked 13th), every one of the last ten years remains in the top 10 warmest years on record.
Average Monthly Conditions
Monthly average SSTs (Table 1.) show how variable conditions were throughout the year. For the first third of the year, monthly SSTs were among the top three warmest for those months. May, June, July, & August average SSTs ranked lower, ranging from 4th to 12th warmest. The final four months were each much cooler, ranging from 12th to 19th warmest for those months.
| Table 1. Monthly Average Sea Surface Temperatures - 2023 | ||||
| Month | Rank (1982-2023) | Observed Temperature | Climatological Average | Temperature Anomaly |
|---|---|---|---|---|
January |
3 |
46.5°F (8.1°C) |
43.5°F (6.4°C) |
3.0°F (1.7°C) |
February |
1 |
45.0°F (7.2°C) |
40.9°F (5.0°C) |
4.1°F (2.3°C) |
March |
1 |
43.7°F (6.5°C) |
40.1°F (4.5°C) |
3.6°F (2.0°C) |
April |
1 |
45.2°F (7.4°C) |
41.8°F (5.5°C) |
3.4°F (1.9°C) |
May |
6 |
48.2°F (9.0°C) |
46.5°F (8.1°C) |
1.6°F (0.9°C) |
June |
9 |
54.9°F (12.7°C) |
53.5°F (11.9°C) |
1.4°F (0.8°C) |
July |
4 |
63.6°F (17.5°C) |
60.5°F (15.8°C) |
3.1°F (1.7°C) |
August |
12 |
64.4°F (18.0°C) |
63.2°F (17.3°C) |
1.2°F (0.7°C) |
September |
14 |
61.8°F (16.6°C) |
61.2°F (16.2°C) |
0.6°F (0.3°C) |
October |
12 |
57.7°F (14.3°C) |
56.7°F (13.7°C) |
1.1°F (0.6°C) |
November |
16 |
52.2°F (11.2°C) |
51.7°F (11.0°C) |
0.5°F (0.3°C) |
December |
19 |
47.3°F (8.5°C) |
47.4°F (8.6°C) |
-0.1°F (-0.1°C) |
| Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data. | ||||
| Climatological Reference Period: 1991-2020. | ||||
Daily SSTs in the Gulf of Maine
The annual cycle of SST in the Gulf of Maine exhibits a pattern common to regions in the Northern Hemisphere, with the lowest temperatures observed in March and the highest values observed in August (Figure 6). Daily SST anomalies in 2023 never fell below +-1.79 °F compared to the long-term (1991-2020) average and reached as high as high as 5 °F above the long-term average.
The largest temperature anomalies were observed during November, which also turned out to be the warmest November on record (as highlighted in our Fall 2022 update). Figure 6 also illustrates that, in 2023, 135 days experienced SST anomalies that exceeded the threshold for being considered marine heatwaves, or MHW (more on MHWs below, including Figure 10)
SST Anomalis & MHW Events
A marine heatwave (MHW) is defined as a period when there are five or more consecutive days when the observed SST is greater than the 90th percentile of the long-term average for that day. Gaps of 2 days or less in this threshold do not constitute a break in the MHW event. Figure 6 illustrates that the Gulf of Maine met the criteria for experiencing a MHW for 135 days in 2023 (or 97% of the year). Superimposing MHW status (black line) over the full timeseries of daily SST anomalies (blue/white/red shading) (Figure 6) reveals that the frequency, duration, and intensity of MHWs has increased in the past decade.
Record Setting Daily Sea Surface Temperatures
Daily SST values in 2023 were some of the highest ever recorded in the Gulf of Maine. Record high SSTs were set for more than half of all days during the months of February, March, & April. February saw the most new records, with 86% of days setting new daily record highs.
Decline of Below Average SST Temperature Days
As SST in the Gulf of Maine continues to rise, the fraction of days when the region experiences below-average temperatures is expected to continue to fall. In a world without human-caused climate change, we would expect, positive (warm) and negative (cool) SST anomalies to more or less balance out over the span of several years, as various patterns of natural climate variability alternate having a dominant influence on Earth’s climate (e.g., La Niña vs El Niño). What is being observed in the Gulf of Maine (and elsewhere around the world), however, is a loss of that balance: larger fractions of recent years are experiencing above average temperatures and cold spells are becoming vanishingly rare.
During 2023 the Gulf of Maine experienced SSTs below the long-term average for a total of 48 days. A period of less than 2 months, and with no days experiencing temperatures that would qualify as marine cold-spells, the cool-water equivalent of a MHW. These events have become rare, with just 5 days of cold spell conditions in the last 20 years.
Gulf of Maine SSTs Out of Balance
The complete picture of above and below average conditions for the Gulf of Maine reveals a region out of balance. Since 2010 the Gulf of Maine has experienced a disproportionate number of anomalously hot SSTs and experienced many prolonged MHW events.
Quarterly SST Trends
Zooming in to sub-annual timescales reveals some interesting patterns: the observed rate of warming in the Gulf of Maine varies throughout the year. Comparisons across the four quarters of the year (Figure 11) reveal that the Gulf of Maine is warming fastest during July-September (~4x the global ocean average)—almost twice the rate seen during January-June (~3x the global ocean average).
Annual SST Anomaly Map
In addition to the sub-annual (or temporal) variation, there can also be significant spatial variability in annually-averaged SST patterns (Figure 12).
This year the impacts of large-scale variability in major oceanic currents, such as the relative influence of the Gulf Stream versus the Labrador Current was quite apparent. With large pockets of above and below average SST anomalies present along the turbulent path of the Gulf Stream to the South of our study area.
Quarterly Average SST Maps
Looking at these spatial patterns across the annual quarters of 2023 (Figure 13) shows not only the large spatial variability in SST conditions, but also the significant variation in those patterns over the course of the year.
Comparison to Global Temperatures
It was not just the Gulf of Maine that experienced exceptional warmth in 2023. Globally, 2023 was the warmest year for both sea surface temperatures and combined land & ocean temperatures.
Figure 15 shows annual average SST anomalies for oceans all over the world in 2023. From this view we can see how much of the Southern Ocean and expanses of the southeastern Pacific were anomalously warm (a general feature of El Niño conditions). Closer to home here in the United States, we see both unusually high and unusually low SSTs along the Gulf Stream’s path as it extends East away from Cape Hatteras.
Warming Rates of the Global Ocean
Percentile Warming Ranks
The Importance of the Gulf of Maine Region
For analyses like these that characterize environmental conditions for a particular area—in this case, the Gulf of Maine—it is important to be clear about the spatial extent that defines the region of study; different domain boundaries could produce different results. The spatial area we use to define the “Gulf of Maine” is displayed in Figure 17, and has remained consistent throughout our seasonal and annual warming updates.
A Note on Data Sources
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.
- NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.
Recommended Citation:
Gulf of Maine Research Institute. 2023. Gulf of Maine Warming Update: 2023. https://gmri.org/stories/warming-22