2021 Gulf of Maine Warming Update: Hottest Year on Record

Announcements | Mar 28, 2022

With an annual average sea surface temperature (SST) of 54.14 °F — more than 4 °F above normal — the Gulf of Maine experienced its warmest year on record in 2021. The region experienced the warmest fall on record and the second warmest summer, amid a marine heatwave event that lasted the entirety of 2021 — a first for the region.

White font is layered on top of wave-like ridges of dark blue, light blue, green, orange, and red (from top to bottom).

Annual Sea Surface Temperatures

The annual average SST in the Gulf of Maine for 2021 was 54.14 °F; this was 4.17 °F above the long-term (i.e., 1982-2011) average. Last year topped the previous warmest year on record — 2012 — by a remarkable 0.5 °F. Since the early 1980s, the rate of warming in the Gulf of Maine has been nearly triple that of the world’s oceans.

Figure 1. A timeseries of annual average sea surface temperature anomalies for the Gulf of Maine (solid black line) from 1982 through 2021, illustrating that 2021 was the warmest year on record. Long-term trendlines for the Gulf of Maine (blue) and the entire globe (green) show how much more quickly the Gulf of Maine is warming compared to the rest of the world.
Figure 1. A timeseries of annual average sea surface temperature anomalies for the Gulf of Maine (solid black line) from 1982 through 2021, illustrating that 2021 was the warmest year on record. Long-term trendlines for the Gulf of Maine (blue) and the entire globe (green) show how much more quickly the Gulf of Maine is warming compared to the rest of the world.

There can be significant spatial variability in annually-averaged SST patterns. The Gulf of Maine in 2021 illustrated this fact with the region southeast of Georges Bank experiencing the highest SST anomalies. This area is more susceptible to large-scale variability in major oceanic currents, such as the relative influence of the Gulf Stream versus the Labrador Current, than other areas of the Gulf of Maine.

Figure 2. Map of annual average sea surface temperature anomalies in 2021. The box outlined by the black dashed line denotes the region of study for the analysis. Darker red regions indicate stronger anomalies.
Figure 2. Map of annual average sea surface temperature anomalies in 2021. The box outlined by the black dashed line denotes the region of study for the analysis presented throughout this report.

When we compare 2021 to other unusually warm years in the Gulf of Maine, we see that in absolute terms (the top stack of bars in the figure below), the difference between the warmest five years may not seem so large, but when we look at the deviation from the long-term average SST (i.e., the annual SST anomaly), 2021 really pops out, putting a finer point on just how unusually warm the Gulf of Maine was this past year.

Figure 3. Bar plots of absolute sea surface temperature (SST) values (top panel) and SST anomalies (bottom panel) for the top five warmest years in the Gulf of Maine.
Figure 3. Bar plots of absolute sea surface temperature (SST) values (top panel) and SST anomalies (bottom panel) for the top five warmest years in the Gulf of Maine.

Worldwide Temperatures

It was not just the Gulf of Maine that experienced exceptional warmth in 2021, however. According to NOAA, 2021 was the 4th warmest year on record for the contiguous United States and the 6th warmest year globally. The figure below shows annual average SST anomalies for oceans all over the world in 2021. While much of the Southern Ocean and expanses of the southeastern Pacific off South America were anomalously cool, most of the world’s oceans experienced unusually warm temperatures in 2021. This is particularly true for James Bay in Canada, the Labrador Sea, the Baltic Sea, and a swath from ~35 °N to ~45 °N across ocean basins — a region that includes the Gulf of Maine.

Figure 4. Map of annual average sea surface temperature anomalies for the world’s oceans in 2021. Warmer anomalies are darker shades of red. Cooler than normal temperatures are light blue.
Figure 4. Map of annual average sea surface temperature anomalies for the world’s oceans in 2021.

Daily Sea Surface Temperatures

The annual cycle of SST in the Gulf of Maine exhibits a familiar pattern with low temperatures in March and high temperatures in August. The average difference between the annual maximum SST in August and the annual minimum SST in March is 22.91 °F. In 2021, the difference between the maximum SST (Aug 22nd, 68.88 °F) and minimum SST (Mar 17th, 41.16 °F) was 27.72 °F, with daily SST anomaly values never falling below +1.51 °F and reaching as high as +7.36 °F above the long-term average.

More Record Hot Days

The figure below shows the highest recorded SST values for every day of the year in the Gulf of Maine. Record-setting days that occurred in 2021 are denoted with red dots. Indeed, record-setting daily high SSTs in the Gulf of Maine were recorded for at least 2 days in each month of 2021, with the most set in October, which experienced record high SSTs for all 31 days. In total, 2021 set 169 record high temperature days in the Gulf of Maine; stated another way, nearly 46% of days in 2021 experienced record high SSTs in the Gulf of Maine.

Figure 5a. Timeseries of the record high daily sea surface temperature in the Gulf of Maine for every day of the year. Record-breaking temperatures that were observed in 2021 are denoted with red dots; when a record-breaking temperature for a given day was observed in any other year, that daily record is in gray.
Figure 5a. Timeseries of the record high daily sea surface temperature in the Gulf of Maine for every day of the year. Record-breaking temperatures that were observed in 2021 are denoted with red dots; when a record-breaking temperature for a given day was observed in any other year, that daily record is in gray.
Figure 5b. A bar plot showing the percentage of days during each month in 2021 when a record-high temperature was observed in the Gulf of Maine (e.g., 100% — or every day — of October 2021 experienced a record-setting high temperature).
Figure 5b. A bar plot showing the percentage of days during each month in 2021 when a record-high temperature was observed in the Gulf of Maine (e.g., 100% — or every day — of October 2021 experienced a record-setting high temperature).

More Persistent, Intense Heatwaves

A marine heatwave (MHW) is defined as a period when there are 5 or more consecutive days when the observed SST is greater than the 90th percentile of the long-term average for that day. In the figure below, the long-term average is illustrated with the gray dashed line and the threshold for being a MHW day is illustrated by the dashed red line. The solid red line in the figure shows the observed SST for that day.

The Gulf of Maine met the criteria for MHW status for 360 days in 2021. Furthermore, successive heatwaves separated by a gap of two days or fewer are considered part of the same event. Because the stretches falling below this threshold lasted no longer than two days (March 17, April 25-26, and July 31-August 1), the Gulf of Maine experienced a marine heatwave event that lasted the entirety of 2021 — a first for the region.

While the warmest temperatures of the year were experienced in August (as we would expect from the climatology for the region), the largest temperature anomalies were observed during the last days of June, when they breached 7 °F above the long-term average.

Figure 6. Annual timeseries of daily average sea surface temperature (SST) for the Gulf of Maine. The annual cycle of the long-term average SST for 1982-2011 (gray dashed line) informs the definition of a “marine heatwave threshold” (red dotted line). Observed daily SST values (solid red line when above heatwave threshold, solid blue line when below) for 2021 illustrate that the Gulf of Maine experienced marine heatwave conditions for almost the entire year.
Figure 6. Annual timeseries of daily average sea surface temperature (SST) for the Gulf of Maine. The annual cycle of the long-term average SST for 1982-2011 (gray dashed line) informs the definition of a “marine heatwave threshold” (red dotted line). Observed daily SST values (solid red line when above heatwave threshold, solid blue line when below) for 2021 illustrate that the Gulf of Maine experienced marine heatwave conditions for almost the entire year.

Comparing daily SST anomalies and MHW status for 2021 to the longer-term record — as in the figure below — it becomes clear that the frequency, duration, and intensity of MHWs has not only increased in the past decade, but reached a peak last year. Under historical levels of natural variability in the region, hot and cold SST anomalies tend to balance out within a year or over several years. What is being observed in the Gulf of Maine, however, is a loss of that balance: larger fractions of recent years are experiencing above average temperatures and cold spells are becoming vanishingly rare.

Figure 7. Heat map of daily sea surface temperature anomalies from the beginning of 1982 through the end of 2021. Not only do more large warm anomalies (darker reds) appear more frequently in recent years, but the frequency and duration of marine heatwave events (black lines) in the Gulf of Maine has become more pronounced in the past decade.
Figure 7. Heat map of daily sea surface temperature anomalies from the beginning of 1982 through the end of 2021. Not only do more large warm anomalies (darker reds) appear more frequently in recent years, but the frequency and duration of marine heatwave events (black lines) in the Gulf of Maine has become more pronounced in the past decade.

Warming Rate

GMRI research was the first to reveal that the Gulf of Maine has been warming faster than the vast majority of the world’s ocean. The figure below updates this historical analysis by including data for 2021. Indeed, the story has not changed: it’s clear that the Gulf of Maine is warming faster than ~98% of the world’s ocean. The reason why this is so important is because the rate of change can have profound consequences for the biology of individual species and for entire food webs. These aspects are areas of ongoing work (and captured well in the recent IPCC Working Group II report on impacts, adaptation, and vulnerability).

Figure 8. Maps illustrating the spatial variability in how fast different areas of the Gulf of Maine (left) and the world’s oceans (right) are warming from 1982 through 2021. Yellow areas indicate regions that are warming faster than 97.5% of the world’s oceans. Areas in black show regions where the warming rate over that time period is at or below the 80th percentile of warming rates across the globe.
Figure 8. Maps illustrating the spatial variability in how fast different areas of the Gulf of Maine (left) and the world’s oceans (right) are warming from 1982 through 2021. Yellow areas indicate regions that are warming faster than 97.5% of the world’s oceans. Areas in black show regions where the warming rate over that time period is at or below the 80th percentile of warming rates across the globe.

Concluding Thoughts

In recent years, SSTs in the Gulf of Maine (and adjacent waters) have increased and the rate of warming has accelerated. Research suggests there are two major factors at play, one oceanographic and one atmospheric:

  1. Oceanographic: The Gulf of Maine is influenced by two primary oceanic currents: the Labrador Current and the Gulf Stream. Historically, a stronger southward flow of the Labrador Current has kept the heat transported northward along the U.S. east coast by the Gulf Stream further out to sea. But changes are occurring in the interplay between the Labrador Current and Gulf Stream. More Arctic-origin freshwater from melting sea ice and land-based ice is constricting the southward flow of the Labrador Current, which is also allowing the Gulf Stream to spread out more at lower latitudes around the Gulf of Maine. This is allowing warmer water to “spill over” into the Gulf of Maine.
  2. Atmospheric: The North Atlantic Oscillation — a large-scale pattern of natural variability in the atmosphere — has been in a “positive phase” more frequently over the past 10-15 years, meaning a more zonal (i.e., west-to-east) atmospheric pattern has dominated in the region. The relative decrease in “waviness” of the jet stream inhibits more consistent intrusions of cooler, Arctic air into the region.

The relative contribution of these two climatological drivers may vary through time, but the result (in sea surface temperature terms) is similar: a warmer Gulf of Maine.

A Note on Data Sources

Recommended Citation: Gulf of Maine Research Institute. 2022. 2021 Gulf of Maine Warming Update: Hottest Year on Record. https://gmri.org/stories/warming-21

Contributing Scientists

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