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Record-breaking North Atlantic Ocean temperatures contribute to extreme marine heatwaves [1]

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Date: 2023-07

Global average sea surface temperatures last month reached unprecedented levels for June. The north Atlantic Ocean recorded exceptionally warm sea surface temperatures, with several extreme marine heatwaves. These were related to short term changes in atmospheric circulation and longer-term changes in the ocean, according to analysis from the Copernicus Climate Change Service (C3S*). So far this year, discussions of our oceans and climate have largely focused on the onset of El Niño, recently declared by the World Meteorological Organization, and its potential for pushing global temperatures into “uncharted territory” by the end of 2023 and into 2024. But in fact, we have already entered uncharted territory due to the exceptionally warm conditions in the north Atlantic Ocean.

North Atlantic View Sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

DOWNLOAD IMAGE Pacific View Sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

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Already in May, sea surface temperatures globally were higher than in any previous May on record, and this continued into June with even larger anomalies compared to the average. Orange and red colours in the map above show the areas where the sea surface temperatures were above the 1991-2020 climatological average last month. The record-breaking sea surface temperature anomalies in the north Atlantic, with localised extreme heatwaves, are clearly visible. On the right-hand map above, the tropical Pacific shows signs of the developing El Niño, with warmer-than-average sea surface temperatures in the central and eastern equatorial Pacific. Several areas, such as the northwest Pacific and Sea of Japan, the southwest Pacific southeast of New Zealand, and the western Indian Ocean southeast of Madagascar, also recorded marine heatwaves.

North Atlantic temperatures off the charts

Average sea surface temperatures for June 2023 across the north Atlantic were the warmest in the ERA5** record for the time of year by a very large margin, at 0.91°C above average. This is around 0.5°C more than the previous warmest June, recorded in 2010. The anomalous warmth is particularly striking in the northeastern Atlantic Ocean, at 1.36°C above average for the month as a whole, as shown in the chart below. Sea surface temperature anomalies (°C) averaged over the northeastern Atlantic region for the month of June from 1979 to 2023. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

DOWNLOAD IMAGE Temperatures in the northeastern Atlantic climbed steadily from the end of May, peaking on 21 June at around 1.6°C above average, as shown in the Daily SST Anomalies chart below. While absolute temperatures do reach higher values in the region during the summer, the average temperatures observed last month are more typical of later in the summer. Before June 2023, the earliest such temperatures have been observed in the ERA5 data record, shown below since 1979, is towards the end of July.

Daily SST Anomalies Daily sea surface temperature anomaly (°C) averaged over the northeastern Atlantic region during 2023 (black line) and for previous years from 1979 to 2022 (red and blue lines). Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

DOWNLOAD IMAGE | DOWNLOAD DATA Daily SSTs Daily sea surface temperature (°C) averaged over the northeastern Atlantic region during 2023 (black line) and for previous years from 1979 to 2022 (grey lines). The average conditions for 1991-2020 are shown by the dashed line. Data source: ERA5. Credit: Copernicus Climate Change Service / ECMWF.

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The region was affected by several marine heatwaves. West of Ireland, NOAA’s Marine Heatwave Watch classified the heatwave locally up to a Category 5 (‘Beyond Extreme’), with sea surface temperatures as high as 4-5°C above average at its peak. Category 1 marine heatwave conditions were already present in this area since early June, reaching extreme conditions between 15 and 24 June before returning to category 1 at the end of the month. Category 4 (‘extreme’) marine heatwave conditions occurred around much of Ireland and the U.K., and in the Baltic Sea. More widely, Category 2 (‘strong’) conditions extended further south and west into the Bay of Biscay and along the northwest coast of Africa into the subtropical and tropical Atlantic. Marine heatwaves result from a combination of atmospheric and oceanographic processes and can significantly impact marine life and bring about extreme weather events.

A combination of contributing factors

While the causes of the anomalous warmth across the northeastern Atlantic are still a matter of research, there are already several contributing factors to take into consideration. These include atmospheric circulation, air pollution and climate change trends.

Circulation Average mean sea level pressure and 10m wind fields for June from 1991-2020 (left) and June 2023 average mean sea level pressure and 10m wind fields. Data source: ERA5. Credit: Copernicus Climate Change Service / ECMWF.

DOWNLOAD IMAGE Anomalies Mean sea level pressure anomalies (hPa) averaged over the Azores High region for the month of June from 1940 to 2023. Data source: ERA5. Credit: Copernicus Climate Change Service / ECMWF.

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During June, the atmospheric circulation over the North Atlantic basin was unusual. The Azores High, a semi-permanent area of high atmospheric pressure over the Atlantic Ocean, was much weaker than average – the weakest in the ERA5 data record for June, by a very large margin, as shown in the map and the chart above. This follows 10 years during which the Azores High was close to or above average at this time of year. A weakened Azores High is associated with a weakening of the winds that are typical of the region. The ERA5 data show that surface wind speeds were below average over most of the northeastern Atlantic during June. Wind speed values that were 20-30% below average often coincided with the largest positive sea surface temperature anomalies, particularly in the subtropical north Atlantic. Averaged over the northeastern Atlantic region, June 2023 saw the largest negative wind speed anomaly in the ERA5 record. Surface wind speeds are closely linked to sea surface temperatures because reduced wind speeds lead to a reduction in the mixing of surface water with the cooler water below, allowing the sea surface temperatures to increase. Lower wind speeds also weaken the upwelling of deeper, colder water along the Canary current, an ocean current flowing southward along the coast of West Africa and then westward into the North Atlantic near the Cape Verde islands. A further consequence of weaker surface winds is a reduction in the movement of Saharan dust westward over the north Atlantic. The lack of vertical mixing in the ocean may have contributed to the rapid increase in the sea surface temperatures, while the reduction in Saharan dust over the North Atlantic also reinforced the increasing temperatures. Typically, Saharan dust has the effect of scattering solar radiation back into space before it reaches the ocean surface. Met Éireann, the state meteorological service of Ireland, also highlighted that in the period leading up to the marine heatwave, there was high pressure to the north, with very little wind and sunny conditions causing the sea surface temperatures to warm significantly. Animation showing both the surface (10m) wind speed anomaly (%) and sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

DOWNLOAD GIF On longer timescales, there are several other factors that may have influenced or reinforced the warm anomalies. One is of course the influence of climate change and global warming due to increasing greenhouse gas concentrations. Another factor is the decreasing particulate pollution over the northern hemisphere, in particular from Europe and North America. While the reduced pollution is beneficial for the environment and human health, less pollution could also have an impact on the amount of solar radiation being scattered back into space, leading to increased warming. A further consideration is the multi-decadal fluctuation of north Atlantic circulation and the transport of heat. These complex fluctuations are a topic of ongoing research and observation, and likely driven partly by global warming and partly by intrinsic variability.

A long term or a temporary shift?

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[1] Url: https://climate.copernicus.eu/record-breaking-north-atlantic-ocean-temperatures-contribute-extreme-marine-heatwaves

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