Record-breaking North Atlantic Ocean temperatures contribute to extreme marine heatwaves (2024)

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.

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.
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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.

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

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.

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?

Following the peak of the North Atlantic sea surface temperature anomalies on 21 June, temperature anomalies began to drop again towards the end of the month, although they remain above average. This follows a shift in the circulation, with stronger winds moving off the coast of west Africa, and northeast towards Ireland and the U.K.

Stronger winds have the effect of mixing the surface water with the cooler water below, cooling the sea surface temperatures. It is likely that the highest temperatures occurred in a relatively shallow layer at the surface of the ocean, meaning that this cooling response to the increased wind speeds can occur relatively quickly.

The marine heatwaves observed during June were likely a result of a combination of the short-term anomalous atmospheric circulation and longer-term changes, including variations in circulation and heat transport in the Atlantic, and the warming of our oceans due to climate change. Globally, mean sea surface temperature has increased, and continues to increase, since the pre-industrial era, while 90% of the extra heat associated with global warming has been taken up by the oceans.

The warm sea surface temperatures in the north Atlantic also impacted the Atlantic hurricane season, which officially began on 1 June, and has already seen three named storms, including two simultaneously, which is exceptionally rare for June.

With El Niño conditions typically suppressing hurricane activity in the Atlantic, these exceptional conditions highlight the complexity of the Earth system, and the importance of considering other aspects of climate variability alongside the developing El Niño.

These anomalous conditions in the north Atlantic have been short-lived compared to El Niño events, with warmer-than-average temperatures seen since March so far, in tandem with the onset stages of the El Niño. With El Niño predicted to become more pronounced through the rest of 2023 and into 2024, it remains to be seen how conditions will develop in the north Atlantic.

*C3S is implemented by the European Centre for Medium-Range Weather Forecasts on behalf of the European Commission.

**About the data

The analysis presented here makes use of the ERA5 reanalysis dataset, available through the Copernicus Climate Data Store. Further information on datasets and long-term evolution of variables mentioned here is available via the European State of the Climate About the data and Climate Bulletin About the data, pages as well as the Copernicus Climate Indicators dashboard.