The waters around Great Britain and Ireland have recently been affected by the 4 category (extreme) marine heatwave, while the North Atlantic Ocean continues to experience extraordinary warmth. In the coming months, we will have a clearer picture of the impact of these heatwaves on ecosystems. In the meantime, we are understanding how frequent this phenomenon is.
In the Black Sea, the average duration of marine heatwaves has been between 13 and 19 days. For the analysed time period (1895-2022), an increase in the frequency of marine heatwave occurrences has been observed, ranging from 0.75 to 2.25 cases per decade, especially in the eastern region of the Black Sea. Therefore, not only do these heatwaves occur in the Black Sea, but their number and duration have increased since 1985.
What are marine heatwaves?
We are already familiar with heatwaves in the atmosphere – periods of high temperatures (compared to the normal values for a specific region and time of year) that extend for several days (generally more than three consecutive days).
According to the latest IPCC report, the frequency, duration, and intensity of heatwaves have increased globally compared to the pre-industrial period. Climate projections indicate that these changes in heatwave characteristics will continue with the increasing global average temperature.
However, there are also heatwaves that occur in seas or oceans, known as marine heatwaves. In the circumstances of a marine heatwave, the temperature at the ocean or sea surface is high – compared to the climatology for a specific region and time period – for an extended period of time (at least five consecutive days).
Recent examples of marine heatwaves include:
(1) The Mediterranean Sea heatwave from May to August 2003, when the sea surface temperature was 2-3°C higher than in previous summers – this event was record-breaking and had a significant impact on ecosystems.
(2) The heatwave in the northwest United States in 2012, when the surface temperature of the Atlantic Ocean was 2.5°C higher than the multi-year average (it was the longest-lasting event for this region).
(3) The heatwave in 2010/2011 in western Australia, when the surface temperature of the ocean was 3°C higher than the multi-year monthly average (another record event), resulting in the migration of some fish species towards the south.
Just like the atmospheric heatwaves, marine heatwaves have a significant impact on ecosystems. For example, they increase mammal mortality, coral bleaching, habitat modifications over time and space affecting the fishing industry, and the proliferation of algae. These heatwaves can also impact the formation of tropical cyclones and contribute to ocean stratification, acidification and deoxygenation.
Causes of marine heatwave occurrences
Marine heatwaves are generated by multiple factors that vary depending on the event and region.
Thus, marine heatwaves are generated by ocean currents that lead to the formation of regions with higher surface temperatures compared to neighbouring regions.
The heat exchange between the atmosphere and the ocean (the atmosphere warming the ocean surface) also contributes to the occurrence of heatwaves.
Wind also has an effect on heatwaves: it can intensify or reduce a marine heatwave. Large-scale atmospheric processes over relatively long periods of time, such as El Niño (shown in the right panel), can modify the likelihood of marine heatwave occurrences for specific regions.
To be inserted: https://youtu.be/WPA-KpldDVc
Distribution of marine heatwaves
In a study published in Nature Communications in 2019, Neil Holbrook and his colleagues examined the characteristics of global marine heatwaves. The study used satellite observations of sea surface temperatures between 1982 and 2016. Figure 1 presents the intensity, duration, and frequency of marine heatwaves. In the Mediterranean Sea, an average of 2-3 marine heatwaves per year were observed, with an average duration of 5-15 days and an average intensity of 1.5-4°C.
Figure 1. Characteristics of marine heatwaves in different regions (a) during 1982 – 2015: (b) intensity, (c) duration and (d) apparition frequency
Marine Heatwaves and Climate Change
With the increase in global average temperature, the temperature of the oceans and seas is also rising. Consequently, the probability of marine heatwave occurrences is increasing. Marine ecosystems can adapt to temperature changes within certain limits, but they cannot adapt to extreme events such as marine heatwaves.
A study published in the journal Nature Communications in 2018 showed that between 1925 and 2016, the frequency of global marine heatwave occurrences increased by 34%, and their duration increased by 17%. This increase can be attributed to the rise in average ocean temperatures. That being said, in the future, we can expect an increase in the number of days with marine heatwaves if global warming continues.
The marine heatwave in the region of the United Kingdom in June 2023
As we have seen above, marine heatwaves are not uncommon in the Mediterranean Sea. However, such events are extremely rare in the northern Atlantic region. Figure 2 shows the sea surface temperature anomaly for June 18, 2023, compared to the long-term average from 1981 to 2016.
Figure 2. Sea surface temperature anomaly compared to the long-term average (1981-2016) for June 18, 2023. Copyright: ESA (Data: NOAA)
We observe that for the northern Atlantic region, particularly in the region of the United Kingdom, the sea surface temperature anomaly is around 5°C for this period, while the northern regions of Europe experience anomalies of up to 8°C. Apart from being extremely rare, this event is very intense and unusual for this time of the year.
As marine heatwaves evolve, they can be classified based on intensity (sea surface temperature anomaly) into Category 1 as moderate events, Category 2 as intense events, Category 3 as severe events, Category 4 as extreme events, and Category 5 as beyond extreme events.
Therefore, the marine heatwave in the region of the United Kingdom belongs to Category 4 (Figure 3). Considering the extent of this heatwave, from southern Iceland to the west coast of Africa, we may be witnessing the most extreme marine heatwave in modern history for the northern Atlantic region.
Figure 3 (animation). Marine heatwaves between May 20 and June 19, 2023. Data source
Before the onset of this marine heatwave, high values (approaching record values) for sea surface temperature were recorded globally in April and May (>21°C starting from April). Additionally, these high temperatures were associated with a decrease in Sahara dust concentration over the northern part of the Atlantic Ocean due to weaker winds.
When Sahara dust moves over the ocean surface, a portion of the solar radiation is reflected. In the absence of Sahara dust, the dark ocean surface absorbs the solar heat. Another factor contributing to the increase in ocean temperature is the reduction of atmospheric pollution associated with maritime transport. The sulphur content in the fuels used in maritime transport is a major air pollutant with adverse effects on human health. Reducing the sulphur content in these fuels also means reducing the aerosols in the atmosphere, which have a cooling effect by reflecting solar radiation.
In the coming months, we will have a clearer picture of the impact of this marine heatwave on the ecosystems in the region of the United Kingdom. However, until then, we need to analyse how we can mitigate the impact of such events. First and foremost, we must reduce greenhouse gas emissions. Additionally, we need to develop monitoring capabilities for heatwaves and invest in research on these extreme events. For example, studies on thermal limits for different species or the development of regional or global programs.
Are there marine heatwaves in the Black Sea?
Figure 4. (a) Average annual number of days that are part of heatwaves, (b) average annual number of heatwaves, and (c) average duration of heatwaves for the Black Sea in the period 1985-2022.
To answer this question, we used a dataset containing daily information on sea surface temperature developed by NOAA, which is also depicted in Figure 3. According to NASA, this dataset “is one of the best and most consistent global daily sea surface temperature products at a 5 km resolution.”
To analyse marine heatwaves in the Black Sea, we used a product based on CoralTemp SST, which indicates the category of each marine heatwave (values ranging from 1 – moderate to 5 – extreme) for each day and each 5 km pixel. The average number of days that are part of a marine heatwave ranges from approximately 40 days per year in the western part of the Black Sea to around 70 days per year in the eastern region (Figure 4a). Marine heatwaves were more frequent (between 3.25 and 3.75 heatwaves per year) in the region of the eastern coast of the Black Sea (Russia and Georgia) and south of the Crimean Peninsula (Figure 4a). In our region, between 2.5 and 3 marine heatwaves are observed annually.
In the Black Sea, the average duration of marine heatwaves ranges from 13 to 19 days, with shorter durations in the western part (13-15 days) compared to the eastern part (17-19 days) (Figure 4c). For the analysed time interval (1985-2022), there has been an increase in the frequency of marine heatwaves between 0.75 and 2.25 cases per decade, especially in the eastern region of the Black Sea (Figure 5a). Additionally, the duration of marine heatwaves has increased since 1985, ranging from 0.5 to 7 days per decade (Figure 5b). Therefore, not only do these heatwaves exist in the Black Sea, but both their number and duration have increased since 1985.
Figure 5. (a) Trend in the number of marine heatwaves and (b) trend in the average duration of marine heatwaves for the Black Sea in the period 1985-2022. The grey dots represent the pixels where results are statistically representative.
Bogdan Antonescu
He is a physicist specialised in atmospheric physics, with an interest in history, climatology, physical processes, and the impact of extreme weather phenomena. Currently, he leads the project “Extreme weather events in the future climate of Romania (ClimExRo)”, which aims, among other objectives, to bring academic research closer to the public. More details about this project can be found on the project’s webpage.
Article first published on InfoClima