As on land, marine heatwaves are periods of abnormally high temperatures relative to the average seasonal water temperature in a particular region of the sea. Hence, marine heatwaves are extreme warm water events that come during shorter periods on top of the general trend of increasing seawater temperatures worldwide. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate found that marine heatwaves globally have doubled in frequency since 1982. While the Arctic is known to warm 2-3 times faster than the rest of the world, so far, only one study had investigated marine heatwaves in the Arctic, and none had focused on the Barents Sea – the epicenter of Arctic warming.
Increased occurrence, duration and intensity
Using daily high-resolution satellite data of sea-surface temperature, Nansen Legacy postdoctoral fellow Bayoumy Mohamed (UNIS) investigated the occurrence of marine heatwaves in the Barents Sea in detail. In total, Bayoumy and co-workers identified 72 marine heatwaves in the Barents Sea between 1982 and 2020, of which 54 took place after 2003. Hence, marine heatwaves have not only become substantially more frequent in the Barents Sea during the last two decades, but the increase in frequency is twice as high as what is seen globally, making the Barents Sea also in this respect to a hotspot of climatic changes.
“The rate of increase in marine heatwave frequency in the Barents Sea is double the global average, making the Barents Sea to a high-risk region for the ecological impact of marine heatwaves.”
Bayoumy Mohamed / UNIS
In addition to occurring more frequent, marine heatwaves in the Barents Sea also have become more long-lived and intensive with respect to water temperature. Local differences in heatwave frequency, duration and intensity exist, with the northern Barents Sea experiencing relatively high frequencies of heatwaves, whereas heatwaves in the southern Barents Sea have a tendency to last longer and be more intense.
2016 – two hundred days with heatwaves
During 2016, seven marine heatwaves took place in the Barents Sea. Taken together, they lasted for entire 200 out of the 365 days that year. The most intense heatwave started in the southern Barents Sea on 28 June and spread over the following 63 days northwards into the northern Barents Sea. At its maximum, sea surface temperatures lay more than 4˚C over the seasonal average water temperature, classifying this heatwave as «strong» compared to other heatwaves worldwide. As a result, fish shifted their geographic distribution, as documented by Nansen Legacy researcher Elena Eriksen (IMR). She and her colleagues documented how cod and herring aggregations shifted eastwards, while capelin aggregations shifted northward, and polar cod nearly disappeared in the core area of the southeastern Barents Sea.
Diverse biological responses
The example from 2016 illustrates the potentially far-reaching consequences of marine heatwaves on marine animals and the ecosystem as a whole. Yet, a recent Nansen Legacy publication under the lead of Bérengère Husson (IMR) demonstrates that the response of single species, populations and food web structure to marine heatwaves is not coherent from event to event. While some events led to geographical redistribution of fish on community-level at the scale of the whole Barents Sea, other events only triggered more local expansions of single species. The work suggests that species thresholds and lagged responses, as well as potential interactions with the general warming trend in the Barents Sea, set up new initial conditions that drive a unique ecological response to each marine heatwave. This hampers the predictability of the impact of marine heatwaves on the Barents Sea ecosystem, and introduces a challenge to the future sustainable management of the region.
References:
Eriksen E, Bagøien E, Strand E, Primicerio R, Prokhorova T, Trofimov A, Prokopchuk I (2020) The record-warm Barents Sea and 0-Group fish response to abnormal conditions. Frontiers in Marine
Science 7: 338. doi.org/10.3389/fmars.2020.00338
Husson B, Lind S, Fossheim M, Kato-Solvang H, Skern-Mauritzen M, Pécuchet L, Ingvaldsen RB, Dolgov AV, Primicerio R (2022) Successive extreme climatic events lead to immediate, large-scale, and diverse responses from fish in the Arctic. Global Change Biology 28: 3728– 3744. doi.org/10.1111/gcb.16153
Mohamed B, Nilsen F, Skogseth R (2022) Marine heatwaves characteristics in the Barents Sea based on high resolution satellite data (1982–2020). Frontiers in Marine Science 9. doi.org/10.3389/fmars.2022.821646