# The marine heatwave west of Ireland in June 2023

Gerard D. Mccarthy, Sandra Plecha, Guillaume Charria, Amélie Simon, Coline Poppeschi, Ana Russo

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## To cite this version:

Gerard D. Mccarthy, Sandra Plecha, Guillaume Charria, Amélie Simon, Coline Poppeschi, et al.. The marine heatwave west of Ireland in June 2023. Weather, 2023, 78 (11), pp.321-323. 10.1002/wea.4498 . hal-04452293

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Brought back my joy in seeing just as soon
as I had left behind the air of death
that had afflicted both my sight and
breast.

## Acknowledgements

This paper is published by courtesy of the Italian Air Force's *Rivista di Meteorologia Aeronautica*, where the original work in Italian was previously published.

## References

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digitaldante.columbia.edu/dante/divin e-comedy/.

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**Stabile G.** 2007. Dante e la filosofia della natura. Percezioni, linguaggi e cosmologie, Sismel, Firenze, Italy.

**Barbero A.** 2021. *Dante*, Translated by Allan Cameron. London: Profile Books.

## Further reading

The Divine Comedy: Inferno, Purgatorio, Paradiso di Dante Alighieri. Translated by Allen Mandelbaum, Everyman's Library, 1995, also available from https://web.archi ve.org/web/20210812201612/https://

Correspondence to: A. Fuccello fuccello.meteoam@gmail.com
© 2023 Royal Meteorological Society.

DOI: 10.1002/wea.4441

## Spotlight

# The marine heatwave west of Ireland in June 2023

Gerard D. McCarthy¹ ID,
Sandra Plecha²,
Guillaume Charria³,
Amélie Simon²,
Coline Poppeschi³ ID and
Ana Russo²

¹ICARUS: Irish Climate Research Unit,
Department of Geography, Maynooth
University, Ireland

²Universidade de Lisboa, Faculdade
de Ciências, Instituto Dom Luiz (IDL),
Lisboa, Portugal

³Ifremer, Univ. Brest, CNRS, IRD,
Laboratory for Ocean Physics and
Satellite Remote Sensing (LOPS), IUEM,
Brest, France

The summer of 2023 had been notable for a number of climate extremes: sea ice in the Antarctic dropped to its lowest for the time of year since satellite records began in the 1970s (NASA Earth Observatory, 2023), terrestrial heatwaves engulfed southern Europe and southern United States/Mexico that would have been 'virtually impossible' without climate change (WorldWea therAttribution, 2023), and sea-surface

temperatures (SSTs) in the North Atlantic reached their highest since satellite records began in 1982 (NOAA, 2023). In concert with these terrestrial heatwaves and high Atlantic SSTs, a severe marine heatwave (MHW) developed in the eastern North Atlantic, west of Ireland (Figure 1a). Except for a narrow band close to the coast from Greenland to Canada, SSTs everywhere in the North Atlantic were above the 41-year average (1982–2023), with many regions experiencing temperatures 2 degC higher than average. In the study region west of Ireland (highlighted in Figure 1b), the SST reached an impressive 4 degC above average.

A MHW is a discrete, prolonged (5 days or longer), intensely warm water (>90th percentile temperature for a given time of year) event (Hobday et al., 2016). MHWs have a direct impact on ocean ecosystems. Famously, extreme heat can lead to widespread coral bleaching in shallow water corals, mass mortalities of marine species and marine deforestation. In Irish waters, there is a recognition of the growing influence of warm water Lusitanian species over the cold water boreal species, such as exemplified by the growing numbers of anchovies in Irish waters (Vaughan et al., 2023). However, while studies of the North Sea and the

English Channel exist, the study of MHWs on ecosystems in Irish waters is limited.

The metric of MHW activity (Simon et al., 2022) allows us to quantitatively characterise MHW events for a certain period and study area. It is computed for each grid cell as the sum, over every event detected, of the product of their duration (days), intensity (degC) and area affected (km²). Therefore, the higher the occurrence, duration, intensity and area are, the higher will be the activity value. For satellite products, the area is the grid cell area of the detected event and for in situ, the area is taken to be 1km². The MHW activity between June 2023 and mid-July is shown in Figure 1(b). It shows a broad warming over the domain with maximum activity near the west coast of Ireland. The activity reached for the domain as a whole approached 60 degC day 10³km², which is approximately equivalent to temperatures of 2 degC above average lasting for the full month of June.

With such elevated temperatures, it is not surprising to find that the June 2023 MHW was confined to a shallow layer, near the sea-surface. Argo floats, which provide profiles of the top 2000m of the deep ocean, show the depth profile of ocean temperature offshore of Ireland during June 2023 (Figure 1c). In June, the North Atlantic has

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<figure><img src="image_5.png" /><figcaption>Figure 1. (a) Sea-surface temperature anomaly observed in June 2023 (degC) relative to 1982–2023. The black rectangle defines the study area (50–60°N, 20–5°W). (b) Activity of 2023 summer (June–July–August–September; JJAS) marine heatwave (MHW) (degC days 10³ km²) in study area. In situ observations from Irish meteorological buoys are shown in white open circles. Locations of Argo floats are shown with coloured open circles. (c) Depth profiles of the MHW based on Argo profiling float profiles from 11 to 15 June. Colours correspond to the locations in (b). (d) Times series of (red) satellite-derived summer (JJAS) and (black) buoy-derived June marine heatwaves mean activity. Dashed line represents the regression of a third-order polynomial. Daily sea-surface temperature (SST) data are from the NOAA Optimum Interpolation SST product (OISSTV2; Huang et al., 2020).</figcaption></figure>

cold, deep mixed layers of the winter keeping temperatures between 11 and 12°C for much of the upper ocean. On top of this cold layer, the MHW developed in the top 25m of water. While the action of wind and waves would typically mix colder deeper water with this warm upper layer, June 2023 was characterised by settled conditions that meant this did not occur (Met Éireann, 2023).

Observations of SSTs from satellites and in situ buoys allow us to contextualise this

MHW in terms of historical events. Figure 1(d) shows that the MHW activity in summer 2023 was the highest of any MHW in the waters west of Ireland since satellite records began in 1982. In situ meteorological buoy data (locations shown in Figure 1b) provide observations closer to the Irish coast. These data show the exceptional nature of this MHW where, looking at all June data, across all five met buoys (M2–M6), the activity in 2023 topped 60 degC days km², when the

previous highest value recorded was in 2014 peaked at approximately 15 degC days km². The increased frequency of warm events in Irish water in recent years is evident in the years 2021–2023 occupying 3 of the top 4 years, in terms of maximum MHW activity. Since 2000, there has been a linear positive trend in MHW in June (or June–July) to the west of Ireland.

Globally, MHWs have been increasing in frequency and duration largely in response
to rising mean ocean temperatures (Frölicher and Laufkötter, 2018; Oliver et al., 2018). However, since 2007, temperatures in Irish waters have been cooling (McCarthy et al., 2023), linked by some authors to a decline in the Atlantic Meridional Overturning Circulation, sometimes referred to as the Gulf Stream System (Caesar et al., 2018). The observed increase in MHWs in Irish waters presented here suggests that these waters are not immune from the effects of extreme ocean warming.

It is unlikely that Irish waters will escape the forecast increased frequency of MHWs for the world's oceans (IPCC, 2021). In this context, the MHW of June 2023 could well be a warning for the future.

## Acknowledgements

GM, SMP, AR and AS are supported by under the 2019 JPI Climate and JPI Oceans Joint Call project ROADMAP, carried out with the support of the Marine Institute and funded by the Irish Government in Ireland (Grant-Aid Agreement No. PBA/CC/20/01) and with the support of the Fundação para a Ciência e a Tecnologia, I.P./MCTES, through National Funds in Portugal (PIDDAC; grant no. UIDB/50019/2020 to Instituto Dom Luiz; project ROADMAP grant no. JPIOCEANS/0001/2019). AR and SMP also acknowledge the Fundação para a Ciência e a Tecnologia, I.P./MCTES: 2022.01167.CEECIND and 2021.00988.CEECIND, respectively. Open access funding provided by IReL. (Correction added on 24 November 2023, after first online publication: funding statement added.)

## Author contributions

**Gerard McCarthy:** Conceptualisation; writing – original draft; writing – review and editing.
**Sandra Plecha:** Formal analysis; writing – review and editing.
**Guillaume**

**Charria:** Formal analysis; writing – review and editing.
**Amélie Simon:** Formal analysis; writing – review and editing.
**Coline Poppeschi:** Formal analysis; writing – review and editing.
**Ana Russo:** Formal analysis; writing – review and editing.

## References

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**Huang B, Liu C, Banzon V et al.** 2020. Improvements of the daily optimum interpolation sea surface temperature (DOISST) version 2.1. *J. Clim.* **34**: 2923–2939. https://doi.org/10.1175/JCLI-D-20-0166

**IPCC.** 2021. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Masson-Delmotte V, Zhai P, Pirani A et al. (eds). Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA.

**McCarthy GD, Caesar L, Ulthaman A et al.** 2023. Chapter 03: physical oceanography. In: Irish Ocean Climate & Ecosystem Status Report. Nolan G, Cusack C, Fitzhenry D (eds). Marine Institute: Galway, Ireland, pp 25–35.

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[accessed 6 October 2023].

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**Oliver ECJ, Donat MG, Burrows MT et al.** 2018. Longer and more frequent marine heatwaves over the past century. *Nat. Commun.* **9**: 1–12. https://doi.org/10.1038/s41467-018-03732-9

**Simon A, Plecha SM, Russo A et al.** 2022. Hot and cold marine extreme events in the Mediterranean over the period 1982–2021. *Front. Mar. Sci.* **9**: 1–12. https://doi.org/10.3389/fmars.2022.892201

**Vaughan L, Minto C, Reid D et al.** 2023. Chapter 06: commercial fisheries. In: Irish Ocean Climate & Ecosystem Status Report. Nolan G, Cusack C, Fitzhenry D (eds). Marine Institute: Galway, Ireland, pp 75–91.

**WorldWeatherAttribution.** 2023. Extreme heat in North America, Europe and China in July 2023 made much more likely by climate change. https://www.worldweatherattribution.org/extreme-heat-in-north-america-europe-and-china-in-july-2023-made-much-more-likely-by-climate-change/ [accessed 6 October 2023].

Correspondence to: G. D. McCarthy
*gerard.mccarthy@mu.ie*

© 2023 The Authors. Weather published by John Wiley & Sons Ltd on behalf of Royal Meteorological Society.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

(Correction added on 24 November 2023, after first online publication: The copyright line was changed and legal statement added.)

doi: 10.1002/wea.4498

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