Exceptional atmospheric conditions in June 2023 generated a northwest European marine heatwave which contributed to breaking land temperature records

Marine heatwaves (MHWs), defined as prolonged periods of anomalously high sea surface temperatures (SSTs), are significant events with potentially severe ecological and socioeconomic consequences. These consequences range from coral bleaching in tropical regions to regime shifts in temperate ecosystems and the intensification of coastal urban heat islands. While MHWs in mid-latitudes tend to be shorter-lived due to the annual temperature cycle and jet stream variability, they are still a cause for concern. Wind speed suppression is frequently implicated in MHW formation, often accompanied by reduced latent heat loss. Summer tends to be a prime season for intense MHWs because of shallower mixed layers, weaker winds, and higher solar radiation variability. Continental shelves, given their shallow depths, exhibit heightened sensitivity to local and regional drivers influencing SSTs. Anthropogenic climate change has considerably increased the likelihood of MHWs, and projections indicate a concerning future trend of increasing intensity, extent, and duration across all warming scenarios, with even extreme events becoming more frequent under high emission scenarios. This paper focuses on the exceptional MHW that affected the Northwest European shelf (NWS) in June 2023, examining its characteristics, origins, feedbacks on the weather, and its implications within the context of a changing climate, particularly in light of globally unprecedented SST levels reached in August 2023.

Existing literature extensively documents the impacts and characteristics of marine heatwaves. Studies highlight the devastating effects on coral reefs (Shlesinger & van Woesik, 2023), the significant alterations to temperate marine ecosystems (Wernberg et al., 2016), and the contribution to coastal urban heat intensification (Hu, 2021). Global assessments of MHWs and their drivers (Holbrook et al., 2019; Oliver et al., 2021; Sen Gupta et al., 2020; Vogt et al., 2022) reveal that wind speed reduction is a primary factor in their formation. The impact of ocean stratification on MHWs is also discussed. The role of anthropogenic climate change in increasing the frequency and intensity of MHWs is well-established (IPCC, 2022; Frölicher et al., 2018; Oliver et al., 2018), with projections indicating a grim future of more frequent and severe events. The study builds upon this existing knowledge, focusing on a specific, high-impact event in the NWS region, and adds to the understanding of regional MHW drivers and their impacts on the interconnected ocean-atmosphere system.

This study employed a multifaceted approach combining observational data and state-of-the-art modeling capabilities to investigate the NWS MHW of June 2023. The analysis utilized several key datasets: Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) provided daily SST data; in-situ glider and Western Channel Observatory (WCO) data offered high-resolution measurements; EN4 data provided objective analyses of ocean temperature and salinity profiles; ERA5 reanalysis data provided atmospheric information; and a coupled model, UKC3, was used to investigate the event's dynamics. The UKC3 system is a high-resolution coupled atmosphere-ocean-wave model, integrating the Met Office Unified Model with NEMO and WAVEWATCH III. Several experiments were conducted, including simulations with observed and climatological SSTs to assess the MHW's impact on the weather. A surface mixed layer heat budget was applied using bulk formulae and an entrainment term to examine the dynamics of SST changes. The study also used regional ocean downscaling of the Hadley Centre Perturbed Parameter Ensemble (PPE) under the RCP8.5 scenario to provide future projections of MHW frequency and intensity. Weather regime classifications were employed to categorize atmospheric conditions. This comprehensive approach enabled the researchers to analyze the interplay between atmospheric forcing, ocean dynamics, and the resulting feedbacks on both oceanic and atmospheric systems, ultimately providing a detailed picture of the 2023 MHW.

The June 2023 MHW in the NWS was exceptional in both intensity and duration. SSTs soared, reaching anomalies up to +5°C locally, and the NWS-averaged anomaly constituted a category II MHW lasting 16 days—unprecedented in the last 40 years. The rapid onset of the MHW, with a transition from category I to II in just 6 days, was remarkable. The study found that persistent anticyclonic weather patterns played a crucial role. These patterns, characterized by weak winds, high sunshine, and the advection of warm, moist air, led to extremely shallow mixed layers, significantly enhancing solar radiative heating. While ocean pre-conditioning contributed to background warming, the primary driver was atmospheric forcing. Once established, the MHW exhibited a positive feedback mechanism with low-level cloud cover: reduced cloud cover caused by warmer SSTs further increased solar radiation into the ocean, maintaining extreme conditions. The MHW exerted a significant influence on weather over land, leading to record-breaking June temperatures in the UK and a 23% increase in rainfall. Analysis showed that the MHW was responsible for a +0.6°C increase in the UK's record June temperature. The increased rainfall was linked to stronger, warmer, and moister sea breezes. Future projections, based on RCP8.5 scenarios, suggest that SSTs observed during the 2023 MHW will become average conditions by mid-century, and cold spells by the end of the century, indicating a dramatic increase in the frequency and intensity of MHWs in the coming decades.

The findings demonstrate the profound and multifaceted impacts of an extreme MHW event on both the ocean and atmosphere. The rapid intensification of the MHW due to the unique interplay of atmospheric conditions highlights the vulnerability of the NWS to extreme weather events. The positive feedback loop between the MHW and reduced cloud cover emphasizes the close coupling between the ocean and atmosphere. The significant impact on land temperatures and rainfall underscores the strong regional connections between oceanic and atmospheric processes, with implications for heatwaves and precipitation patterns. The substantial increase in future MHW frequency and intensity, as projected in this study, poses serious ecological and socioeconomic risks and necessitates comprehensive adaptation and mitigation strategies. The study's findings also emphasize the need for advancements in regional weather prediction models to account for time-varying SSTs, as this can significantly impact forecast accuracy during rapid warming events.

The June 2023 MHW in the NWS was an unprecedented event driven primarily by exceptional atmospheric conditions. The MHW's feedback mechanisms and substantial impact on land temperatures and rainfall highlight the strong ocean-atmosphere coupling and the far-reaching consequences of marine heatwaves. Future projections indicate a stark increase in MHW frequency and intensity, underscoring the urgency of addressing climate change. Further research should focus on improving regional climate models and conducting detailed impact assessments to understand the broader effects of such events on ecosystems and society. Continued monitoring of specific weather regimes could improve MHW forecasting.

While this study provides a comprehensive analysis of the June 2023 MHW, several limitations should be noted. The study relies on model simulations which involve inherent uncertainties, and although the models are validated against observation, there are still limits to their capability to perfectly replicate the physical processes. The focus on the NWS region limits the generalizability of some findings to other regions. Future studies could explore the MHW's impacts on specific ecosystems and the longer-term consequences of frequent, intense MHWs. Further refinements in coupled models could provide even more accurate projections of future MHW occurrences.