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

Marine heatwaves (MHWs) are prolonged periods (>5 days) of anomalously high sea surface temperature (SST) exceeding the 90th percentile of the daily climatology, with major ecological and socioeconomic impacts. In mid-latitudes they are typically short-lived (10–15 days) and linked to large-scale atmospheric pressure anomalies, with wind speed suppression commonly driving events. The shallow Northwest European Shelf (NWS) is particularly sensitive to regional drivers. Anthropogenic climate change has increased the likelihood, intensity, extent and duration of MHWs, with extreme events projected to become far more frequent under high-emission scenarios. This study investigates the origins, evolution, and impacts of the June 2023 NWS MHW, its feedbacks on regional weather, and its context within ongoing climate change.

Prior work defines and categorizes MHWs and documents their global drivers and impacts, including wind suppression and reduced latent heat loss as primary mechanisms in many regions. MHWs tend to be most intense in summer due to shallow mixed layers and higher solar variability. Studies show increasing trends in MHW intensity, duration, and spatial extent under anthropogenic warming, with rare events becoming common in high-emissions scenarios. The NWS’s shallow bathymetry enhances SST sensitivity to local forcing and tides. Background conditions in 2023 followed anomalously warm North Atlantic conditions in spring, with subsequent Mediterranean MHW in July. The study builds on satellite-based SST products (OSTIA), reanalyses (ERA5, EN4), long-term observatories (Western Channel Observatory), and previous analyses of weather regimes over Europe to contextualize the June 2023 event and explore its drivers and feedbacks.

The study integrates observations and high-resolution modeling. Datasets: OSTIA satellite-based SST analyses (near-real time and climate versions) provided 1982–2023 reference and anomalies; in situ gliders (Seaglider and Slocum platforms) in the Rockall Trough and east of Scotland provided 4 m near-surface temperatures, vertical profiles, and surface mixed layer (SML) depths; Western Channel Observatory (WCO) stations E1 and L4 supplied long-term subsurface temperature time series at 2 m and 50 m; EN4 objective analyses provided hydrographic context; WAVamm15_RAN wave reanalysis characterized wave climate; ERA5 reanalysis provided atmospheric fluxes and winds. A one-dimensional Price–Weller–Pinkel (PWP) mixed-layer model, initialized from an EN4 profile and forced by ERA5 fluxes, assessed the role of stratification and air–sea fluxes. A regional coupled prediction system (UKC3; atmosphere–ocean–waves) produced kilometer-scale coupled simulations (CPL) from 1 June 2023 with operational initial conditions and lateral boundaries, evaluating the evolution of SST, SML, tidal modulation, and mixed-layer heat budget across six stages using a standard European weather regime classification. Counterfactual regional atmospheric simulations used observed June 2023 SSTs (ATMostia) versus climatological SSTs (ATMclim) to quantify impacts on near-surface air temperature, winds, cloud, humidity, and precipitation; 18-member ensembles (ATMostia_ens, ATMclim_ens) starting 19 June assessed short-range predictability impacts on clouds and convective precipitation. A high-resolution regional wave reanalysis (WAVamm15_RAN) assessed wave activity anomalies. A set of sensitivity coupled runs initialized from different ocean years but with the same atmospheric forcing tested the role of ocean preconditioning and Atlantic advection. Future projections employed a 12-member regional ocean downscaling of the Hadley Centre Perturbed Parameter Ensemble (HadGEM3-GC3.05 PPE) under RCP8.5 (OCN_amm7_proj), providing transient 1990–2098 SSTs; a two-dimensional Hobday MHW algorithm quantified future MHW frequency, duration, and intensity. Mixed-layer heat budget: assuming a fully mixed SML, a bulk temperature tendency equation included surface fluxes (shortwave, longwave, latent, sensible), entrainment associated with SML deepening, and a residual R (including advection). Terms were computed hourly per grid point, using de Boyer Montégut SML depth (0.2 °C criterion, 3 m reference). Diagnostics included cumulative tendencies and tests with fixed versus evolving SML depth. Weather regime attribution used a 30-cluster classification for Northwest Europe. All analyses emphasized June 2023 stages (build-up, peak, persistence, breakdown) and spatial heterogeneity across the shelf and adjacent waters.

- Intensity and duration: NWS-average SST anomaly in June 2023 was +2.9 °C relative to climatology, constituting a shelf-averaged category II MHW that lasted 16 days—the longest in the 1982–2023 record. Local anomalies reached +5 °C (central North Sea and Irish shelf), with some coastal category IV conditions. The 7-day warming rate peaked at 2.4 °C (second-highest in 40 years). - Vertical structure: Gliders showed extremely shallow SML (<10 m) from ~10–20 June, deepening to 10–20 m by late June; WCO stations indicated reduced anomalies at 50 m as the mixed layer later deepened. - Drivers and evolution: Anticyclonic regimes brought high insolation (>600 W m⁻²), weak winds, weak waves (lowest June wave activity in 40 years), and advection of warm, moist tropical air reducing turbulent and longwave cooling. Neap tides contributed to SML shoaling (~1 m) and enhanced surface heating. Surface heat budget during build-up (stages 1–3) showed shortwave heating contributed +10.0 °C to the SML, partly offset by entrainment (−2.7 °C), longwave (−3.0 °C), and modest sensible/latent cooling (−1.6 °C). SML shoaling amplified surface heat flux impact (total surface flux contribution increased by ~85%). - Positive feedbacks: Once established, the MHW reduced low cloud by ~15% over the shelf (stage 4), increasing shortwave into the ocean by ~11% (up to +78 W m⁻²) and adding ~+0.4 °C to SST during stage 4 despite enhanced cooling by LW/SH/LH—maintaining the MHW. - Land weather impacts: The MHW increased 1.5 m air temperature over the British Isles by +1.1 °C on average during 19–25 June (stages 4–5), with peaks up to +1.5 °C in afternoon sea-breeze conditions. Winds over land increased via enhanced marine boundary layer momentum and stronger sea breezes. Specific humidity increased by ~7%, and precipitation increased by ~23% over the British Isles during stage 4, with higher convective rainfall probabilities linked to sea-breeze convergence; some events would likely not have occurred without the preceding marine warming. - Role of preconditioning: High-resolution coupled sensitivity simulations initialized from different ocean states all developed the MHW under the same atmospheric forcing, indicating ocean preconditioning and Atlantic advection were secondary to atmospheric forcing; salinity had limited impact on June stratification. - Climate context: A background +0.9 °C June warming over the last 20 years elevated the 2023 event from category I to category II (for 16 days). Trends show fewer cold spells and more warm spells across the NWS and at WCO stations, with a pronounced shift in the last ~8 years and disappearance of the NE Atlantic cold blob of 2015–2021. No clear recent trend in the specific weather regimes (WRs 5, 6, 9, 16) linked to the MHW build-up. - Projections: Under RCP8.5 PPE downscaling, NWS daily mean SSTs similar to June 2023 would be a small warm anomaly by 2040–2059, average by 2050–2069, and a cold spell by 2079–2098. Fraction of year experiencing MHWs rises from ~8% (2000–2019) to ~66% (2040–2059) and ~93% (2079–2098); by century’s end, ~39% of the NWS experiences MHWs >95% of the time. Average temperature above the 90th percentile threshold increases from +0.44 °C (2000–2019) to +0.89 °C (2040–2059) and +2.11 °C (2079–2098). Early summer warming trends strengthen relative to winter. - Forecasting implication: Fixed-SST regional NWP over 7 days degrades forecast quality during rapid SST change; time-varying SST improves short-range forecasts, as implemented at the Met Office.

The study demonstrates that exceptional atmospheric conditions—anticyclonic regimes yielding high insolation, weak winds, and suppressed wave activity, augmented by advection of warm, moist air—were the primary drivers of the rapid onset and intensity of the June 2023 MHW on the NWS. Ocean preconditioning and large-scale advection played secondary roles, as shown by sensitivity experiments. The shallow mixed layer was crucial, both enabling rapid surface warming through amplified shortwave absorption and limiting turbulent heat losses. Once formed, the MHW fed back on the atmosphere: over the sea it reduced low cloud, boosting incoming shortwave and maintaining the anomalous warmth; over land it enhanced sea-breeze circulations, increased near-surface air temperature and humidity, and elevated convective precipitation probabilities. These findings link oceanic extremes directly to impactful land weather, quantitatively attributing a portion of the record UK June warmth (+0.6 °C of the +0.9 °C record) to the MHW. In a broader climate context, a multi-decade warming background elevated the event’s category, and projections indicate that SSTs like those of June 2023 will become typical by mid-century under high emissions, with MHWs occupying most of the year by century’s end. Operationally, accurate representation of evolving SSTs and ocean–atmosphere coupling is essential for short-range hazard prediction and for realistic regional climate projections.

The June 2023 NWS marine heatwave was unprecedented in duration and intensity at the shelf-average scale, driven primarily by anticyclonic weather with high solar radiation, weak winds and waves, and warm, moist air that collectively shoaled the mixed layer and accelerated surface warming. The event exhibited strong ocean–atmosphere coupling: it reduced low cloud to maintain ocean heating and amplified land-sea breezes, contributing to record-breaking UK June temperatures and increased convective rainfall. A +0.9 °C background June warming over the last two decades elevated the event’s category, and high-emissions projections indicate that similar SSTs will be commonplace by mid-century, with MHW conditions dominating the annual cycle by century’s end. Key contributions include disentangling the roles of atmospheric forcing versus ocean preconditioning, quantifying mixed-layer heat budget terms and positive cloud feedback, and attributing land weather impacts to the MHW. Future directions include: monitoring weather regimes 5, 6, 9, and 16 for early MHW warnings; advancing coupled, time-varying SST approaches in NWP and regional climate projections; and undertaking ecosystem impact and resilience studies to inform adaptation to more frequent and persistent MHWs.

- Attribution of the anticyclonic period to climate change remains uncertain; no clear trend in the specific weather regimes (WRs 5, 6, 9, 16) was identified. - Mixed-layer heat budget includes a residual term R (e.g., advection), indicating not all processes are explicitly resolved in the diagnostic budget. - Land and ocean feedback estimates rely on regional model experiments (ATMostia/ATMclim, CPL) and short ensemble forecasts; results are conditioned on model physics and initial/boundary conditions. - Future projections are based on a high-end scenario (RCP8.5) with a high climate sensitivity PPE; quantitative frequencies and intensities may differ under other scenarios or models. - Some author affiliations and auxiliary details for all institutions were not fully enumerated in the provided text; however, this does not affect the scientific conclusions.