Exceptional atmospheric conditions in June 2023 generated Northwest European marine heatwave which contributed to breaking land temperatures records

The Northwest European shelf experienced unprecedented surface temperature anomalies in June 2023 (anomalies up to 5 °C locally, north of Ireland). The shelf average underwent its longest recorded category II marine heatwave (2.9°C anomaly for 16 days). With state-of-the-art observation and modelling capabilities, we completed an in-depth analysis of its origins, its feebacks on the weather and its likelihood of future recurrence. We showed the marine heatwave developed quickly due to strong atmospheric forcing (record high level of sunshine, weak winds, tropical air) and record weak wave activity under two weeks of anticyclonic weather regimes. Additionally, tidal activity (neap/spring tide) modulated the marine heatwave and contributed to its spatial heterogeneity. Once formed, this shallow marine heatwave fed back on the weather: over the sea it reduced cloud cover, helping to sustain the marine heatwave itself. Over land, it contributed to breaking June mean temperature records and to enhanced convective rainfall through stronger, warmer and moister sea breezes. This marine heatwave was intensified by the last 20-year warming trend in sea surface temperatures. Such sea surface temperatures are projected to become commonplace by the middle of the century under a high greenhouse gas emission scenario. This study was enabled by strong collaborations between observers and modellers of the ocean, waves and atmosphere. It highlights the necessity of long-term ocean observation for monitoring regional seas, complemented by regional reanalyses and climate projections. It also illustrates the usefulness of regional coupled systems for process understanding, short term weather forecasting and their potential for adding value to regional climate information.