Unlucky Weather Could Have Made the 2023 North Atlantic Marine Heatwave Far Worse

In the summer of 2023, sea surface temperatures across the North Atlantic surged to levels that stunned oceanographers and climate scientists alike. The event, which unfolded against a backdrop of already record-breaking global warmth, was widely described as unprecedented in the observational record. Yet a new modeling study suggests that the 2023 North Atlantic marine heatwave, dramatic as it was, may have been a relatively restrained version of what the atmosphere and ocean were actually capable of producing. Under slightly different weather patterns, the same starting conditions could have generated an event of far greater intensity, duration, and cumulative ecological stress.

The research, published in Nature Communications by Catherine H. Gregory, Urs Hofmann Elizondo, Thibault Guinaldo, and Thomas L. Frölicher, uses a technique known as ensemble boosting to explore physically plausible alternative versions of the 2023 event. Rather than asking what did happen, the team asked what could have happened under the same large-scale climate conditions. The answer is sobering. In their boosted simulations, marine heatwaves emerged that lasted up to 579 days, reached maximum intensities of 4.5 degrees Celsius above the long-term climatology, and accumulated a total heat stress of 717 degree-days. By comparison, the observed 2023 event lasted roughly 240 days, peaked at about 2.9 degrees Celsius above normal, and accumulated 224 degree-days of cumulative intensity.

Ensemble boosting is a relatively new approach in the study of extreme events, and it addresses a persistent problem in climate science: state-of-the-art Earth system models are run as large collections of simulations, but only a handful of members will ever reproduce a truly rare extreme. The technique works by taking a coupled Earth system model and selectively perturbing the initial conditions of individual model members during periods that resemble the preconditioning of a real event. These perturbations are tiny, physically consistent nudges that represent the natural chaos of the atmosphere and ocean. If a nudged simulation goes on to produce an extreme event, that event is not an artifact; it is a legitimate alternative evolution of the same climate system, one that the dice of weather variability could plausibly have rolled.

Applied to the North Atlantic in 2023, the method produced a family of storylines that bracket the observed event and extend well beyond it. The most extreme simulated heatwaves were not freak occurrences requiring exotic physics. They arose from the same oceanic and atmospheric ingredients that produced the real event, arranged in somewhat less favorable configurations. This is the central message of the study: the margin between the 2023 heatwave we experienced and a substantially more severe outcome was narrower than most people would assume, and it was governed largely by the luck of day-to-day and season-to-season weather.

The detailed analysis of the longest and most intense boosted events reveals a set of contrasting mechanisms that conspire to build and sustain extreme ocean warmth. A central player is persistent high atmospheric pressure over the Euro-Atlantic region. When a high-pressure system settles over the North Atlantic, it suppresses cloud formation and calms the winds. Calmer winds mean less evaporative cooling, so the ocean loses less heat to the atmosphere through turbulent fluxes. Clearer skies, meanwhile, allow more incoming shortwave radiation to penetrate the sea surface and warm the upper ocean directly. The combination of reduced turbulent heat loss and enhanced solar heating acts like a lid and a magnifying glass at once, allowing heat to accumulate in the surface layers week after week.

The third mechanism identified by the researchers involves the redistribution of heat within the upper ocean. Marine heatwaves are not simply a story of local heating; they also depend on how heat is stirred, mixed, and transported. In the most extreme simulated events, changes in ocean circulation and mixing patterns kept warm water trapped near the surface or advected additional warmth into the affected region, reinforcing the surface anomaly. The interplay between these atmospheric drivers and oceanic redistribution helps explain why some boosted events persisted for well over a year and a half, far outlasting the observed event, while others of comparable peak intensity faded more quickly.

These findings matter because the consequences of a marine heatwave scale with both its intensity and its duration. Peak temperature anomalies determine the immediate physiological stress on organisms such as corals, fish larvae, and plankton, while cumulative intensity, often measured in degree-days, captures the prolonged exposure that can push ecosystems past tipping points. A heatwave lasting 579 days with a cumulative intensity more than three times that of the observed 2023 event would represent a fundamentally different level of stress for North Atlantic ecosystems, fisheries, and the communities that depend on them. The boosted storylines therefore function as a stress test, revealing the upper range of what present-day climate variability can deliver.

The study also places these present-day possibilities in the context of future warming, and this is where the implications become most striking. The researchers show that events comparable to the observed 2023 heatwave, which today still qualifies as an extreme outlier, become increasingly common as global warming progresses. In a warmer climate, the baseline temperature of the North Atlantic rises, so the same weather patterns that produced the 2023 anomaly would operate on a warmer starting point, pushing temperatures into ranges that would today seem almost implausible. In effect, the boosted ensemble offers a preview of the kinds of events that routine greenhouse warming will make ordinary. What was a near-miss in 2023 could, within decades, fall within the range of typical summer conditions.

The North Atlantic holds particular importance in the Earth system, influencing weather over Europe and North America, absorbing substantial amounts of heat and carbon dioxide, and hosting some of the most productive fisheries in the world. Prolonged extreme warmth in this basin can alter stratification and nutrient supply, shift the distribution of commercially important species, and contribute to atmospheric circulation anomalies that affect terrestrial heatwaves and droughts. The 2023 event already prompted intense scientific debate about its causes, with proposed explanations ranging from reduced aerosol pollution from shipping to El Niño development and unusual atmospheric circulation. This new study does not settle that debate, but it reframes it: whatever the underlying preconditioning was, the weather-dependent upper tail of possible outcomes was much more severe than what materialized.

Methodologically, the work demonstrates the growing value of storyline-based approaches to climate risk. Instead of assigning a single probability to an observed extreme, ensemble boosting generates a spectrum of physically self-consistent alternatives that illuminate which processes control an event’s severity and how close reality came to a worse outcome. For decision-makers in fisheries management, coastal planning, and marine conservation, such storylines are arguably more useful than abstract return periods, because they describe concrete scenarios that could plausibly unfold next time the same preconditioning arises. As computing resources expand and coupled models improve, techniques of this kind are likely to become a standard tool for assessing the risks posed by compound and record-shattering extremes, on land as well as at sea.

The broader takeaway is a reminder that the record books of the last few years should not be read as a ceiling. The 2023 North Atlantic marine heatwave was extraordinary, but the physics of the present-day climate system, probed systematically by the boosted simulations of Gregory and colleagues, permits events that are longer, hotter, and more cumulative in their impact. As global warming continues to raise the baseline, the gap between what is observed and what is possible will narrow from the other direction, with once-hypothetical extremes becoming the new benchmark. Preparing for the worst plausible outcome, rather than merely the worst recorded one, is emerging as an essential principle for understanding and adapting to the changing ocean.

Subject of Research: Alternative climate model scenarios of the 2023 North Atlantic marine heatwave and its potential for greater extremity

Article Title: The 2023 North Atlantic marine heatwave could have evolved into a far more extreme event

Article References: Gregory, C. H., Elizondo, U. H., Guinaldo, T., & Frölicher, T. L. (2026). The 2023 North Atlantic marine heatwave could have evolved into a far more extreme event. Nature Communications. https://doi.org/10.1038/s41467-026-78863-5

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78863-5

Keywords: marine heatwave, North Atlantic, ensemble boosting, Earth system model, ocean warming, climate extremes, sea surface temperature, Euro-Atlantic high pressure, cumulative intensity, climate change, physical oceanography, future warming