Unrealised extreme storylines of the 2023 North Atlantic marine heatwave revealed through ensemble boosting

In 2023, the North Atlantic experienced a marine heatwave (MHW) of unprecedented intensity and spatial extent, with sea-surface temperatures breaking records across large parts of the basin. This event raises the question of whether it reached the upper bound of MHW severity under current climate conditions. Here, we apply an ensemble boosting framework to MHWs for the first time, generating physically consistent storylines of the 2023 event conditioned on the observed pre-warming of the North Atlantic. This approach explores the upper tail of physically plausible extremes within a modelled climate system, revealing unrealised but credible extreme outcomes.

The boosted ensemble indicates that the 2023 MHW could plausibly have been both more intense and longer-lived than observed. In extreme storylines, events can persist for up to 579 days and reach a peak intensity of 4.5°C above climatology, compared with 240 days and 2.9°C in observations, likely implying substantially greater risks for marine organisms and ecosystems. Importantly, these amplified outcomes arise from the same mechanisms as the observed one, suggesting that no novel drivers are required. The most extreme storylines are predominantly atmospherically driven. They are characterised by persistently weak near-surface winds and shallow mixed-layers that reduce turbulent heat loss, together with sustained high-pressure conditions over the Euro-Atlantic region, particular in late spring, that suppress cloud cover and enhance shortwave radiative heating. Depending on their persistence and strength, these same drivers can give rise to either exceptionally intense or exceptionally long-lived MHWs. While events of this level of extremity are very rare under current climate conditions, we show that they become more apparent under global warming levels of 2.5 °C or more. By explicitly sampling the physically consistent upper tail, ensemble boosting provides a new perspective on extreme MHWs in a warming North Atlantic.