| 11 Mar 2026
The past evolution of marine heatwaves and their drivers in the southern North Sea
Abstract. Marine heatwaves (MHWs) are defined as prolonged periods of anomalously high ocean temperatures. These events can have severe impacts on marine ecosystems and, if they occur at the surface, can feed back on the atmosphere, changing inland air temperatures and precipitation.
We use a comprehensive set of model, reanalysis, and observational datasets to investigate recent changes in North Sea MHWs. All datasets show a significant warming trend, accompanied by a marked increase in the frequency of MHWs. In contrast, the maximum intensity of MHWs has decreased in many regions of the North Sea, including the German Bight. If the linear trend in temperature is removed, only few MHWs have been detected after 2019, suggesting natural variability has damped the effect of the long-term warming.
While distinct weather patterns are associated with the onset of MHWs, their occurrence alone is not sufficient to trigger them. As the heat content is an integrated quantity, the ocean temperature at the beginning of the season (ocean preconditioning) is a key factor, in addition to prevailing weather patterns during the season. As a consequence, only in winter we find a significant dependency of MHWs on established climate indices. Instead, MHWs result from a combination of short-term, weather-related, variability and longer-term seasonal to decadal variability.
Furthermore, we find that the evolution of the surface temperature in the German Bight is largely determined by local atmospheric conditions rather than remote variability in the Atlantic. Although the inflow of warm water through the English Channel is important, it is the atmosphere that controls its volume transport and temperature. Whether the atmospheric conditions themselves are linked to remote variability in the Atlantic Ocean remains to be studied.
This paper uses a wealth of data sources, principally models and re-analyses, to consider the relative influence of atmospheric and oceanic drivers on the generation of marine heatwaves (MHW) in the North Sea, specifically in the German Bight. They report that, save for in winter, weather patterns and climate indices (e.g. the NAO) in a given season cannot alone explain the incidence of MHW: the effects of ocean pre-conditioning must also be taken into account. They also report that ocean-atmosphere heat fluxes tend to dampen the influence of heat transport anomalies, lessening the frequency with which heat transport alone generates MHW, at least in their study region.
This is a nice paper that contains some interesting results that I think will be of interest to the community. I particularly think that the consideration of both vertical (atmospheric) and horizontal (oceanic) heat fluxes is timely. I have only a few fairly minor comments, so I recommend publication after minor revisions. Please accept my apologies for the delay in submitting this review.
COMMENTS
Introduction. It would be nice to have a few sentences justifying the choice of study region. Why was the German Bight chosen as the focus of this study? Why is it important? And a bit of background about the region’s oceanography would be good too – for instance, annual cycle of stratification, circulation patterns etc.
Line 129. Is there likely to be an impact of not including tidal forcing? This is presumably a region with relatively strong tidal currents? Apologies if I’ve missed something, but a word or two on this would be welcome.
Line 148. “Dynamically” implies a temporal change, but do you mean a spatial change in this context?
Also line 148. Taking the sub-surface temperature as the surface temperature: how does this fit with the finding of Berthou et al (2024) that heatwaves peak in a thin surface layer, and anomalies are much smaller below this? Is the depth of your uppermost interpolated value still within this surface layer?
Line 176. The model might not include the effects of tidal forcing directly, but presumably the observations that are fed into GLORYS12 will include the effects of tidal forcing?
Line 283. Apologies if I’ve missed something here, but will the degree of surface heat lost to deeper model levels be at least partly determined by the degree of turbulent mixing in the model? Which will be influenced by tides – or the lack thereof? That is, will not including tides in products/models such as VIKING20X reduce the amount of vertical heat transport?
Figure 2. I know this is a common way of plotting things, so I don’t want to insist – but I do think it makes it harder to interpret these sorts of plots when the bits that are significant are stippled out, rather than the bits that aren’t.
Line 484. In what direction are the latent heat anomalies? I’d state here for clarity.
Line 735. This sentence doesn’t make a lot of sense to me: there was a warming trend that was followed by a warming trend? I’d re-phrase for clarity.
Line 805. Temperature is an integrated quantity – do you mean spatially or temporally? (Or both?!) From later context, I assume you mean temporally, but I’d state it here explicitly.