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 a 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. In our study region, 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.