Amplified Warming and Marine Heatwaves in the North Sea Under a Warming Climate

Abstract. The Northeast Atlantic and adjacent regions, such as the North Sea, are among the fastest-warming areas in the world. However, the role of climate change and internal variability on marine heatwaves (MHWs) in this region remains poorly understood. This study aims to quantify the relevant changes in sea surface temperature (SST) and MHWs in the North Sea, as well as to identify the leading patterns of interannual MHW variability over more than four decades (1982–2024). Our results indicate a new regime shift in the annual mean SST in the North Sea since 2013. Therefore, we examine the relationships between MHW trends and long-term SST warming trends to quantify the role of climate change in the intensification of MHWs. We found that the increase in MHWs is related to the significant decadal change in SST over the North Sea, and we have revealed that large-scale climate modes, such as the Atlantic Multidecadal Oscillation and the East Atlantic Pattern, play a crucial role in this decadal change in SST. In particular, the SST trend has doubled in recent years (post-2013) compared to the previous period (1982–2012: pre-2013), leading to more intense and frequent MHWs. The SST and MHW frequencies have significantly increased by 0.38 °C/decade and 1.04 events/decade, respectively, over the entire study period. After removing the long-term SST warming trend before MHW detection, all MHW features exhibited insignificant trends, indicating that the long-term SST trend is the primary driver of the observed long-term MHW trend in the North Sea region, thereby confirming the crucial role of mean SST changes in MHW in this region. Furthermore, we found that 80 % of the observed trend in MHW frequency is attributed to long-term warming, while the rest is attributed to internal variability. The SST record in May 2024, manifested by the longest (27 days) and most intense (2.2 °C) MHW event, is attributed to an anomalous anticyclonic atmospheric circulation over the Baltic Sea and southern Norway, which enhances solar radiation over the North Sea. Finally, our results showed an opposite response of chlorophyll-a concentrations to MHWs, with an increase in the coastal areas of the southern part and a decrease in the northeastern part of the North Sea.