HIGH RESOLUTION MARINE HEAT WAVES IN THE NORTH SEA: HISTORY, IMPACTS, CAUSES

Programme: STEREO 4

Contract: SR/00/404

Start - End: 1 December 2022 - 31 July 2025

Project type: Exploration

Context and objectives

Long-term ocean and atmospheric warming driven by a changing climate are making common the occurrence of prolonged Marine Heat Waves (MHWs) while reducing the incidence of cold spells. These two extremes have now been well documented, and have shown to provoke severe impacts on marine ecosystems globally – and on near-shore, shallow environments specifically – with in turn huge ecological and socioeconomic consequences. There is however a lack of knowledge on the impacts of these extreme events at the regional level, among others because of the lack of adequate datasets at the required high spatial and temporal resolutions.

North-Heat (2022-2024) will address these gaps by focusing on the following objectives:

Determine the Marine Heat Waves and Cold Spells in the southern North Sea.
Develop a methodology, based on DINEOF, to improve the spatial resolution of SST data using high-resolution SST data from Landsat/TIRS data.
Study the influence of specific MHWs on the different habitats of the southern North Sea, through the study of very high spatial resolution sea surface temperature and ocean color variables like chlorophyll.

Project outcome

The project provides valuable insights into the historical changes, causes, and effects of MHWs in the North Sea. The project results have been presented at international conferences and published in peer-reviewed journals (two articles). In the first article, we analysed the long-term trends of MHW and MCS in the southern North Sea from 1982 to 2021. The results show a significant increase in MHW frequency and total MHW days, while MCS frequency and days have decreased. The study highlights the influence of large-scale climate modes on these trends and shows that global warming is likely to lead to more frequent MHWs and fewer MCSs. In the second article, we quantified the changes in SSTs and MHWs across the North Sea, finding a significant regime shift in mean annual SST since 2013 that has led to more intense and frequent MHWs. The article shows that long-term warming is identified as the primary driver of the observed trends in MHW frequency, with 80% of the increase attributed to this warming, while the remaining 20% is linked to internal variability. The most intense MHW event recorded in May 2024 was characterized by a 27-day duration and a SST increase of 2.2 °C, influenced by anomalous atmospheric conditions that enhanced solar radiation over the North Sea. Furthermore, in this article, we found a contrasting 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