Dimitry Van der Zande

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 in 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 4 decades (1982–2024). Our results indicated a new regime shift in the annual mean SST in the North Sea since 2013. Therefore, we examined the relationships between MHW trends and long-term sea surface 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 the post-2013 period (0.8 °C per decade), compared with the pre-2013 period (0.4 °C per decade), leading to longer and more frequent MHWs. The SST, MHW frequency, and MHW days increased significantly by 0.38 °C per decade, 1.04 events per decade, and 17.27 d per decade, respectively, over the entire study period. After removing the long-term sea surface 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 MHWs 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, manifest by the longest (27 d) and most intense (2.2 °C) MHW event, is attributed to an anomalous anticyclonic atmospheric circulation over the Baltic Sea and southern Norway, which enhanced solar radiation over the North Sea. Finally, we also investigated how the chlorophyll a concentration responded to the MHW, revealing a decrease in the deep and cold-water regions of the northern North Sea and an increase in the shallow and warm-water areas of the southern North Sea.

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.

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.

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.

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.

In this work we present a super-resolution approach for deriving high-spatial-resolution and high-temporal-resolution ocean colour satellite datasets. The technique is based on DINEOF (data-interpolating empirical orthogonal functions), a data-driven method that uses the spatio-temporal coherence of analysed datasets to infer missing information. DINEOF is used here to effectively increase the spatial resolution of satellite data and is applied to a combination of Sentinel-2 and Sentinel-3 datasets. The results show that DINEOF is able to infer the spatial variability observed in the Sentinel-2 data to the Sentinel-3 data while reconstructing missing information due to clouds and reducing the amount of noise in the initial dataset. In order to achieve this, the Sentinel-2 and Sentinel-3 datasets have undergone the same pre-processing, including a comprehensive, region-independent, and pixel-based automatic switching scheme for choosing the most appropriate atmospheric correction and ocean colour algorithm to derive in-water products. The super-resolution DINEOF has been applied to two different variables (turbidity and chlorophyll) and two different domains (Belgian coastal zone and the whole of the North Sea), and the sub-mesoscale variability of the turbidity along the Belgian coastal zone has been studied.

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.

Dissolved CO2 and buried organic matter budgets have been studied in shelf seas to identify carbon fractions that are exported to the ocean interior or preserved in the sediment. However, the fate of suspended particulate organic matter remains less understood, particularly because its lability is difficult to identify. Analysis of the different fractions of particulate organic matter in the North Sea could contribute to understanding its fate. The particulate organic carbon (POC) concentration follows coastal-offshore gradients that can be predicted with the suspended particulate matter (SPM) concentration. The POC:SPM ratio indeed features a typical exponential decrease with the SPM concentration. While that ratio is higher offshore where SPM concentrations are minimum, it reaches low asymptotic values at the coast where SPM concentrations are high. Such a relationship is actually found in many different systems (coastal zones, estuaries) and at different latitudes. A semi-empirical model has been proposed to fit the observed data of that relationship in the southern North Sea (German Bight: Schartau et al., 2019; Belgian zone: Fettweis et al., 2022). Based on the model assumptions, it is possible to separate two fractions of POC: the fresh fraction, that is assumed to accumulate during the bloom and to be degraded within the season, and a more refractory POC fraction. More detailed calculations allow this latter fraction to be divided into a slow POC, which includes the refractory detritus, and a mineral POC, that is the POC adsorbed on the surfaces of clay minerals. We assume that suspended mineral particles in the North Sea provide a total surface area saturated with adsorbed organic matter, also considering an underlying dynamic equilibrium between adsorption and desorption of organic matter. We then calculate the SPM budget in the North Sea from satellite remote sensing and vertical concentration profiles obtained from in situ observations. On this basis, we can use the semi-empirical model to establish a budget of the fresh, refractory detrital and mineral fractions of POC on the shelf.

High-quality satellite-based ocean colour products can provide valuable support and insights in the management and monitoring of coastal ecosystems. Today’s availability of Earth Observation (EO) data is unprecedented including traditional medium resolution ocean colour systems (e.g. Sentinel-3/OLCI) and high-resolution land sensors (e.g. Sentinel-2/MSI). Each of these sensors offers specific advantages in terms of spatial, temporal or radiometric characteristics, enabling the provision of different types of ocean colour products. This is also reflected in the portfolio of the Copernicus Marine service providing a number of different OC products to support different types of end users. While “traditional” ocean colour sensors like Sentinel-3 OLCI provide daily temporal resolution, the sensors onboard these satellites do not measure at the necessary high spatial resolution to resolve complex coastal dynamics. High spatial resolution sensors, like MSI onboard Sentinel-2 (10m - 60m resolution), are able to resolve these small scales, but their temporal revisit time is far from optimal (about 5 days considering the Sentinel-2 A & B constellation). Additionally, both high spatial resolution datasets and traditional ones are hindered by the presence of clouds, resulting in a large amount of missing data.Given the high complementarity of these two measurement strategies, we present a methodology to derive daily cloud-free multi-resolution ocean colour products from the synergistic use of Sentinel-2 and Sentinel-3 data by applying DINEOF (Data Interpolating Empirical Orthogonal Functions). A key processing step in the gap-filling procedure is the harmonization between the ocean colour products (e.g. chlorophyll-a concentration -CHL-, Turbidity -TUR-) between the Sentinel-2/MSI and Sentinel-3/OLCI sensor. Due to differences between the sensors regarding spectral band sets and viewing geometry, CHL algorithms used for Sentinel-3/OLCI cannot be directly transferred to Sentinel-2/MSI resulting in the chlorophyll-a products often having diverging values between both sensors. To increase the coherency between the Sentinel-2/MSI and Sentinel-3/OLCI CHL products, a machine learning technique (LightGBM) was used to transfer the more complex CHL algorithms (e.g. band ratio algorithms, switching algorithms, other machine learning algorithms) from Sentinel-3/OLCI to Sentinel-2/MSI. Subsequently, DINEOF is used to generate the daily multi-resolution products by using the daily OLCI products to support gap-filling in the high-resolution 5-daily coastal products and retaining the high spatial resolution of Sentinel-2/MSI data and the high temporal resolution of OLCI data in the final product.The machine learning and multi-resolution gap-filling approach will be demonstrated and validated in different regions (e.g. North Sea, North Adriatic Sea) to ensure suitability of the approach for integration into the operational high resolution COPERNICUS Marine Service. An analysis of TUR and CHL daily products at 100m resolution will be presented, alongside an analysis of the spatial and temporal scales retained by the approach. This work was performed in the Copernicus Marine Service Evolution Project MultiRes (21036-COP-INNO-SCI)

Marine Heat Waves (MHWs) are defined as discrete periods of anomalously warm water temperature at a given location. MHWs can have a huge impact on marine ecosystems, already under stress because of the effects of a warming ocean under climate change and high anthropogenic pressure. This work will assess the spatio-temporal evolution of MHWs in the southern North Sea, with an emphasis on the 2022 events. Studying the impact of MHWs on coastal marine ecosystems is currently hampered by the resolution mismatch between traditional satellite data (typically 1 km spatial resolution for SST and CHL) and species habitat/substrate. In the southern North Sea, a multitude of shallow sandbanks, sand, mud and coarser sediment substrats are for instance present, offering a multitude of habitats to different species. With ocean dynamics, and hence water mass and temperature distribution being impacted by the presence of these sandbanks, fine spatial resolution data are required for accurate analysis of the consequences of MHWs and cold spells on the ecosystem. The Thermal InfraRed Sensor (TIRS) sensor onboard the Landsat constellation provides SST at a spatial resolution of 30 m with an accuracy of 0.1 to 0.2K, and can allow the study of the evolution of small-scale dynamics in coastal regions, including the development of MHWs. However, Landsat data have a very low revisit time (7-9 days), not optimal to study specific MHW events, which can evolve on a daily basis. This work will assess the synergy between Landsat data and daily, low-spatial resolution SST data to analyse the evolution of MHWs at coastal regions. DINEOF (Data Interpolating Empirical Orthogonal Functions) will be used to merge these tow data sources and provide high spatial and temporal resolution SST data. This work is a first attempt at linking MHW variability and their consequences on marine ecosystems at very fine spatio-temporal scales, and is part of the North-Heat project. We aim at providing key insights for our comprehension of MHWs in the southern North Sea, a region where marine ecosystems are already under high anthropogenic pressure.