Abstract The Northwest European shelf experienced unprecedented surface temperature anomalies in June 2023 (anomalies up to 5 °C locally, north of Ireland). Here, we show the shelf average underwent its longest recorded category II marine heatwave (16 days). With state-of-the-art observation and modelling capabilities, we show the marine heatwave developed quickly due to strong atmospheric forcing (high level of sunshine, weak winds, tropical air) and weak wave activity under anticyclonic weather regimes. Once formed, this shallow marine heatwave fed back on the weather: over the sea it reduced cloud cover and over land it contributed to breaking June mean temperature records and to enhanced convective rainfall through stronger, warmer and moister sea breezes. This marine heatwave was intensified by the last 20-year warming trend in sea surface temperatures. Such sea surface temperatures are projected to become commonplace by the middle of the century under a high greenhouse gas emission scenario.

Abstract. Oceanic exchanges across the Greenland–Scotland Ridge (GSR) play a crucial role in shaping the Arctic climate and linking with the Atlantic meridional overturning circulation. Most considered ocean reanalyses underestimate the observed 1993–2020 mean net inflow of warm and saline Atlantic Water of 8.0 ± 0.5 Sv by up to 15 %, with reanalyses at 0.25∘ resolution additionally exhibiting larger biases in the single inflow branches compared to higher-resolution products. The underestimation of Atlantic Water inflow translates into a low bias in mean oceanic heat flux at the GSR of 5 %–22 % in reanalyses compared to the observed value of 280 ± 18 TW. Interannual variations in reanalysis transports correlate reasonably well with observed transports in most branches crossing the GSR. Observations and reanalyses with data assimilation show a marked reduction in oceanic heat flux across the GSR of 4 %–9 % (compared to 1993–2020 means) during a biennial (2-year-long) period centered on 2018, a record low for several products. The anomaly was associated with a temporary reduction in geostrophic Atlantic Water inflow through the Faroe–Shetland branch and was augmented by anomalously cool temperatures of Atlantic Water arriving at the GSR. The latter is linked to a recent strengthening of the North Atlantic subpolar gyre and illustrates the interplay of interannual and decadal changes in modulating transports at the GSR.

Wind driven circulation in the North Sea is revisited with a specific focus on locally modified winds and their impacts. We show for the first time that local extrema of the wind stress curl (WSC), generated by orography and ocean-atmosphere interactions, help regulate circulation in the northern North Sea. While calculated transports are strongly coupled with wind stress, which itself is driven by large-scale forcing, transports through the Norwegian Trench have higher correlations with the WSC field due to local extrema. Such WSC extrema regulate the eddy activity around the Norwegian Trench. We conclude that orography and ocean-atmosphere interaction are two important mechanisms contributing to the generation of the WSC extrema around the Norwegian coast. Ocean-atmosphere interaction is considered a potential mechanism developing the WSC extrema. Our results show that local winds are more important than previously documented, with important implications for regional circulation likely to result from future changes to local surface gradients, such as may arise from changing meteorological or hydro-climatic forcing. These are additional impacts on North Sea circulation that may not be accounted for from changes in wind stress alone. Plain Language Summary North Sea circulation is investigated with a specific focus on the local winds and their impacts. We show for the first time that the local extrema of the wind field, generated by the coastline and oceanic contributions, help regulate circulation in the northern North Sea. Wind is driven by large-scale forcing mechanisms, and is closely related with volume transports. Volume transports through the Norwegian Trench are better related to the wind field, due to local extrema of wind. Local extrema of wind, and their direction, stimulates rotation in the region, controlling the eddy activity around the Norwegian Trench. We conclude that interaction of winds with the land and an ocean-atmosphere feedback mechanism contribute to the generation of the local extrema of wind along the Norwegian coastline. Our results show that local winds are more important than previously known and have important implications for regional circulation. Therefore, future changes to local density differences from changing meteorological conditions may have further impacts on North Sea circulation, which may not be accountable from changes in wind stress alone.

There is an urgent need to reduce global greenhouse gas emissions. One method of achieving this is through Carbon Capture and Storage (CCS). Geological structures that lie offshore under continental shelf seas offer huge CCS storage potential. An emerging marine industry is developing to exploit this potential and national marine monitoring agencies will soon need to consider the potential impacts of this emerging industry. This review of published literature is aimed at generalists responsible for the delivery of national marine monitoring, as well as those involved in the management of the marine environment. It briefly summarizes why the emerging offshore CCS industry is needed, how large it may be and what marine infrastructure may be involved. For the purposes of this paper, a hypothetical 20 Mtpa industry has been used to gauge the potential impact of a developing offshore CCS industry. The probability of CO2 leaks from such an industry is low. If they do occur, the spatial scale of impact will be small, and the potential environmental impacts will be low. Irrespective of how CO2 is transported or stored within shelf seas, leaked CO2 will enter the sea as a gas or as a solution dissolved in sediment pore water. CO2 as a gas will dissolve into seawater and/or directly vent to the atmosphere, depending on the initial conditions of the leak. The most probable source of leaks in a developed CCS industry is from pipelines (currently a 2-year event per 1000 km pipeline). The most probable source of leakage from geological storage is through abandoned wells (a 20- to 80-year event for a 20 Mtpa industry). The source of leaks from a CCS scheme with the potential to release the greatest mass of CO2 is through geological faults, as these may go undetected (if they occur) for long periods. The probability of leaks from geological storage, through faults or abandoned wells, is site dependent and minimized by the site selection process. The review concludes with recommended priorities for future marine science development.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v1]Go to new versionLocally modified winds regulate North Sea circulationAuthorsAnılAkpınariDMatthew R.PalmerMark EInallBarbaraBerxiDJeff A.PoltonSee all authors Anıl AkpınariDCorresponding Author• Submitting AuthorNational Oceanography CentreiDhttps://orcid.org/0000-0002-0271-407Xview email addressThe email was not providedcopy email addressMatthew R. PalmerNational Oceanography Centreview email addressThe email was not providedcopy email addressMark E InallScottish Association For Marine Scienceview email addressThe email was not providedcopy email addressBarbara BerxiDMarine Scotland ScienceiDhttps://orcid.org/0000-0001-5459-2409view email addressThe email was not providedcopy email addressJeff A. PoltonNational Oceanography Centreview email addressThe email was not providedcopy email address

Oil spills at sea are one of the most disastrous anthropogenic pollution events, with the Deepwater Horizon spill providing a testament to how profoundly the health of marine ecosystems and the livelihood of its coastal inhabitants can be severely impacted by spilled oil. The fate of oil in the environment is largely dictated by the presence and activities of natural communities of oil-degrading bacteria.

Ocean circulation redistributes heat, freshwater, carbon, and nutrients all around the globe.Because of their importance in regulating climate, weather, extreme events, sea level, fisheries, and ecosystems, large-scale ocean currents should be monitored continuously.

The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic.

The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel.

Losses due to mortality are a serious economic drain on Scottish salmon aquaculture and are a limitation to its sustainable growth. Understanding the changes in losses, and associated drivers, are required to identify risks to sustainable aquaculture. Data on losses were obtained from two open source data sets: monthly losses of biomass 2003 - 2018 and losses of salmon over production cycles (numbers input minus output harvest) 2002-2016. Monthly loss rates increased, accelerating after 2010, while losses per production cycle displayed no trend. Two modelling frameworks were investigated to produce an early warning tool for managers about potential increases in losses. Both linear regression and beta regression showed that monthly losses related to biomass and minimum winter air temperatures with high precision and low bias. These relationships apply at both the national and regional levels where the beta regression best fit model explain 82 % and 69 % of variation in mortality, some regional differences apply, particularly for the Northern Isles. The lack of trend in losses per production cycle may have been due to shorter production cycles as more salmon were harvested earlier, and possibly increasing losses of larger salmon (which affects biomass but not numbers lost). In the long-term, the models predict that milder winters and increased biomass will be associated with increased mortality, which will need to be managed. In the short-term, given relatively little year-to-year variation in biomass, minimum winter temperature is a powerful early warning of the likely extent of losses in the Scottish salmon farming industry.

Conditions in the Arctic are in part driven by the ocean state in the Arctic Mediterranean (AM), the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Exchange between the lower latitude ocean basins and this region occurs through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow). These waters are subsequently modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water (surface outflow) on both sides of Greenland. These exchanges transport heat and salt into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, the observational coverage has become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this EGU contribution, we focus on the observations (incl. volume transport time series) of all the main AM-exchange branches collected in the last 20 to 30 years.

Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring–neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being “fished out” by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimize these risks, with u measured by a low-frequency acoustic Doppler current profiler (ADCP) moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the semidiurnal internal tide radiating from the Wyville Thomson Ridge in the North Atlantic. The observed energy flux (4.2±0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by subsampling potential density in an idealized internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (<10 %) if all the glider data are contained within a “watch circle” with a diameter smaller than 1∕8 the mode-1 horizontal wavelength of the internal tide. Energy flux is biased low because the glider samples density with a broad range of phase shifts, resulting in underestimation of vertical isopycnal displacement and available potential energy. The negative bias increases with increasing watch circle diameter. If watch circle diameter is larger than 1∕8 the mode-1 horizontal wavelength, the negative bias is more than 3 % and all realizations within the 95 % confidence interval are underestimates. Over the Wyville Thomson Ridge, where the semidiurnal mode-1 horizontal wavelength is ≈100 km and all the glider dives are within a 5 km diameter watch circle, the observed energy flux is estimated to have a negative bias of only 0.4 % and an error of less than 3 % at the 95 % confidence limit. With typical glider performance, we expect energy flux error due to imperfect co-location to be <10 % in most mid-latitude shelf slope regions.

Quantifying the variability of North Sea inflows and understanding the temporal variability of their physical properties are essential for understanding, modelling and managing the ecosystems of the North Sea. The Joint North Sea Information System (JONSIS) line hydrographic section crosses the path of the main inflows of Atlantic water into the northwestern North Sea. We use observations from an autonomous underwater glider to observe the inflows at high spatial and temporal resolutions. The glider completed 10 partial sections of the JONSIS line in October and November of 2013. Key water masses of the inflow are identified; their spatial distribution varies greatly from section to section. This is not apparent from long-running ship surveys of the JONSIS line, which are generally several months apart. In particular, the distribution of water of most recent Atlantic origin varies as summer stratification decays throughout autumn: at the start of the deployment it is present as a thin layer beneath the thermocline; at the end of the deployment, it occupies the full depth of the water column. Thermohaline flow, i.e. that which is driven by horizontal density gradients, is focused into three or four jets (approximately 10 km wide). Jets as narrow as these have not previously been observed in the region. We also observe baroclinic eddies. The thermohaline transport of the inflows is compared with the absolute transport that is derived by referencing geostrophic shear to the glider's dive-average current. Thermohaline transport (approximately 0.2 Sv) is consistently smaller than absolute transport (approximately 0.5 Sv). The week-to-week variability in hydrography and flow structure identified in this study is relevant to on-going efforts to define a background state against which the nature of anthropogenic changes can be assessed, and future modelling efforts should represent the spatial and temporal variability that we have identified.

Discovery and understanding of fragile deep-sea habitats like sponge aggregations, are being outpaced by anthropogenic resource exploitation. Sustainable ocean development in the Faroe-Shetland Channel Nature Conservation Marine Protected Area (FSC NCMPA; northeast Atlantic), which harbors sponge aggregations, now requires adaptive management in the face of encroachment of multisectorial activities in this area (e.g., fishing, oil and gas, shipping) and climate change. We examined sponge morphotype composition, richness, diversity, density and body-size distribution inside and outside the FSC NCMPA, and the role of environmental variability and human impact in these sponge aggregations. Analyses were based on the examination of 465 high resolution images from 13 towed-camera transects. A catalog for regional sponge morphotypes was also developed and applied for these analyses. Analysis revealed that morphotype composition did not differ between inside and outside the FSC NCMPA but richness, diversity and densities of massive/spherical/papillate and flabellate/caliculate sponges were higher inside than outside the boundary. The sponge aggregations occurred within a narrow zone between 450 and 530 m depth, within relatively warm and saline water masses. Furthermore, multiple size cohorts of sponges were recorded inside the FSC NCMPA, in contrast to the single cohort outside. Distance-based linear modeling showed that demersal fisheries, substratum, salinity and temperature explained a statistically-significant amount of variation (48%, p < 0.001) of sponge density across the study area. Findings on density and size cohorts suggest that the FSC NCMPA boundary currently encloses the most vulnerable area, which also demonstrates normal ecosystem functions (e.g., recruitment). However, sponges were constrained to a narrow environmental niche shaped by fisheries and interactions of FSC NCMPA water masses with the slope that in turn likely determine, food supply to the sponge aggregations. Our study illustrated the vulnerability of the FSC NCMPA sponge aggregations to fisheries and changes to water mass properties over time. The morphotype catalog and suite of indicators (i.e., density and body-size distribution) allow for baseline and future assessments of anthropogenic and climate change impacts on sponge aggregations’ environmental status in the FSC NCMPA, thus guiding management as sectoral encroachment continues in this area.

Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. Ocean gliders enable the observation of fronts and tide-dominated flows at high resolution. We use dive-average currents from a 2-month (12 October–2 December 2013) glider deployment along a zonal hydrographic section in the north-western North Sea to accurately determine M2 and S2 tidal velocities. The results of the glider-based method agree well with tidal velocities measured by current meters and with velocities extracted from the TPXO tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We then use the glider-derived tidal velocities to investigate tidal controls on the location of a front repeatedly observed by the glider. The front moves offshore at a rate of 0.51 km day−1. During the first part of the deployment (from mid-October until mid-November), results of a one-dimensional model suggest that the balance between surface heat fluxes and tidal stirring is the primary control on frontal location: as heat is lost to the atmosphere, full-depth mixing is able to occur in progressively deeper water. In the latter half of the deployment (mid-November to early December), a front controlled solely by heat fluxes and tidal stirring is not predicted to exist, yet a front persists in the observations. We analyse hydrographic observations collected by the glider to attribute the persistence of the front to the boundary between different water masses, in particular to the presence of cold, saline, Atlantic-origin water in the deeper portion of the section. We combine these results to propose that the front is a hybrid front: one controlled in summer by the local balance between heat fluxes and mixing and which in winter exists as the boundary between water masses advected to the north-western North Sea from diverse source regions. The glider observations capture the period when the front makes the transition from its summertime to wintertime state. Fronts in other shelf sea regions with oceanic influence may exhibit similar behaviour, with controlling processes and locations changing over an annual cycle. These results have implications for the thermohaline circulation of shelf seas.