Sea surface temperature (SST) fronts were analyzed in the Levantine Basin of the Mediterranean Sea over a 20-year period (2003–2022) using a high-resolution (~1 km) satellite dataset. Frontal gradients were strongest in regions of freshwater influence and around the Ierapetra eddies and Rhodes Gyre. Seasonally, maximum frontal activity was observed in fall and summer. Empirical orthogonal function (EOF) analysis revealed both monthly-to-seasonal variability and interannual variability in frontal gradients. Seasonal frontal variability is partially explained by atmospheric forcing; that is, wind stress curl (WSC) and net air–sea heat flux. The maximum frontal activity was observed in 2006, coinciding with the strongest WSC magnitude. The minimum frontal activity was observed in 2017, which saw the largest winter heat loss to the atmosphere. The highest frontal activity was typically observed in years with mild winters followed by strong Etesian winds. Over the study period (2003–2022), frontal gradients declined in the Levantine Basin. Our results suggest that years with a strong frontal boundary current (Asia Minor Current; AMC) coincide with reduced cross-shelf transport. Subsequent studies are recommended to concentrate on the variability in the frontal intensity of the AMC and associated cross-shelf transports, which are important for the oligotrophic Levantine Basin.

Fronts are ubiquitous features in the ocean, having significant implications for oceanic and atmospheric environments, including; water masses, currents, ocean-atmosphere interactions, and ecosystems, particularly through cross-front exchanges of water masses, materials and biota. In this work, we investigate thermal fronts in the Levantine Basin of the Mediterranean Sea, using remotely sensed sea surface temperature data. First, a frontal detection algorithm is used to determine the fronts. Then the spatial and temporal variability of the fronts are presented. A specific focus lies on the fronts associated with the Asia Minor boundary current, due to its frontal instabilities and associated eddy activity. Detected fronts are used as a basis to investigate these features and their contribution to cross-frontal exchanges. Further work includes identification of in-situ data gaps in existing observatories around fronts to provide an effective monitoring strategy of fronts in the region. This work has been produced benefiting from the 2236 Co-Funded Brain Circulation Scheme2 (CoCirculation2) of TÜBITAK (Project No: 121C411). However, the entire responsibility of the publication belongs to the owner of the publication. The financial support received from TÜBITAK does not mean that the content of the publication is approved in a scientific sense by TÜBITAK.

Shelf seas play a key role in both the global carbon cycle and coastal marine ecosystems through the draw-down and fixing of carbon, as measured through phytoplankton net primary production (NPP). Measuring NPP in situ and extrapolating this to the local, regional, and global scale presents challenges however because of limitations with the techniques utilised (e.g. radiocarbon isotopes), data sparsity, and the inherent biogeochemical heterogeneity of coastal and open-shelf waters. Here, we introduce a new data set generated using a technique based on the synergistic use of in situ glider profiles and satellite Earth observation measurements which can be implemented in a real-time or delayed-mode system (https://doi.org/10.5285/e6974644-2026-0f94-e053-6c86abc00109; Loveday and Smyth, 2022). We apply this system to a fleet of gliders successively deployed over a 19-month time frame in the North Sea, generating an unprecedented fine-scale time series of NPP in the region. At a large scale, this time series gives close agreement with existing satellite-based estimates of NPP for the region and previous in situ estimates. What has not been elucidated before is the high-frequency, small-scale, depth-resolved variability associated with bloom phenology, mesoscale phenomena, and mixed layer dynamics.

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.

We introduced feedback from the simulated biogeochemistry to physics in the framework of the CMEMS operational physical-biogeochemical model of the North-West European Shelf. Using this development we demonstrate that light attenuation by the biogeochemical tracers has a significant impact on ocean heating in the upper 200m of the water column. We also show that the simulated temperature is sensitive to the modelling scheme representing the underwater light attenuation, i.e in how it resolves spectra, direction and the optically active tracers. We will discuss in detail the impact of these developments on the research version of the CMEMS operational model that includes assimilation of temperature, salinity and chlorophyll.

Cross-shelf exchanges in the Black Sea were investigated using remote sensing data and an ocean circulation model to which an eddy-tracking algorithm and Lagrangian particle tracking model was applied. An anticyclonic eddy in 1998 and a cyclonic eddy in 2000 were investigated in detail. Eddy-induced cross-shelf transport of low salinity and high Chl-a waters reached a maximum in the presence of filaments associated with these eddies. The daily mean volume transport by the eddies was comparable with the previously documented transport by eddies of similar size in the north-western shelf region. Lagrangian particle tracking results showed that 59% of particles initially released over the shelf were transported offshore within 30 days by the 1998 anticyclone and 27% by the 2000 cyclone. The net volume transport across the Black Sea shelf-break reached the maxima in winter, coinciding with the increase in wind stress curl and mean kinetic energy that is a measure of the intensity of the boundary current. Ekman transport directly influences the cross-shelf exchanges in the surface layer. The south-eastern Black Sea is presented as an important area for cross-shelf transport. The total cross-shelf transport can be divided into its “large-scale” and “eddy-induced” components. Eddy-induced transport was 34% and 37% of the total cross-shelf transport (1998–2014) in the Black Sea in the off-shelf and on-shelf directions, respectively, but these values ranged between 25% and 65% depending on the eddy activity over time.

We use a recently developed spectrally resolved bio-optical module to better represent the interaction between the incoming irradiance and the heat fluxes in the upper ocean within the (pre-)operat...

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

A recognized global increase in the extent of shelf sea and coastal oxygen deficiency calls for an urgent need to increase the spatial and temporal measurement of oxygen and a better understanding of the processes that lead to oxygen deficiency. This need is severely impeded by the natural complexity of ecosystem functioning, the impact of a changing climate, connectivity between different regions of our shelf seas and large-scale external forcing from ocean and atmosphere. Currently, methods are severely restricted in resolving this complexity due to poor resolution in observational coverage, which calls for the development of new strategies for observing and monitoring marine ecosystem and environmental status to better enable national and regional assessments. AlterEco is a UK based project that has been jointly funded by academic and government agencies and the WWF to address this challenge using a novel monitoring framework to deliver improved understanding of key shelf sea ecosystem drivers. This framework capitalizes on recent UK investments in marine autonomous vehicles, such as ocean gliders and wave-driven surface vehicles, and state-of-the-art chemical sensors to investigate the physical and biogeochemical functioning in the North Sea from autumn 2017 to spring 2019. The chosen area is known to undergo variable physical, chemical and biological conditioning and includes areas previously identified to experience seasonal bottom layer oxygen depletion. We will present analysis of the effectiveness of the chosen framework to meet assessments of good environmental status and will discuss the global transferability of this approach.

The spatio-temporal distribution of cross-shelf exchanges in the northern Bay of Biscay from 2007 to 2010 were investigated using a high-resolution three-dimensional model as well as sea-surface temperature and chlorophyll-a concentration satellite observations. Our results show that the net yearly mean transport was upslope each year, with 2010 showing the highest value (0.93 Sv upslope). Bottom fluxes showed peak values near Chapel Bank, with mean values of 0.1 m(3)s(-1)m(-2) and maximum values of 0.2 m(3) s(-1)m(-2). Our model de- monstrated that cross-shelf exchanges can be divided into three vertical layers. At the surface, cross-shelf transports are driven by wind forcing (Ekman transport accounts for about 60% of the total cross-shelf transport) and mesoscale activity (eddy advection accounts for about 30% of transport). In the absence of mesoscale activity, Ekman transport at the surface is typically balanced out by a downslope flux at the bottom boundary layer. Exchanges at mid-depths are regulated by mesoscale activity and tides. A ubiquitous feature appeared at the bottom boundary with a cross-shelf flow in the downslope direction. Numerical simulations suggest bottom fluxes of 0.1 m(3)s(-1) m(-2), in agreement with previous in situ observations. We discuss the impact of winds, tides and mesoscale eddies on cross-shelf exchanges using different examples. The eddy census was obtained using an eddy-tracking algorithm. The shelf break was shown to be an important area of eddy presence due to slope current instabilities. The impact of eddies on surface and mid-depth transport is illustrated with a shelf-break eddy as an example, from its generation to dissipation and its contribution to cross-shelf exchanges. Results suggest that the largest magnitudes of downslope transports occur in the presence of both strong winds and intense eddy activity. Our detailed and quantitative exploration of cross-shelf transports in the northern Bay of Biscay highlights the relative contribution of intermittent processes (e.g. wind-driven events, eddies, frictional bottom layer) and confirms the complex links between the coastal and open ocean over shelf breaks.

On-set of spring stratification is one of the physical factors that influence the productivity of the continental shelves. Atmospheric convective mixing determines the on-set of spring stratification. This is particularly important in seasonally stratified shelf seas, where stratification constrain nutrient injection to the water column. Higher productivity in stratified period relies on intermittent diapycnal mixing events. Thus, the on-set and intensity of stratification is important for the functioning of the shelf-sea ecosystem. In this study, we investigate on-set of stratification, and its relation with the atmospheric conditions as well as imprints of sub-mesoscale features. We use high resolution in-situ measurements from 10 glider deployments, spanning over 18 months in the central North Sea. Focusing on two consecutive winters, we present year to year variability in the timing and intensity of stratification. An early initiation of stratification is observed in 2018/2019, which is also intense compared to the previous year of 2017/2018. We find that reduced wind stress and net air-sea heat fluxes result in an early on-set of stratification in 2018/2019. In February 2019, intermittent increases in chlorophyll are observed, corresponding to a minimum in sea-to-air heat loss. Similarly, in 2019 an earlier spring bloom is observed. We investigate this period with NEMO model outputs at 7km resolution (AMM7) and show a similar response, emphasizing the influence of atmospheric variability on dynamics of the shelf-sea.

Cold intermediate layers (CILs) constitute a key indicator of the subsurface thermal signature in many marginal and subarctic seas. Towards better understanding the impacts of natural and anthropogenic climate variability in the Black Sea, the present study elucidates processes leading to the observed changes of CIL characteristics at mesoscale and monthly-to-interannual scales by analyzing weekly temperature-salinity profiles provided by Argo floats for 2002–2015. The continuous time series data allowed to elucidate different types of CIL formation episodes. For the first time, it was possible to trace their subsequent evolution depending on the number and intensity of successive, but often intermittent, autumn-winter cooling events in addition to the ambient flow and stratification characteristics within the cyclonic interior and anticyclonic coastal eddies/gyres. The spatial distribution of CIL formation in the cold years 2003, 2006 and 2012 covered much of the Black Sea, even coastal anticyclonic eddies as a new feature which has not been documented before. The data also recorded modification of CIL during the rest of the year in terms of its thickness, position, temperature and salinity ranges in response to advective transport by the circulation system, interior turbulent mixing, and quasi-lateral intrusions associated with mesoscale features.