Bruno Zakardjian

Between 9 June and 6 July 2023 a SeaExplorer glider equipped with an Underwater Vision Profiler (UVP6), surveyed frontal structures between a cyclonic and two anticyclonic eddies in the Northeast Atlantic as part of the APERO cruise (Assessing marine biogenic matter Production, Export and Remineralization: from the surface to the dark Ocean) that was designed to quantify the spatial and temporal variability of the biological carbon pump (BCP) from the surface to the mesopelagic zone. Combined high‐resolution physical, biogeochemical, and imaging observations resolved particulate organic carbon (POC) distributions and fluxes across (sub)mesoscale features. Particle concentrations, POC stocks, and fluxes showed pronounced spatial heterogeneity, with sharp gradients at frontal interfaces and enhanced variability in larger particle size classes. Estimated POC fluxes varied by up to an order of magnitude over distances <10 km at the surface, with variations of up to a factor 2–3 at 1,000 m. Lagrangian backward trajectories from altimetry revealed that these signals detectable at submesoscale resulted from convergence of outer water masses in frontal zones, whereas vertical processes were primarily responsible for biogeochemical variability within eddy cores. Secondarily it is shown that optical backscattering underestimated and poorly resolved deep variability of large‐particle POC when compared with UVP‐derived ones, but it is proposed that combining both sensors may improve total POC estimates. These results demonstrate that glider‐UVP6 systems provide a powerful approach to capture fine‐scale BCP variability and constrain vertical carbon export estimates.

We present an assessment of the water mass dynamics in a reanalysis of the Mediterranean Sea with a focus on the Algero‐Provencal basin. We use a θ‐S‐based algorithm to compute the fractions of the main western Mediterranean water masses: Atlantic and modified Atlantic Waters (AW and mAW), Western and Eastern Intermediate Waters (WIW and EIW), Tyrrhenian and Western Mediterranean Deep Water (TDW and WMDW). The reanalysis retains the known mean characteristics of the water masses, their seasonal to interannual variabilities, and main circulation patterns when compared with the literature. The imprint of winter mixing is particularly obvious with coherent variations of water mass volumes, mainly the yearly creation of WIW from mAW on northernmost shelves and of WMDW from all surface and intermediate layers during years of deep water formation (DWF). The results also highlight some unrealistic events of variability of the WMDW volume that are likely due to the data assimilation process. Recomputation of water mass volumes and transports without these altered years allowed to highlight the possible disruption of large‐scale barotropic cyclonic circulation in the East Algerian basin in response to major DWF events over the Gulf of Lion and the induced surface consequence on Algerian Eddies' trajectories.

We present an assessment of the water mass dynamics in a reanalysis of the Mediterranean Sea with a focus on the Western basin. We use a θ-S based algorithm to compute the fractions of the main western Mediterranean water masses : Atlantic and modified Atlantic Waters (AW, mAW), Western and Levantine Intermediate Waters (WIW and LIW)and Western Mediterranean Deep Waters (WDW). The reanalysis retains the known mean characteristics of the water masses, their seasonal to interannual variability and main circulation patterns when compared with the literature. The imprints of winter mixing is particularly obvious with coherent variations of water mass volumes, mainly the yearly creation of WIW from mAW on northernmost shelves and of WMDW from all surface and intermediate layers during years of deep water formation. The results also highlight some unrealistic events of variability of the WMDW volume that are likely due to the data assimilation process. Re-computing the water mass volumes and transports without these altered years allowed to highlight the possible disruption of the large-scale barotropic cyclonic circulation in the Eastern Algerian basin in response to major DWF events over the Gulf of Lion. The reanalysis also showsan overtopping ofWMDW in the Sardinia Channel in 2009 leading to a major backward flow of mAW from the Tyrrhenian to the Algero-Provençal basin. Both processes affects the circulations of AW and mAW over the whole western Mediterranean.

During the last 15 years, substantial progress has been achieved in altimetry data processing, now providing data with enough accuracy to illustrate the potential of these observations for coastal applications. In parallel, new altimetry techniques improve data quality by reducing land contamination and enhancing the signal-to-noise ratio. Satellite altimetry provides more robust and accurate measurements ever closer to the coast and resolve shorter ocean signals. An important issue is now to learn how to use altimetry data in conjunction with other coastal observing techniques. Here, we cross-compare and combine the coastal currents provided by large datasets of ship-mounted acoustic Doppler current profilers (ADCPs), gliders, high-frequency (HF) radars and altimetry. We analyze how the different available observing techniques, with a particular focus on altimetry, capture the Northern Current variability at different timescales. We also study the coherence, divergence and complementarity of the information derived from the different instruments considered. Two generations of altimetry missions and both 1 Hz and high-rate measurements are used: Jason-2 (nadir Ku-band radar) and SARAL/AltiKa (nadir Ka-band altimetry); their performances are compared. In terms of mean speed of the Northern Current, a very good spatial continuity and coherence is observed at regional scale, showing the complementarity among the types of current measurements. In terms of current variability, there is still a good spatial coherence but the Northern Current amplitudes derived from altimetry, glider, ADCP and HF radar data differ, mainly because of differences in their respective spatial and temporal resolutions. If we consider seasonal variations, 1 Hz altimetry captures ∼60 % and ∼55 % of the continental slope current amplitude observed by the gliders and by the ADCPs, respectively. For individual dates this number varies a lot as a function of the characteristics of the Northern Current on the corresponding date, with no clear seasonal tendency observed. Compared to Jason-2, the SARAL altimeter data tend to give estimations of the NC characteristics that are closer to in situ data in a number of cases. The much noisier high-rate altimetry data appear to be more difficult to analyze but they provide current estimates that are generally closer to the other types of current measurements. Thus, satellite altimetry provides a synoptic view of the Northern Current circulation system and variability, which helps to interpret the other observations. Its regular sampling allows for the observation of many features that may be missed by irregular in situ data.

A 20 year coastal altimetry data set (X‐TRACK) is used, for the first time, to gain insight into the long‐term interannual variations of the surface circulation in the Sicily Channel. First, a spectral along with a time/space diagram analysis are applied to the monthly means. They reveal a regionally coherent current patterns from track to track with a marked interannual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Second, an empirical altimetry‐based transport‐like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well‐balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 ± 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.

Technological advances in the recent satellite altimeter missions of Jason-2, SARAL/AltiKa and CryoSat-2 have improved their signal-to-noise ratio, allowing us to observe finer-scale ocean processes with along-track data. Here, we analyse the noise levels and observable ocean scales in the northwestern Mediterranean Sea, using spectral analyses of along-track sea surface height from the three missions. Jason-2 has a higher mean noise level with strong seasonal variations, with higher noise in winter due to the rougher sea state. SARAL/AltiKa has the lowest noise, again with strong seasonal variations. CryoSat-2 is in synthetic aperture radar (SAR) mode in the Mediterranean Sea but with lower-resolution ocean corrections; its statistical noise level is moderate with little seasonal variation. These noise levels impact on the ocean scales we can observe. In winter, when the mixed layers are deepest and the submesoscale is energetic, all of the altimeter missions can observe wavelengths down to 40–50 km (individual feature diameters of 20–25 km). In summer when the submesoscales are weaker, SARAL can detect ocean scales down to 35 km wavelength, whereas the higher noise from Jason-2 and CryoSat-2 blocks the observation of scales less than 50–55 km wavelength. This statistical analysis is completed by individual case studies, where filtered along-track altimeter data are compared with co-located glider and high-frequency (HF) radar data. The glider comparisons work well for larger ocean structures, but observations of the smaller, rapidly moving dynamics are difficult to co-locate in space and time (gliders cover 200 km in a few days, altimetry in 30 s). HF radar surface currents at Toulon measure the meandering Northern Current, and their good temporal sampling shows promising results in comparison to co-located SARAL altimetric currents. Techniques to separate the geostrophic component from the wind-driven ageostrophic flow need further development in this coastal band.

This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time.

The Arctic Ocean (AO) undergoes profound changes of its physical and biotic environments due to climate change. In some areas of the Beaufort Sea, the stronger haline stratification observed in summer alters the plankton ecosystem structure, functioning and productivity, promoting oligotrophy. A one-dimension (1-D) physical–biological coupled model based on the large multiparametric database of the Malina project in the Beaufort Sea was used (i) to infer the plankton ecosystem functioning and related nitrogen fluxes and (ii) to assess the model sensitivity to key light-driven processes involved in nutrient recycling and phytoplankton growth. The coupled model suggested that ammonium photochemically produced from photosensitive dissolved organic nitrogen (i.e., photoammonification process) was a necessary nitrogen source to achieve the observed levels of microbial biomass and production. Photoammonification directly and indirectly (by stimulating the microbial food web activity) contributed to 70% and 18.5% of the 0–10 m and whole water column, respectively, simulated primary production (respectively 66% and 16% for the bacterial production). The model also suggested that variable carbon to chlorophyll ratios were required to simulate the observed herbivorous versus microbial food web competition and realistic nitrogen fluxes in the Beaufort Sea oligotrophic waters. In face of accelerating Arctic warming, more attention should be paid in the future to the mechanistic processes involved in food webs and functional group competition, nutrient recycling and primary production in poorly productive waters of the AO, as they are expected to expand rapidly.