The ocean surface mixed layer represents a critical interface linking the ocean and atmosphere. The physical processes determining the surface mixed layer properties and mediate atmosphere-ocean exchange. Submesoscale processes play a key role in cross-scale oceanic energy transformation and the determination of surface mixed-layer properties, including the enhancement of vertical nutrient transport, leading to increased primary productivity. Herein, we presented observations of the spiral chlorophyll-a filament and its influence on turbulence within an anticyclonic eddy in the western South China Sea during August 2021. The filament had a negative Ertel potential vorticity associated with strong upwelled/downward currents (approximately 20-40 m/day). Across-filament sections of the in-situ profiles showed turbulent dissipation rates enhanced in the filament. We suggested this enhancement values can be attributed to submesoscale processes, which accounted for 25% of the total parameterized turbulent dissipation rates. The present parametrized submesoscale turbulent scheme overestimated the in-situ values. The filament transferred kinetic energy upward to anticyclonic eddy via barotropic instability and gained energy from the anticyclonic eddy via baroclinic instability. After kinetic energy budget diagnostic, we suggested besides symmetric instability, centrifugal instability and mixed layer baroclinic instability should also be included in the turbulence scheme to overcome the overestimation. The observed dual energy transfers between the anticyclonic eddy and filament, and the observed high turbulent energy dissipation within the filament, emphasized the need for these processes to be accurately parameterized regional and climate models. 

How to cite: Qiu, C. and Wang, D.: Observational energy transfers of a spiral cold filament within an anticyclonic eddy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6911, https://doi.org/10.5194/egusphere-egu24-6911, 2024.

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic with the overall goal of understanding the role of fine-scale processes in internal ocean dynamics and air-sea interaction. Four oceanographic ships, the French Atalante, the German Maria S. Merian and Meteor, and the US Ron Brown, were closely coordinated with airborne observations and autonomous ocean platforms (gliders, ©Saildrones, Argo floats, and drifters) to simultaneously measure the ocean and atmosphere from east of Barbados to the northern border of French Guyana. The multiple observations of the ocean, atmosphere, and their interface have revealed more complex ocean dynamics than expected, in particular a strong interaction between the Amazon River outflow (despite its reduced winter discharge), the North Brazil Current (NBC), and several mesoscale eddies (including the highly energetic NBC rings). This leads to even richer submesoscale dynamics that shape an important fraction of the air-sea exchange of heat, momentum, and CO₂, and efficiently isolates the NBC northward flow waters from intense and continuous interactions with the atmosphere. Owing to the many complementary observations from ships and autonomous platforms, we have been able to quantify some of these processes, including the diurnal cycle and the 3D dynamics of different mesoscale eddies, as well as to map and quantify different terms of the air-sea fluxes and their impacts on the marine atmospheric boundary-layer water budget. The results have been widely used not only to validate numerical simulations of the region, but also to guide their analyses and to improve various numerical parameterizations.

The collection of these observations was the result of an important international coordination between many different groups of ocean and atmospheric scientists. In addition, the special strategy for targeted data collection of meso- and submesoscale processes relied on daily planning of the field experiment and on detailed analysis of the near-real-time satellite data and the observations already obtained during the experiment, which was essential for providing the right snapshots of the ocean and atmosphere for the quantification of many processes. The lessons learned from this experiment will be implemented and extended in the upcoming major high-resolution oceanographic endeavor, the WHIRLS experiment, which will take place in June-July 2025, southwest of Africa.

How to cite: Speich, S., Karstensen, J., Carton, X., Barbedo Rocha, C., Bellenger, H., Pasquero, C., Parodi, A., Gula, J., Bourras, D., Davy, R., renault, L., del Moral-Méndez, A., Zhang, D., Fairall, C., Farrell, D., von Storch, J.-S., Giordani, H., Reverdin, G., Horstmann, J., and Keenlyside, N. and the EUREC4A-OA/ATOMIC Ocean-atmosphere processes: Multi-platform high resolution in situ observations for understanding mesoscale and sub-mesoscale processes and their role in the air-sea exchanges: Experiences and prospects from the EUREC4A-OA/ATOMIC field experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19188, https://doi.org/10.5194/egusphere-egu24-19188, 2024.

Located in the southwest Indian Ocean, between Madagascar and the African continent, the Mozambique Channel is a western boundary current system characterized by an intense eddy activity (Halo et al. 2014). Large anticyclonic rings, reaching up to 300 to 350 km in diameter and 2000 m of vertical extension, are structuring the marine ecosystems from phytoplankton to top predators (de Ruijter et al., 2002; Ternon et al., 2014, Weimerskirch et al., 2004). They impact the environmental conditions on the Mozambican shelves by promoting the upwelling of nutrient rich deeper waters (Lamont et al., 2010; Malauene et al, 2014). Coastal waters, generally rich in plankton and nutrients, can be also be transported offshore along the edges of the rings. The occurrence of an eddy dipole with the anticyclonic ring in the northern side of the cyclonic eddy can enhance the processes (Roberts et al., 2014). Mesoscale eddy flux is supposed to be the dominant source of nutrients for the central Mozambique Channel (José et al., 2016). The first leg of the RESILIENCE (fRonts, EddieS and marIne LIfe in the wEstern iNdian oCEan) multidisciplinary oceanographic cruise on board R/V Marion Dufresne II in April-May 2022 was focusing on the central structure of a dipole composed by a Mozambique Channel Ring and a cyclonic spiral eddy. The goals were here to observe at high resolution the mesoscale and submesoscale structures in the core of the dipole, their origins and evolution, and their potential implications for biogeochemical and ecological processes in the Mozambique Channel. To do so we crossed several times the eastern side of the dipole, towing a moving vessel profiler in addition to SADCP continuous observations and multidisciplinary stations and trawls at regular intervals. The dipole event commenced on 24 April 2022 and endured for 24 days. Existence of strong currents, reaching speeds of 150 cm/s, leads to the prevalence of horizontal stirring as the dominant process. This results in an efficient and fast transport of material from the shelf to the central Mozambique Channel. The Omega equation was used to show the dominance of a smaller scale meander for the vertical velocities. Layering is evident in the frontal structure. This first documentation of the in-situ central structure of a dipole, formed by the convergence of a Mozambique Channel Ring and a spiral eddy, lays the foundation for subsequent ecological investigations.

How to cite: Penven, P., Ternon, J. F., Noyon, M., and Herbette, S.: Central structure of a Mozambique Channel mesoscale eddy-ring dipole, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12815, https://doi.org/10.5194/egusphere-egu24-12815, 2024.

In this study, we compare global coupled climate simulations (1950’s and abrupt 4xCO2) with different ocean resolution in the Atlantic sector of the Southern Ocean (ASO), including the Agulhas, with parameterised and explicitly simulated eddies respectively. We find that the eddy-rich 1950’s simulation has a reduced South Atlantic warm bias, because of a more defined Agulhas retroflection, and coupled climate effects are observed outside the region with increased ocean resolution where e.g. equatorial precipitation changes markedly.

The Agulhas leakage plays a key role in connecting the Indian and the Atlantic oceans, with mesoscale eddies carrying heat and salt into the South Atlantic. In most state-of-the-art coupled climate models, the ocean resolution is insufficient to explicitly simulate those eddies, and they are instead represented through a parameterisation of the eddy induced flow. We use the coupled climate model FOCI, which combines a NEMO3.6 ocean with an ECHAM6 atmosphere, LIM2 sea ice, and a JSBACH land module, via an OASIS coupler. Through AGRIF nesting, we increase the ocean resolution from 1/2° to 1/10° in the Atlantic sector of the Southern Ocean.

The eddy-rich 1950’s simulation exhibits a reduced warm bias in the South Atlantic compared to the simulation without it. The bias reduction is a result of a more defined Agulhas retroflection which reduces ocean heat transport into the South Atlantic while increasing heat transport poleward. This change in ocean temperature distribution is anticipated from previous studies with ocean-only models. However, we also see coupled climate effects extending to the equatorial region, well outside the region with increased ocean resolution. We observe changes in precipitation and surface wind fields over both the tropical Atlantic and tropical/South Pacific. The changes over the tropical Atlantic are likely linked to a direct response to changes in sea surface temperature that extend across the South Atlantic. The eddy-rich 1950’s simulation also shows significant reduction of surface air temperature (SAT) biases, mostly in the Northern Hemisphere, and winds in the Southern Hemisphere, w.r.t. observationally based reanalysis products. In the strong warming scenario (abrupt 4xCO2), the eddy-rich simulation shows less SAT increase over the Atlantic and a larger seasonality in the response of the westerly wind fields over the Southern Ocean. In conclusion, increased resolution of the ASO, allowing for explicit simulation of mesoscale eddies e.g. in the Agulhas, leads to reduction of model biases and coupled climate effects.

How to cite: Ödalen, M., Savita, A., Kjellsson, J., Wahl, S., Ferreira, D., Ayres, H., Roquet, F., and Park, W.: Coupled climate effects of eddy rich model resolution in and south of the Agulhas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19696, https://doi.org/10.5194/egusphere-egu24-19696, 2024.

Amidst the global challenge of plastic pollution, the marine environment surrounding the Canary Islands is not immune to this pressing issue. Besides, the northwestern African upwelling system is an ideal environment for fisheries, which eventually become potential contributors to marine floating debris. Entanglement in large marine floating debris of fisheries origin represents a prevalent cause of stranding incidents for sea turtles. However, connecting the fisheries activity with the offshore flow of this debris towards the open ocean and the Canary Islands proves challenging due to the high mesoscale variability in the region, which hampers a straightforward visualization of clear patterns of distribution.

This study aims to investigate the offshore transport of marine floating debris originating from the upwelling zone and elucidate the underlying driving mechanisms. Additionally, the study also aims to uncover the upwelling-related origins of marine debris observed in proximity to the Canary Islands.

To analyse the oceanward transport of marine debris, OceanParcels is used, a Lagrangian tool to estimate the trajectories of virtual particles released into the ocean. These particles are released along the African coast, and their trajectories are computed following two different approaches. Firstly, seasonally averaged surface velocities are used to account for the mean seasonal fields leading to the marine debris distribution. Secondly, daily-varying surface velocities are used to simulate real ocean conditions as closely as possible. Jointly, these views provide insights into the key features responsible for transporting particles offshore. Lastly, Stokes drift is incorporated to account for its impact on particle trajectories.

The results using seasonally-averaged surface velocities reveal the formation of offshore-orientated corridors through which particles, representing marine debris, are advected oceanward. This is confirmed following the daily-varying simulations. These corridors are hypothesized to be formed by the recurrent detachment of the coastal jet stream at certain key locations of the African coastline, then leading the transport of marine debris offshore. Furthermore, virtual particles are observed that are advected offshore via upwelling filaments, i.e. cold-water tongues that extend oceanward from the inner continental shelf. Importantly, Stokes drift appears to counterwork the offshore transport of marine debris likely due to a prevailing strong southward and coastward surface advection. However, it is noted that accounting for the Stokes drift is an ongoing field of research and its effect may be overestimated as currently implemented.

On the one hand, the upwelling zone north of Cape Ghir seems to be responsible for the largest amount of upwelling-related marine debris of a northern origin, reaching the Canary Islands through a northeast-to-southwest orientated corridor. On the other hand, the upwelling zone between Cape Ghir and Cape Bojador appears to be mostly responsible for the marine debris reaching the Canary Islands with a southern origin.

How to cite: Rader, L., Aguiar-González, B., Price, T., Fraile-Nuez, E., Vega-Moreno, D., and Machín, F.: Surface circulation and marine debris: exploring the impact of northwestern African upwelling on offshore transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12927, https://doi.org/10.5194/egusphere-egu24-12927, 2024.

The Oxygen Minimum Zone (OMZ) off the coast of Mauritania and Senegal is characterized by a shallow and a deep oxygen minimum, each with potentially different formation mechanisms. Although the shallow OMZ has been linked to the en-route degradation of organic matter within highly productive, coastal-generated eddies, less attention has been paid to hypoxic Dissolved Oxygen (DO) concentrations observed along the coastal region, where low-oxygen eddies are formed. This study aims to clarify the spatio-temporal dynamics and underlying mechanisms that lead to the formation of the shallow OMZ along the northwestern African coast.

To achieve this, a Eulerian-Lagrangian numerical framework was employed by combining a coupled physical-biogeochemical model with a Lagrangian particle-tracking simulation. The model domain covers the entire Tropical Atlantic with an horizontal resolution of 3 km, achieving a good representation of the horizontal and vertical structure of the North Atlantic OMZ. To assess the pathways and evolution of the water masses that form the shallow OMZ, lagrangian particles were released in grid cells with DO < 40 μmol.l^-1 and traced backwards in time.

Our results reveal distinct seasonal and latitudinal variations of DO concentrations along the coast, with DO concentrations significantly decreasing in the transition from the upwelling to the relaxation season (from May to July). Associated with the transport of more oxygenated South Atlantic Central Waters (SACW), the influence of the Poleward Undercurrent (PUC) on the ventilation of the coastal region is evident, especially when the current loses intensity and becomes a surface-intensified feature in summer. When the PUC reaches its maximum intensity in autumn, its core deepens below the mixed layer and replaces the older, oxygen-poor waters with ventilated waters of southern origin.

The impact of eddies on coastal dynamics was also explored. A quasi-permanent Anticyclonic Modewater Eddy (ACME) formed during the upwelling season by the interaction of the PUC with the Cap-Vert headland is the main mechanism behind the import of offshore waters to the coastal region. Lagrangian particle trajectories suggest that this eddy prevents the direct northward transport of SACW by the PUC. Whilst some of the particles are trapped and subsequently transported offshore inside the eddy, other particles are stirred with an older, less oxygenated SACW variety in the offshore region and re-circulate to the coastal region. Similar particle re-circulation patterns are also observed further north, coinciding with cyclonic and ACME formation hotspots.

Our findings suggest that in addition to their role in the formation and advection of oxygen-depleted waters to offshore, coastal-generated eddies play a crucial role in modulating DO levels along the northwestern African coast.

How to cite: Cardoso, C., Calil, P., Caldeira, R. M. A., and Peliz, Á.: A model perspective on the drivers of the shallow oxygen minimum zone off the northwestern African coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6196, https://doi.org/10.5194/egusphere-egu24-6196, 2024.

Modelling the distribution of biogeochemical components in the ocean is essential for further understanding climate change impacts and assessing the functioning of marine ecosystems. This requires robust and efficient physical-biological simulations of coupled ocean-ecosystem models, which are often hindered by limited data availability and computational resources. The option of running biological tracer fields offline, independently from the physical ocean simulation, is appealing due to increased computational efficiency. Here, we present an assessment and implementation of an offline biogeochemical model — the Offline Fennel model — within the Regional Ocean Modeling System (ROMS). Our methodology employs ROMS hydrodynamic outputs to run the biogeochemical model offline. This work also includes the first evaluation exercise of the referred offline biogeochemical model. We used a variety of skill metrics to compare the simulated surface chlorophyll to an ocean colour dataset (CMEMS-Mediterranean Ocean Colour) and BGC-ARGO floats for the 2015-2020 period. The model is able to reproduce the temporal and spatial structures of the main chlorophyll fluctuation patterns in the study area, the Northwestern Mediterranean Sea, as well as the vertical distribution of chlorophyll and nitrate. This area is of particular interest as it is one of the most productive regions in the entire Mediterranean Basin, with open-ocean upwellings and deep winter convection events occurring seasonally. The typical behaviour of the region is likewise effectively represented in the implementation, including offshore primary production, nutrient supplies from the Rhone and Ebro rivers, and mesoscale hydrographic structures. This study provides a baseline for ROMS users in need of executing more biogeochemical simulations independently from more computationally demanding physical simulations.

How to cite: Crespin Esteve, J., Solé Ollé, J., and Canals Artigas, M.: An Offline Biogeochemical Model within the Regional Ocean Modelling System (ROMS): application to the Northwestern Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19719, https://doi.org/10.5194/egusphere-egu24-19719, 2024.

Much of our understanding of inertial instability in geophysical flows comes from atmospheric physics, and these studies have neglected the impact of lateral boundaries. To address this shortcoming, we performed a series of high-resolution 3D numerical simulations in Oceananigans in the context of the nonhydrostatic Boussinesq equations assuming a rigid-lid approximation. An inertially unstable baroclinic jet was investigated both far away and adjacent to a vertical boundary. The jet was chosen to be in thermal-wind balance and the buoyancy field was perturbed to instigate the instability.

We found that when the unstable jet is sufficiently close to the vertical boundary, the wavenumber of the fastest-growing unstable mode nearly doubled when compared to the jet far away from the boundary. We have not observed this shift to smaller scales in the context of a barotropic jet. The growth rates of the instability, measured by taking the l² norm of the velocity components, showed an initial linear growth phase in the first few days with no significant differences regarding the positioning of the jet. After this period, non-linear saturation stabilized the jet to inertial instability, and a secondary baroclinic instability developed. These findings suggest a previously unaccounted factor that can influence the bio-physicochemical properties of the ocean in proximity to coastal boundaries, contributing to the current understanding of the importance of submesoscale phenomena.

How to cite: Ferreira Azevedo, M., Poulin, F., and Lamb, K.: What happens when an inertially unstable jet approaches a lateral boundary?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16202, https://doi.org/10.5194/egusphere-egu24-16202, 2024.