Spreading of Lagrangian Particles in the Black Sea: A Comparison between Drifters and a High-Resolution Ocean Model

The Lagrangian dispersion statistics of the Black Sea are estimated using satellite-tracked drifters, satellite altimeter data and a high-resolution ocean model. Comparison between the in-situ measurements and the model reveals good agreement in terms of the surface dispersion. The mean sub-basin coherent structures and currents of the Black Sea are well reproduced by the model. Seasonal variability of the dispersion in the upper (15 m), intermediate (150 m) and deep (750 m) layers are discussed with a special focus of the role of sub-basin scale structures and currents on the turbulent dispersion regimes. In terms of the surface relative dispersion, the results show the presence of the three known turbulent exponential, Richardson and diffusive-like regimes. The non-local exponential regime is only detected by the model for scales <10 km, while the local Richardson regime occurs between 10 and 100 km in all cases due to the presence of an inverse energy cascade range, and the diffusive-like regime is well detected for the largest distance by drifters (100–300 km) in winter/spring. Regarding the surface absolute dispersion, it reflects the occurrence of both quasi-ballistic and random-walk regimes at small and large times, respectively, while the two anomalous hyperbolic (5/4) and elliptic (5/3) regimes, which are related to the topology of the Black Sea, are detected at intermediate times. At depth, the signatures of the relative and absolute dispersion regimes shown in the surface layer are still valid in most cases. The absolute dispersion is anisotropic; the zonal component grows faster than the meridional component in any scenario

Generation mechanisms of mesoscale eddies in the Mauritanian Upwelling Region

The physical processes driving the genesis of surface- and subsurface-intensified cyclonic and anticyclonic eddies originating from the coastal current system of the Mauritanian Upwelling Region are investigated using a high-resolution (~1.5 km) configuration of GFDL’s Modular Ocean Model. Estimating an energy budget for the boundary current reveals a baroclinically unstable state during its intensification phase in boreal summer and which is driving eddy generation within the near-coastal region. The mean poleward coastal flow’s interaction with the sloping topography induces enhanced anticyclonic vorticity, with potential vorticity close to zero generated in the bottom boundary layer. Flow separation at sharp topographic bends intensifies the anticyclonic vorticity, and submesoscale structures of low PV coalesce to form anticyclonic vortices. A combination of offshore Ekman transport and horizontal advection determined the amount of SACW in an anticyclonic eddy. A vortex with a relatively dense and low PV core will form an anticyclonic mode-water eddy, which will subduct along isopycnals while propagating offshore and hence be shielded from surface buoyancy forcing. Less contribution of dense SACW promotes the generation of surface anticyclonic eddies as the core is composed of a lighter water mass, which causes the eddy to stay closer to the surface and hence be exposed to surface buoyancy forcing. Simulated cyclonic eddies are formed between the rotational flow of an offshore anticyclonic vortex and a poleward flowing boundary current, with eddy potential energy being the dominant source of eddy kinetic energy. All three types of eddies play a key role in the exchange between the Mauritanian Coastal currents system and the adjacent eastern boundary shadow zone region

Eastern boundaries survey

Report on glider surveys in eastern boundary regions, key regions for the Atlantic fishery and connected with the WP4. Survey, sampling and data delivery in these regions is investigated and reported in here

Links between the Seychelles-Chagos thermocline ridge and large scale climate modes and primary productivity; and the annual cycle of chlorophyll-a

Includes bibliographical references.The Seychelles-Chagos Thermocline Ridge (SCTR) is a region of upwelling present at 55°E- 90°E and 5°S-12°S in the southwest tropical Indian Ocean. It is a region of strong ocean-atmosphere interactions due to the high variability of the thermocline depth caused by the local Ekman pumping. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has shown high variability of surface chlorophyll-a (SChl-a) in the SCTR region. The Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) have also driven significant interannual variation of the depth of 20°C isotherm (D20) and SChl-a in the southern tropical Indian Ocean. A 50-years hindcast (RUN58-07) from a coupled bio-physical model was used to study the SChl-a concentration on an annual time scale and the interannual variability of D20 and SChla in the SCTR in response to IOD and ENSO events. Initial analysis revealed a high overestimation of SChl-a in the 50-year run. Therefore, a 44-years hindcast (RUN58-01) of the same coupled model was taken into consideration. Comparisons with observations show that the RUN58-07 reproduces the D20 and SSH better than the RUN58-01 but the RUN58-01 shows better agreement with SeaWiFS. Results reveal that the SCTR exhibits an annual cycle of SChl-a concentration, with a peak in austral winter (June-August) due to the strong southeasterlies, increasing wind stirring and induced upwelling. Vertical sections of the SCTR also indicate that an increase in surface concentration in austral winter is compensated by a decrease in subsurface phytoplankton blooms. Composite figures show that IOD events exhibit a greater influence on the subsurface and surface variability in the SCTR region. The IOD deepens and shoals the D20 in the SCTR and eastern Indian Ocean respectively whereas ENSO displays a weaker and less-extensive influence on the D20. The spatial distribution of SChl-a in the Indian Ocean is completely disrupted by IOD during which the SCTR becomes oligotrophic whereas the eastern Indian Ocean becomes highly productive. ENSO, however, does not display any significant biogeochemical signature in the SCTR. This study should improve our understanding of the interannual variability of the thermocline depth and chlorophyll-a in the SCTR region; and for the optimization of the management of fishery resources and marine ecosystems

The biophysical processes controlling the South-East Madagascar Bloom

Phytoplankton blooms are ecological hotspots in the ocean, and are fundamental to the biogeochemical cycling of elements, the storage of carbon and the ability to regulate the atmospheric carbon dioxide; and the life in the ocean. The South-East Madagascar Bloom, one of the largest blooms in the global ocean, coexists with the poleward flowing South-East Madagascar Current (SEMC), the eastward flowing South Indian Ocean Countercurrent (SICC) as well as westward-propagating surface and subsurface-intensified eddies. This austral summer bloom extends largely towards the open ocean, from the Madagascan coasts up to ~65°E and it exhibits an intriguing interannual variability. A variety of observational datasets as well as a high resolution coupled physical-biogeochemical model, based on CROCOPISCES, are used to explore the biophysical processes associated with the bloom and these westward-propagating eddies. Based on historical observational data, the bloom is shown to occur in a region of shallow mixed layer, with the surface layer exhibiting lower salinity, a possible signature of the coastal poleward flowing SEMC waters. The testing of various hypotheses revealed a dampening of the coastal current-driven upwelling south-east of Madagascar during bloom months. A dipole mesoscale feature is also prevalent close to the Madagascan coast during the bloom, from which a new hypothesis emerges. This new hypothesis states that the region south/south-east of Madagascar, influenced by local mesoscale turbulence, acts as a gate for the SEMC to flow either towards the African continent, or into the bloom region through an early retroflection, hence fertilizing the bloom. The model produces a sporadic enhancement of chlorophyll-a in the subsurface levels, associated with a low-salinity surface signature. The mean local circulation associated with the simulated bloom also reveals a dipole structure, as in observed datasets. Nitrate from subsurface levels (upwelling) as well as from the Madagascan coast (advection) is shown to influence the simulated bloom. A Lagrangian experiment shows dispersion of higher percentages of particles in the bloom region during bloom years and south of Madagascar during non-bloom years. Mesoscale eddies, originating close to Australia and which propagate westward towards southern Africa, can potentially impact the South-East Madagascar Bloom. In this study, a vast majority of these features have been shown to be subsurface-intensified eddies. A co-located eddy tracking dataset with Argo profiling floats are used to devise a subsurface-eddy identification method, which is based on the steric dynamic height anomaly of a specific eddy. Adding to the `eddy-zoo', these eddies are termed `SIDDIES' (South Indian ocean eDDIES), occurring as surface (surfSIDDIES) and subsurface (subSIDDIES) features. They travel along the latitudinal band range of 15°S to 35°S which we name the ‘SIDDIES corridor’. Advecting warm and fresh water during their propagation, cyclonic (anticyclonic) subSIDDIES contribute about 58% (32%) of the total eddy-heat flux in the South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole, high-saline water eddy-conveyor towards the western South Indian Ocean. These eddies could also possibly transport nutrients throughout their journey, impacting the biogeochemistry of the ocean near Madagascar

1. Wochenbericht MSM129/2

FS Maria S Merian – MSM129/2 07.06.2024 – 06.07.2024 St. John’s (Kanada) – Reykjavik (Island) 1st Wochenbericht (07.06. – 09.06.2024

Les processus biophysiques liés aux floraisons phytoplanctoniques au Sud-Est de Madagascar

Using observational datasets and a high resolution coupled biophysical model (CROCOPISCES), the main aims of this thesis is to study the biophysical processes associated with one of the largest phytoplankton blooms in global ocean, southeast of Madagascar, and the possible role of mesoscale eddies.The study has shown that the bloom occurs in a region of shallow-stratified mixed layer water, with low-salinity waters at the surface possibly associated with the South-East Madagasacar Current (SEMC), and dipole structure in the mean circulation. Observations show that curren-driven upwelling south of Madagascar is reduced during bloom months. It is shown in the model that nitrate from subsurface levels (upwelling) as well as from the Madagascan coast (advection) fertilize the simulated bloom. A Lagrangian analysis shows dispersion of higher percentages of particles in the bloom region during bloom years and south of Madagascar during non-bloom years.Using co-located Argo profiles and an eddy detected algorithm dataset, surface and subsurface-intensified eddies are studied. Subsurface eddies are identified using a detection method based on their steric dynamic height anomaly. Referred to as `SIDDIES’ (South Indian ocean eDDIES), they occur as surface (surfSIDDIES) or subsurface (subSIDDIES) and propagate along a latitudinal band (15°S-35°S) termed as `SIDDIES Corridor’. Advecting warm and fresh water during their propagation, cyclonic (anticyclonic) subSIDDIES contribute about 58% (32%) of the total eddy-heat flux in the South Indian Ocean.A partir d'un ensemble de données d'observation ainsi qu'un modèle couplé physiquebiogéochimique à haute résolution (CROCO-PISCES), cette thèse explore les processus biophysiques associés à l’une des plus grandes floraisons phytoplanctoniques de l’océan global, au Sud-Est de Madagascar, et le possible rôle des tourbillons sur ces blooms. L’étude montre que ce phénomène se produit dans une région caractérisée par une couche de mélange peu profonde, avec des eaux de surface moins salées probablement associées au courant Sud-Est de Madagascar (SEMC), et avec une structure dipolaire dans la circulation moyenne. Les observations ont révélé une diminution des remontées d’eaux froides (upwelling) le long des côtes sud-est de Madagascar pendant les mois de bloom. Dans le modèle, les nitrates provenant des niveaux de subsurface (advection verticale ; upwelling) ainsi que de la côte malgache (advection horizontale) favorisent la production phytoplanctonique simulée. Une expérience lagrangienne de particules montre une plus forte advection de ces dernières dans la zone de floraison pendant les périodes de bloom alors qu’elles sont déviées vers le sud de Madagascar vers le continent Africain pendant les années sans floraison. Une étude est réalisée à partir d’un jeu de données de suivi des tourbillons co-localisés avec des flotteurs de profilage Argo, pour mieux comprendre des tourbillons intensifiés en surface et subsurface. Une méthode d’identification des structures tourbillonnaires de subsurface a été mise en place en se basant sur l’anomalie de la hauteur dynamique stérique. Ces tourbillons, appelés ‘SIDDIES’ (South Indian ocean eDDIES), se produisent en tant que tourbillon intensifié en surface (surfSIDDIES) et en subsurface (subSIDDIES). Ils se déplacent le long d’une bande de latitude située entre 15°S et 35°S appelée « couloir SIDDIES ». Au cours de leurs déplacements, les subSIDDIES cycloniques (anticycloniques) transportent via les processus d’advection, des masses d’eaux chaudes et peu salées de l’Est vers l’Ouest de l’Océan Indien, contribuant ainsi à environ 58% (32%) du flux total de chaleur par tourbillons dans le sud de l'océan Indien

3. Wochenbericht MSM129/2

FS Maria S Merian – MSM129/2 07.06.2024 – 06.07.2024 St. John’s (Kanada) – Reykjavik (Island) 3. Wochenbericht (17.06. – 23.06.2024

Apparent oxygen utilization and relative percentage of South Atlantic Central Water (SACW) during METEOR cruise M156

This dataset includes calculated data over the epi-mesopelagic layer (0-800 m depth) of 26 stations with 14 of them inside or in the vicinity of a cyclonic eddy that formed off Mauritania along the ∼ 900 km zonal corridor between Mauritania and the Cabo Verde islands in the eastern Tropical North Atlantic during the M156 cruise on the RV Meteor from July 3rd to August 1st 2019. The apparent oxygen utilization was calculated as the difference between saturation concentrations of O2 and measured O2 concentrations: AOU= [O2sat ({*}{*}θ{*}{*}, S)] − [O2], with S = salinity and θ = potential temperature (Redfield, 1942; Redfield et al., 1963; Pytkowicz, 1971). The saturated oxygen (O2sat) was computed using measured temperature and salinity following 212 Garcia and Gordon (1992). Relative percentage of South Atlantic Central Water was determined as in Klenz et al., (2018)