Turbulent mixing processes and circulation in the upwelling region off Northwest Africa

Das mauretanische Auftriebsgebiet ist eine der biologisch produktivsten Region im Weltozean und ist ferner als Quelle verschiedener klimarelevanter Spurengase bekannt, wodurch ihm eine wichtige Rolle im globalen Ökosystem zukommt. Diese Studie untersucht klimarelevante physikalische und biogeochemische Prozesse anhand von Mikrostruktur-, CTD- und ADCP-Messungen, die auf fünf Forschungsfahrten zwischen 2005 und 2008 erhoben wurden. Ziel dieser Arbeit ist die Identifikation dominanter turbulenter Vermischungsprozesse und die Quantifizierung ihrer Bedeutung für das Auftriebsgebiet vor Nordwestafrika. Hierzu wurde sowohl Stärke als auch räumliche Verteilung des vertikalen Austauschkoeffizienten anhand von Energiedissipationsraten bestimmt. Die während der verschiedenen Reisen durchgeführten Turbulenzmessungen zeigten, dass die Vermischung im mauretanischen Auftriebsgebiet nahe des Kontinentalhangs am größten ist. Die beobachtete mittlere Dissipationsrate küstenwärts der 500-m-Isobathe (entfernt von der Oberfläche) beträgt 5 × 10-8 W kg-1. Der resultierende mittlere Austauschkoeffizient in dieser Region ist mit Kρ = 12 × 10-4 m2 s-1 um mehr als eine Größenordnung stärker gegenüber den Werten, die weiter entfernt von der Küste bestimmt wurden. Diese verstärkte Vermischung liefert einen erheblichen Anteil des Austausches von Wärme, klimarelevanten Spurengasen und von Nährstoffen zwischen der Deckschicht und dem tieferen Ozean. Eine Vielzahl von Prozessen ist verantwortlich für die erhöhte Vermischung, wobei die Wechselwirkungen zwischen internen Gezeiten und Topographie dominieren. Die präsentierten Beobachtungen zeigen ferner das hohe Maß an räumlicher und zeitlicher Variabilität der Vermischung im oberen Ozean. Zusätzlich ergaben die Untersuchungen zur Zirkulation ein modifiziertes Bild der Oberflächenströmungen im Vergleich zu vorangegangenen Untersuchungen. Insbesondere konnte eine südwärtige Fortsetzung des Kanarenstroms auf dem Schelf nicht beobachtet werden. Die Existenz eines östlichen Randstroms, der in der Literatur als Upwelling Undercurrent (UUC) beschrieben wird, konnte bestätigt und sein mittlerer Transport mit etwa 1 Sv abgeschätzt werden. Er stellt den wesentlichen Versorgungspfad für das küstennahe Auftriebsgebiet mit nährstoffreichem Südatlantischen Zentralwasser aus dem Süden dar. Im Allgemeinen wird die Zirkulation jedoch durch Wirbel dominiert. Eine zyklonale Zirkulationszelle zwischen der afrikanischen Küste und den Kapverdischen Inseln konnte insofern bestätigt werden, als dass küstennah verstärkt eine nordwärtige und küstenfern verstärkt eine südwärtige Strömungstendenz beobachtet wurde.The upwelling region off Mauritania is an important region for the global ecosystem as it is one of the most productive areas of the world ocean. Additionally it is an oceanic source for several climate relevant trace gasses such as CO2 and N2O. This study determines physical and biogeochemical processes from microstructure, CTD and ADCP measurements taken on five ship surveys during the period 2005-2008. The main focus of this study is to identify turbulent mixing processes and to quantify their importance for the upwelling region off Mauritania. Therefore magnitude and spatial distribution of vertical diffusivities has been determined from energy dissipation rates. The turbulence measurements taken from these surveys show that the mixing is greatest close to the shelf break. The observed mean dissipation rates inshore of the 500\,m isobath (away from the surface) is 5 × 10-8 W kg. The resulting mean diffusion coefficient of Kρ = 12 × 10-4 m2 s-1 is more than one magnitude higher compared to the values found at greater distances from the coast. This increased mixing is responsible for an increased exchange of heat, climate relevant trace gases and nutrients between the surface layer and the deeper ocean. Many processes are responsible for the increased mixing dominated by the interaction between internal tide and topography. The presented observations also show high variabilities, both spatial and temporal, in the mixing rates of the upper ocean. Additionally, studies of the circulation show a modified surface circulation in comparison to previous studies. A southward continuation of the Canary Current on the shelf was not observed. The existence of an easterly boundary current, also known as Upwelling Undercurrent (UUC), could be confirmed with a mean transport of 1\,Sv. It describes the main supply of the nutrient rich South Atlantic Central Water for the coastal upwelling region. Generally the circulation is dominated by eddies. The observed intensified inshore northwards and offshore southwards current tendency between the African coast and the Cape Verde Islands give the confirmation for a cyclonical circulation cell

Sea-to-air and diapycnalnitrous oxide fluxes in the eastern tropical North Atlantic Ocean

Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-toair and diapycnal N2O fluxes indicated that the mean sea-toair flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones

Oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands

The open-ocean oxygen minimum zone (OMZ) south and east of the Cape Verde Islands is studied from CTD hydrography, ADCP velocities, Argo float trajectories, and historical data, with a focus on the zonal supply and drainage paths. The strongest oxygen minimum is located north of the North Equatorial Countercurrent (NECC) at about 400 to 500-m depth just above the boundary between Central Water and Antarctic Intermediate Water (AAIW). It is shown that the NECC, the North Equatorial Undercurrent at 4 to 6°N, and a northern branch of the NECC at 8 to 10°N are the sources for oxygen-rich water supplied to the OMZ in summer and fall. A weak eastward NECC at 200-m depth also exists in winter and spring as derived from Argo floats drifting at shallow levels. Historical oxygen data from 200-m depth confirm this seasonality showing high (low) oxygen content in summer and fall (spring) within the supply paths. Compared to the strong oxygen supply at 150 to 300-m depth, the ventilation of the OMZ at 300 to 600-m depth is weaker. Westward drainage of oxygen-poor water takes place north of the Guinea Dome, i.e., north of 10°N, most pronounced at 400 to 600-m depth. In July 2006 the total eastward transport of both NECC bands above σ θ = 27.1 kg m−3 at 23°W was about 13 Sv (1 Sv = 106 m3 s−1). About half of this water volume circulates within the Guinea Dome or recirculates westward north of the Guinea Dome

Tidal-induced mixing and diapycnal nutrient fluxes in the Mauritanian upwelling region

The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to ɛ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2 μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region

Water masses and currents in the upper tropical Northeast Atlantic off Northwest Africa

Recent current measurements in the tropical eastern North Atlantic reproduce the components of the large scale flow field. However, the observations as well as the 1/12°-FLAME model computations indicate that a lot of eddy scale variability is superimposed on the mean flow field. Despite of the disturbance by variability the signature of the Guinea Dome is well present. In November 2002 the Guinea Dome transport from direct observations was about 2.8 Sv above σ θ = 25.8 kg/m3 and 4 Sv between σ θ = 25.8 and 27.1 kg/m3. The oxygen minimum in the shadow zone comprises the central water and the Antarctic Intermediate Water (AAIW) layers and is located between the equatorial current system and the North Equatorial Current. The North Equatorial Counter- and Undercurrents at 3° to 6°N are major oxygen sources for the central water layer of the low-oxygen regions in the northeastern tropical Atlantic. A second, northern North Equatorial Countercurrent (nNECC) band exists at 8° to 10°N. The nNECC carries oxygen rich water from the southern hemisphere eastward but with an admixture of water from the northern hemisphere. A float at 200 m depth was spreading eastward in the North Equatorial Undercurrent (NEUC), at 28°W it shifted northward into the nNECC, and then was trapped in the Guinea Dome region for more than 3 years. The model indicates the region 22° to 32°W as the area of exchange between the NECC/NEUC and the nNECC bands. In the AAIW layer the northern Intermediate Countercurrent acts as oxygen source for the oxygen minimum zone

Turbulente Vermischungsprozesse und Zirkulation im Auftriebsgebiet vor Nordwestafrika

The upwelling region off Mauritania is an important region for the global ecosystem as it is one of the most productive areas of the world ocean. Additionally it is an oceanic source for several climate relevant trace gasses such as CO2 and N2O. This study determines physical and biogeochemical processes from microstructure, CTD and ADCP measurements taken on five ship surveys during the period 2005-2008. The main focus of this study is to identify turbulent mixing processes and to quantify their importance for the upwelling region off Mauritania. Therefore magnitude and spatial distribution of vertical diffusivities has been determined from energy dissipation rates. The turbulence measurements taken from these surveys show that the mixing is greatest close to the shelf break. The observed mean dissipation rates inshore of the 500\,m isobath (away from the surface) is 5 × 10-8 W kg. The resulting mean diffusion coefficient of Kρ = 12 × 10-4 m2 s-1 is more than one magnitude higher compared to the values found at greater distances from the coast. This increased mixing is responsible for an increased exchange of heat, climate relevant trace gases and nutrients between the surface layer and the deeper ocean. Many processes are responsible for the increased mixing dominated by the interaction between internal tide and topography. The presented observations also show high variabilities, both spatial and temporal, in the mixing rates of the upper ocean. Additionally, studies of the circulation show a modified surface circulation in comparison to previous studies. A southward continuation of the Canary Current on the shelf was not observed. The existence of an easterly boundary current, also known as Upwelling Undercurrent (UUC), could be confirmed with a mean transport of 1\,Sv. It describes the main supply of the nutrient rich South Atlantic Central Water for the coastal upwelling region. Generally the circulation is dominated by eddies. The observed intensified inshore northwards and offshore southwards current tendency between the African coast and the Cape Verde Islands give the confirmation for a cyclonical circulation cell