ALBERT

Description: 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.

Description: We investigated turbulence and vertical transport at the “Tommeliten” site in the Norwegian sector of the central North Sea during the R/V Celtic Explorer (CE0913) cruise from 8 - 11 August 2009. The sediments at this site are rather flat, sandy and non-permeable, with the presence methane seeps, as well bacterial mats and seep-related fauna. The hydrography of the ~70 m deep water column was characterized by a mixed surface layer extending to about 20 m depth and a well-mixed ~30 m thick bottom layer that was separated by a stratified interior layer (Figure 1). Amplitudes of tidal velocities were as large as 0.3 m s-1 in the bottom boundary layer. Dissipation rates of turbulent kinetic energy (ε) etermined from microstructure shear profiles was weak (~10-9 W kg-1 - the detection limit of the profiler) in the thermocline but increased to 10-7-10-6 W kg-1 approaching the sea floor and the surface (Figure 1). Vertical turbulent eddy diffusivities (KZ, Figure 2) ranged from 10-6 m2s-1 in the stratified interior to 10-3 m2s-1 and 10-4 m2s-1 in surface and bottom boundary layers respectively; the pseudo-velocity, defined as t =L2 / 2Kz with L=1m (Figure 2) was on the order of hours to several weeks/months in the stratified interior. High-resolution dissolved oxygen (DO) profiles were measured with a fast galvanic AMT oxygen sensor (response time 0.2 s) mounted on the microstructure probe. The sensor is capable of resolving oxygen fine structures (1 cm scale), i.e. the structures in the stratified interior, that are completely overlooked by standard slow DO sensors (Figure 1). Vertical turbulent DO fluxes were calculated using the gradient method with locally-measured dissipation rates of turbulent kinetic energy. The average downward turbulent DO flux from the thermocline to the bottom water was estimated to be 4.4 ± 1.4 mmol m-2 d-1. The AMT sensor now allows us to resolve the before unrealized steep gradient in DO, and to properly characterize the downward fluxes. With benthic DO fluxes from chambers on the order of ~7 mmol m-2 d-1, the water column depletion should therefore be about 3-4 mmol m-2 d-1. This agrees with the observed DO concentrations of about 200 μmol L-1 (67% sat) and points to the thermocline being a significant source of DO. Previously, fluxes would have been grossly underestimated due to the inadequate response time of the traditional membrane sensors. The results of the study show that the acquisition of high-resolution constituent profiles together with local microstructure measurements are necessary to characterized the dynamics of a system with regard to constituent fluxes and to set proper boundary conditions for modeling applications.

Description: The oceans account for an estimated 5 to 40 percent of the total supply of potent greenhouse gas nitrous oxide (N2O) to the atmosphere. Our contribution explores oceanic diapycnal N2O flux in the area of the oxygen minimum zone (OMZ) off West Africa in order to determine N2O sea-to-air-flux more precisely. Within the OMZ, nitrification in regions of low oxygen concentration below the thermocline is suggested to be the main N2O source. In the open ocean far from coastal upwelling, upward transport of N2O from source regions across the thermocline to mixed layer should mainly be driven by turbulent mixing. Collected data during SOPRAN phase 1 comprise N2O profiles and associated turbulent dissipation profiles of the upper 500m at the OMZ and TENATSO time series station, complemented by oxygen and CTD profiles. This data set allows direct calculation of N2O diapycnal flux, especially across the thermocline where very low turbulent mixing meets steep N2O gradients.

Description: 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.

Description: 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.