Diapycnal Nutrient Fluxes in the Cape Ghir upwelling region

European Geosciences Union General Assembly 22-27 April 2012, Vienna, Austria.-- 1 pageAn oceanographic survey was carried out from 18 to 29 October 2010 in the Canary Basin (PROMECA project). Near Cape Ghir, in the Northwest Africa coastal upwelling, 17 CTD casts were made to obtain continuous records of conductivity and temperature with depth, and to collect waters samples for nutrients analyses. Additionally, free-fall turbulence profiles were obtained at each station. 14 Expandable bathythermographs (XBTs) were deployed between stations to increase the grid resolution of the temperature field. Velocity data were acquired with a vessel-mounted Acoustic Doppler Current Profiler (ADCP) with a vertical bin size of 8 m. Water samples for nutrients: nitrate + nitrite (N+N), phosphates and silicates, were collected from 12 depths (down to 2000 m or the maximum depth), with 12-l Niskin bottles mounted on the rosettes sampler. The first results show low to moderate concentrations of nutrients offshore, with average values in the upper 150 m of 2.45 ± 1.98, 0.37 ± 0.18 and 1.47 ± 0.94 μM for N+N, phosphate and silicate, respectively. However, for stations near the Cape Ghir upwelling filament or influenced by this feature, a significant increase in the concentration of nutrients (up to 10 µM of N+N at 200 m) was observed. We have estimated and compared the diapycnal nutrient flux in the region by using two different approaches. First, we used the dissipation rates of turbulent kinetic energy and thermal variance estimated from microstructure data acquired from turbulence profilers, and applied a model based on the dissipation ratio. This way we obtain net turbulence diffusivities in regions where there is an interaction of processes of double diffusion and turbulence induced by vertical shear of the flow. The second approach is based on obtaining diapycnal diffusivities with parameterizations of the gradient Richardson number and density ratioPeer Reviewe

Importance of salt fingering for new nitrogen supply in the oligotrophic ocean.

The input of new nitrogen into the euphotic zone constrains the export of organic carbon to the deep ocean and thereby the biologically mediated long-term CO2 exchange between the ocean and atmosphere. In low-latitude open-ocean regions, turbulence-driven nitrate diffusion from the ocean’s interior and biological fixation of atmospheric N2 are the main sources of new nitrogen for phytoplankton productivity. With measurements across the tropical and subtropical Atlantic, Pacific and Indian oceans, we show that nitrate diffusion (171±190 mmolm 2 d 1) dominates over N2 fixation (9.0±9.4 mmolm 2 d 1) at the time of sampling. Nitrate diffusion mediated by salt fingers is responsible for ca. 20% of the new nitrogen supply in several provinces of the Atlantic and Indian Oceans. Our results indicate that salt finger diffusion should be considered in present and future ocean nitrogen budgets, as it could supply globally 0.23–1.00 TmolNyr 1 to the euphotic zone.MALASPINA (CSD2008-00077)Versión del editor10,015

The Contribution of Double Diffusion versus Shear-Driven Turbulent to Diapycnal Mixing in the Cape Ghir Upwelling Region

IV Encuentro Oceanografía Física Española, celebrado del 20 al 22 de julio de 2016 en Alicante,España.-- 1 pageOne oceanographic survey was carried out within a project multidisciplinary (PROMECA) from 18 to 29 October 2010 in the Canary Basin. During three days, conductivity–temperature-depth (CTD), expandable bathytermograph temperature (XBT), microstructure turbulence (free-fall turbulence profiler) and acoustic Doppler current profiler (ADCP) data were collected in several stations in the Cape Ghir upwelling region. The results show how the vertical distribution of the dissipation rate of the turbulent kinetic energy, ɛ, and of the dissipation rates of thermal variance, ᵡ, are modified by the filaments present in the region. These oceanographic structures intensify the vertical shear of the flow and modify the vertical gradients of temperature and salinity, thus influencing the sources of mixing processes associated (Kelvin-Helmholtz instabilities and double diffusion). The ɛ values range between 10-7 and 10-10 W/kg in the first 500 m of the water column with maximum values near the surface or associated to the filaments. By examining vertical distributions of density ratio, Rρ, and gradient Richardson number, Ri, together with ɛ and ᵡ, we found zones of the water column where dominates salt fingers or shear-driven turbulence. Applying an analysis based on the intermittency factor with a model based on the dissipation ratio, we estimate the net turbulence diffusivities when processes of double diffusion and shear-driven turbulence are present in the same regionPeer Reviewe

Estimación de la capa de mezcla y parametrización de los procesos de entrainment en el filamento de cabo Ghir

II Encuentro de la Oceanografía Física Española (EOF): Conociendo los mares para el beneficio de la sociedad, 14-16 de noviembre de 2012, MadridPeer Reviewe

Diapycnal nutrient fluxes on the northern boundary of Cape Ghir upwelling region

10 pages, 9 figures, 1 tableIn this study we estimate diffusive nutrient fluxes in the northern region of Cape Ghir upwelling system (Northwest Africa) during autumn 2010. The contribution of two co-existing vertical mixing processes (turbulence and salt fingers) is estimated through micro- and fine-structure scale observations. The boundary between coastal upwelling and open ocean waters becomes apparent when nitrate is used as a tracer. Below the mixed layer (56.15±15.56m), the water column is favorable to the occurrence of a salt finger regime. Vertical eddy diffusivity for salt (Ks) at the reference layer (57.86±8.51m, CI 95%) was 3×10-5 (±1.89×10-9, CI 95%) m2s-1. Average diapycnal fluxes indicate that there was a deficit in phosphate supply to the surface layer (6.61×10-4mmolm-2d-1), while these fluxes were 0.09 and 0.03mmolm-2d-1 for nitrate and silicate, respectively. There is a need to conduct more studies to obtain accurate estimations of vertical eddy diffusivity and nutrient supply in complex transitional zones, like Cape Ghir. This will provide us with information about salt and nutrients exchange in onshore-offshore zones. © 2013 Elsevier Ltd.This study was supported by projects PROMECA-2010 (CTM2009-06993-E/MAR) and PROMECA (CTM2008-04057). [...] The fellowship was supported by Ministerio de Asuntos Exteriores y de Cooperación y Agencia Española de Cooperación Internacional del Gobierno Español (MAEC-AECID). J.A. and M.B. were funded by project CAIBEX (CTM2007-66408-C02-02)Peer Reviewe