ALBERT

Description: Oceanic upwelling velocities are too small to be measured directly. Deviations of the He-3/He-4 ratio in the mixed layer from solubility equilibrium provide an indirect means to infer vertical velocities at the base of the mixed layer. This method is applied to the Mauritanian upwelling region for data from three cruises in summer 2006 and winter 2007 and 2008. Diapycnal mixing coefficients are estimated from microstructure measurements, reaching from 10**-3 m**2/s over the shelf break to 10**-5 m**2/s in the open ocean. The resulting upwelling velocities in the onshore region (upto 50 km from the 50 m isobath) are of the order of 2 x 10**-5 m/s}, in agreement with Ekman theory. Further offshore, in some cases the vertical velocities inferred from the helium isotope disequilibrium exceed the values derived from the wind stress curl by one order of magnitude. The Mauritanian coastal area as part of the Canary Current upwelling system belongs to the most productive ocean regions in the world. Nutrient fluxes into the mixed layer (both advective and diffusive) are equivalent to a net community production of about 1 g C/d, and associated heat fluxes vary between 183 +/- 62 W/m**2 in summer and 97 +/- 25 W/m**2 in winter. Regarding the flux into the mixed layer, the contribution of diffusion and advection are of similar magnitude for both heat and nutrients. The upwelling, however, provides the supply of cold and nutrient rich water from below. The large offshore vertical velocities inferred from the helium method are associated with nutrient fluxes of the same order as for the onshore region, and may be responsible for observed patches of high productivity in that area. The offshore heat fluxes due to upwelling and diapycnal mixing are smaller than 70 W/m**2 for all cruises.