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Subproject 1.11

Upwelling velocities inferred from helium isotope disequilibrium

Upwelling is an important factor for the exchange of biogeochemical properties between the mixed layer and the ocean interior. It plays an important role for the nutrient supply of the euphotic zone and also has a cooling effect on the mixed layer. Climate relevant trace gases such as nitrous oxide (N2O) are produced underneath the oceanic mixed layer, transported towards the surface by upwelling and outgassing into the atmosphere.

As upwelling velocities are in the order of 10^-5 m/s, they cannot measured directly. Within SOPRAN, the helium-3 disequilibrium of the surface waters has been used to indirectly infer the magnitude of the upwelling. This method has been applied to the equatorial eastern Atlantic and the area off the Mauritanian coast during the earlier stages of SOPRAN. The mean upwelling in the coastal region is determined by the Ekman divergence, whereas further offshore the helium method indicates eddy induced upwelling.

During SOPRAN phase III, the strength of the Peruvian upwelling will be investigated and compared with the other two SOPRAN regions. By including diapycnal diffusivities, the total fluxes of heat, nutrients and trace gases into the mixed layer will be estimated.
Fig.1: Vertical velocities in the Mauritanian upwelling area.

PI: R. Steinfeldt
Contact: Reiner Steinfeldt (