24.06.2019: Joint FB1/SFB754 Seminar

Dr. Sören Thomsen, LOCEAN-IPSL, Sorbonne Universités, Paris (France): "Let´s stop seeing the upwelling problem through Ekman lenses"

11:00 Uhr, Hörsaal, Düsternbrooker Weg 20


In the classical view newly upwelled water moves offshore via Ekman transport in upwelling regimes. But eddies, filaments and fronts, strongly modulate this offshore transport and a significant fraction of the upwelled water is directly subducted back again into the thermocline. In this study we investigate how air-sea buoyancy fluxes modulate the cross-shore circulation and in particular the subduction process. We force an idealized upwelling model configuration of CROCO (Coastal and Regional Ocean COmmunity model) with constant winds but varying heat fluxes. Simulations both at mesoscale resolving (dx = 8 km, 1/13.4º) and submesoscale rich horizontal resolution (dx = 800 m, 1/134º) are carried out. Typical features of a subtropical upwelling system such as eddies, filaments and fronts are well represented by the model. The changing heat flux forcing clearly impacts the cross-shore circulation and the subduction process: At zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest. The Eulerian mean cross-shore circulation is canceled out by the eddies which result in a vanishing residual transport near the coast (i.e. cold waters are, on average, not subjected to upwelling). With increasing heat fluxes the classical Ekman transport residual circulation recovers due to a weakening of the eddy fluxes. Eddy driven buoyancy fluxes are about 25% stronger in the upper 50 m at higher horizontal resolution. The heat flux effects reach down to 125 m and are of the same magnitude as the resolution effect. Our findings might have strong implications for the overall understanding of upwelling systems functioning, given the importance of air-sea buoyancy flux variability on synoptic but also longer time scales.


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