Eddy Effects on Biological Productivity in the Eastern South Pacific: a 3D Physical-Biogeochemical Modeling and Eddy Tracking Approach

Mesoscale eddies are ubiquitous in the ocean and their contribution to the global transport of heat, salt and biogeochemical tracers has appeared to be comparable to those of the large-scale wind- and thermohaline-driven circulations. However, their effect on the ocean productivity is still debated. In eastern boundary upwelling systems, eddies may play an essential role in the coastal upwelling production and cross-shore transport. Here the effects of mesoscale eddies on biological productivity in the Peru-Chile upwelling system (PCUS) and, more generally, the eastern South Pacific (ESP), are tackled with a three-dimensional physical-biogeochemical modeling approach (ROMS-PISCES). Satellite data of sea surface height, temperature and chlorophyll are used to validate interannual simulations run over the period 2000-2008. Eddy-resolving and eddy-permitting solutions are compared to make a first assessment of the effect of mesoscale activity on biological productivity. Surface and subsurface (intra-thermocline) mesoscale eddies are then followed in the eddy-resolving simulation from their formation at the coast to their spreading in the subtropical gyre, while nutrient and carbon fluxes (phytoplankton, zooplankton and detritus) due to both biological production/loss terms inside the eddy volumes and horizontal/vertical exchanges at the eddy boundaries are tracked. In particular, the impact of nutrient injection by subsurface eddies on productivity in the surface layer is evaluated along the trophic gradient from the coastal upwelling to the oligotrophic gyre. Furthermore, the contribution of mesoscale eddies to the spatio-temporal variability of biological productivity is described on seasonal and interannual scales, and the respective roles of surface and subsurface eddies are discussed. Finally, we seek to evaluate the insight offered by this method for our understanding of the “net” eddy effect on biological productivity in the ESP.