Observed and modeled biogeochemistry of filaments off Peru

Abstract

The biogeochemistry of an observed cold filament off Peru and the representation of filaments in submesoscale-permitting (1/45°) coupled biogeochemical model simulations are the focus of this thesis. Furthermore, we use the simulations to investigate the effect of submesoscale frontal processes on the distribution of biogeochemical tracers in the shallow oxygen minimum zone off Peru. The observed filament contains relatively cold, fresh and nutrient-rich waters originating in the coastal upwelling. Enhanced nitrate concentrations and offshore velocities of up to 0.5 m/s within the filament suggest an offshore transport of nutrients. Despite low chlorophyll α concentrations in the core of the filament, depth integrated primary production is 40% higher than at the upwelling front and 25% higher than offshore. The highly variable relationship of surface chlorophyll α and depth-integrated primary production highlights the inherent uncertainty of primary production estimates based on ocean-color measured by satellites. The observations are used to assess the results of two different biogeochemical model simulations (PISCES / BioEBUS). Both simulations exhibit filaments that are similar in lateral scale, horizontal and vertical structure and offshore extent to those observed, but differences exist in the biogeochemistry: While the PISCES simulation exhibits nitrate concentrations within filaments comparable to observations, filaments are largely depleted of nitrate in the BioEBUS simulation. This difference can be related to a higher pyhtoplankton growth rate and faster nitrate uptake in BioEBUS. The importance of sufficiently slow phytoplankton growth for maintaining realistic concentrations of upwelled nutrients offshore is therefore stressed. Furthermore, the simulations suggest that submesoscale frontal processes increase subduction and offshore export of nitrate which leads to reduced primary production. An increase in oxygen that resembles the pattern of the decrease in nitrate suggests a ventilation of the shallow oxygen minimum zone off Peru by vertical and horizontal eddy-fluxes.