Filaments and fronts play a crucial role for a net offshore and downward nutrient transport in Eastern Boundary Upwelling Systems (EBUSs) and thereby reduce regional primary production. Most studies on this topic are based on either observations or model simulations, but only seldom are both approaches are combined quantitatively to assess the importance of filaments for primary production and nutrient transport. Here we combine targeted interdisciplinary shipboard observations of a cold filament off Peru with submesoscale-permitting (1/45g ) coupled physical (Coastal and Regional Ocean Community model, CROCO) and biogeochemical (Pelagic Interaction Scheme for Carbon and Ecosystem Studies, PISCES) model simulations to (i) evaluate the model simulations in detail, including the timescales of biogeochemical modification of the newly upwelled water, and (ii) quantify the net effect of submesoscale fronts and filaments on primary production in the Peruvian upwelling system. 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 0.5gmgs-1 within the filament suggest an offshore transport of nutrients. Surface chlorophyll in the filament is a factor of 4 lower than at the upwelling front, while surface primary production is a factor of 2 higher. The simulation exhibits filaments that are similar in horizontal and vertical scale compared to the observed filament. Nitrate concentrations and primary production within filaments in the model are comparable to observations as well, justifying further analysis of nitrate uptake and subduction using the model. Virtual Lagrangian floats were released in the subsurface waters along the shelf and biogeochemical variables tracked along the trajectories of floats upwelled near the coast. In the submesoscale-permitting (1/45g ) simulation, 43g% of upwelled floats and 40g% of upwelled nitrate are subducted within 20gd after upwelling, which corresponds to an increase in nitrate subduction compared to a mesoscale-resolving (1/9g ) simulation by 14g%. Taking model biases into account, we give a best estimate for subduction of upwelled nitrate off Peru between 30g%-g40g%. Our results suggest that submesoscale processes further reduce primary production by amplifying the downward and offshore export of nutrients found in previous mesoscale studies, which are thus likely to underestimate the reduction in primary production due to eddy fluxes. Moreover, this downward and offshore transport could also enhance the export of fresh organic matter below the euphotic zone and thereby potentially stimulate microbial activity in regions of the upper offshore oxygen minimum zone.
Bibliografisk noteFunding Information:
Financial support. This research has been supported by the DFG
Acknowledgements. This work is a contribution of the Sonder-forschungsbereich 754 Climate–Biogeochemistry Interactions in the Tropical Ocean (https://www.sfb754.de/), which is funded by the Deutsche Forschungsgemeinschaft (DFG). Soeren Thomsen received funding from the European Commission (Horizon 2020, MSCA-IF-2016, WACO 749699: Fine-scale Physics, Biogeochem- istry and Climate Change in the West African Coastal Ocean). Yonss S. Jose received funding from the BMBF (Humboldt Tipping Points, 01LC1823B). We are grateful to the Peruvian authorities for the permission to carry out scientific work in their national waters. Special thanks go to the captain and the crew of the R/V Meteor for their support during the M136 cruise and to the chief scientist Marcus Dengler. For processing the nutrient samples used for the NO3 sensor calibration, we would like to thank the group of Stefan Sommer and their technicians. We would also like to thank Clarissa Karthäuser and Gabriele Klockgether for help with sample analysis for the primary-production measurements.
(SFB; grant no. 754), the European Commission Horizon 2020 Framework Programme (WACO; grant no. 749699), and the BMBF (Humboldt Tipping Points; grant no. 01LC1823B).