14 Apr 2020

14 Apr 2020

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

The fate of upwelled nitrate off Peru shaped by submesoscale filaments and fronts

Jaard Hauschildt1, Soeren Thomsen1,2, Vincent Echevin2, Andreas Oschlies1, Yonss Saranga José1, Gerd Krahmann1, Laura A. Bristow3,4, and Gaute Lavik3 Jaard Hauschildt et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Laboratoire d’Océanographie et du Climat, Expérimentations et Approches Numériques (LOCEAN), Institut de Recherche pour le Développement (IRD), Institut Pierre-Simon Laplace (IPSL), Université Pierre et Marie Curie (UPMC), Paris, France
  • 3Max Planck Institute for Marine Microbiology (MPI), Bremen, Germany
  • 4Department of Biology / Nordcee, University of Southern Denmark (SDU), Odense, Denmark

Abstract. Filaments and fronts play a crucial role for a net offshore and downward nutrient transport in Eastern Boundary Upwelling Regions (EBUS) and thereby reduce primary production. Often studies are either based on observations or model simulations but seldom 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 (1/45°) coupled physical (CROCO) and biogeochemical (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 of 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 0.5 m s−1 within the filament suggest an offshore transport of nutrients. Surface chlorophyll in the filament is a factor 4 lower than at the upwelling front while surface primary production is a factor 2 higher, highlighting the additional value of direct rate measurements for model validation. The simulation exhibits filaments that are similar in horizontal and vertical scale compared to the observed filament. Nitrate concentrations and primary pro- duction 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 (1/45°) simulation 43.0 % of upwelled floats and 40.6 % of upwelled nitrate is subducted within 20 days after upwelling, which corresponds to an increase of nitrate subduction compared to a mesoscale-resolving (1/9°) simulation by 13.9 %. This suggests 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 photic zone and thereby potentially stimulate microbial activity in the upper offshore oxygen minimum zone.

Jaard Hauschildt et al.

Jaard Hauschildt et al.

Data sets

ADCP current measurements (38 and 75 kHz) during METEOR cruise M136 M. Dengler, R. Czeschel, J. Lüdke, and G. Krahmann

Physical oceanography (CTD) during METEOR cruise M136 G. Krahmann and M. Dengler

Underway CTD data collected during METEOR cruise M136 M. Dengler, S. Thomsen, and G. Krahmann

Jaard Hauschildt et al.


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Short summary
In this paper we quantify the subduction of upwelled nitrate due to physical processes on the order of several kilometers in the coastal upwelling off Peru and its effect on primary production. We also compare the prepresentation of these processes in a high-resolution simulation (~2.5 km) with a more coarsely resolved simulation (~12 km). To do this, we combine high-resolution shipboard observations of physical and biogeochemical parameters with a complex biogeochemical model configuration.