The influence of chlorophyll shading on ocean dynamics has been usually disregarded in eastern boundary upwelling systems modeling studies in spite of their very high primary productivity. Here, we ...study how this effect impacts on the Peru upwelling system using a regional mesoscale‐resolving physical biogeochemical coupled model. We show that the shading effect leads to a surface cooling of up to 1°C on the shelf due to subsurface cooling of the source waters during their transit toward the shelf. The shading effect leads to a more realistic subsurface stratification, a slowdown of the alongshore currents, and a shoaling of the oxycline. Impacts on the regional model biases show that the shading effect needs to be taken into account in both physical and coupled physical‐biogeochemical regional models of upwelling systems.
Plain Language Summary
The chlorophyll pigments of phytoplankton capture the downward penetrating solar energy to produce photosynthesis and warm the surface of the ocean. However, this effect is seldom taken into account in ocean models, in particular in upwelling systems where chlorophyll concentration is very high. In this study, we show that taking into account this effect in a model of the Peruvian upwelling system, one of the most productive systems in the world, modifies not only the temperature, circulation, and turbulence, but also stimulates nearshore phytoplankton production and deoxygenation over the shelf. This study shows that this effect needs to be parameterized in future modeling studies.
Key Points
The influence of surface chlorophyll shading on the Peru upwelling system is investigated using a physical biogeochemical coupled model
The shading effect leads to surface cooling, nutricline and oxycline shoaling, and phytoplankton increase on the shelf
The shading effect should be taken into account in high‐resolution eastern boundary upwelling System models
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 (1/45∘) 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
0.5 m s−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 (1/45∘) simulation, 43 % of upwelled floats and 40 % of upwelled nitrate are subducted within 20 d after upwelling, which corresponds to an increase in nitrate subduction compared to a mesoscale-resolving (1/9∘) simulation by 14 %. Taking model biases into account, we give a best estimate for subduction of upwelled nitrate off Peru between 30 %– 40 %. 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.
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