Long range offshore transport of organic carbon from the Canary Upwelling System, an eddy-resolving modeling study

Abstract

The marine biological pump, consisting of organic carbon production, export and remineralization, is an essential component of the global carbon cycle and exerts an important control on atmospheric carbon concentrations. Lateral fluxes of organic carbon out of their region of production, especially from coastal regions, may explain the large rates of remineralization in regions of low productivity, but they are currently unaccounted by export and budget estimates. Here I quantify and characterize this lateral offshore transport of organic carbon from the north-western African coast of the Canary Upwelling System (CanUS) up to the middle of the oligotrophic North Atlantic Gyre. To this aim, I run the physical Regional Oceanic Modeling System (ROMS) coupled with the biogeochemical-ecosystem Nutrient Phytoplankton Zooplankton Detritus (NPZD) model on an Atlantic telescopic grid. This newly developed grid covers the entire Atlantic basin, allowing to study the lateral transport on very large spatial scales, and is eddy resolving in the region of interest thanks to a strong grid refinement towards the north-western African coast. With the use of climatological model simulations I study the magnitude, reach, pattern and impact of the total long range offshore flux of organic carbon, and study in detail the special role of mesoscale eddies and filaments in this transport, with a particular attention to the evolution of the eddy organic carbon budget. The results of my work indicate that, on average, more than 1/3 of the net community production of the first 100 km from the CanUS coast is laterally exported away from this region towards the open waters in the euphotic layer (100 m depth). On average in the CanUS, the divergence of this offshore flux is the largest lateral input of organic carbon in the open waters up to 1500 km from the coast and represents up to 1/3 of the large-scale mean offshore production in the open waters. This divergence even exceeds half of the mean offshore production in the euphotic layer of the central CanUS subregion, a key zonal band for the alongshore collection and offshore redistribution of the biogeochemical tracers. In the 100 m - 200 m depth layer, the offshore transport continues to dominate the lateral fluxes of organic carbon, displacing it further towards the center of the north Atlantic gyre. The long-range offshore transport of organic carbon results in a spatial decoupling of its sources and sinks, corresponding to a strongly net autotrophic watercolumn on the shelf, and a net heterotrophic watercolumn offshore, the latter stemming from the fueling of extra deep heterotrophic activity. My results demonstrate also that mesoscale activity has a fundamental role in the offshore redistribution of the organic carbon in the euphotic layer. Coastal upwelling filaments, identified with a newly developed algorithm, contribute 80% to the zonal fluxes at 100 km offshore and are the main advective source of organic carbon in the first 500 km from the coast, but their contribution declines to zero at 1000 km offshore. Mesoscale eddies, instead, represent about 20% of the offshore flux between 500 km from the coast and the middle of the North Atlantic gyre. Despite their slow drift, eddies account for 30 % of the of the total organic carbon of the offshore euphotic layer (2/3 found in cyclonic eddies), thus contributing significantly to the carbon reservoir in the open ocean. In the eddy-rich northern region of the CanUS, the long lasting organic carbon reservoir of cyclonic eddies is sustained in large part by the efficient trapping of organic carbon and nutrients at formation. These core-centric and laterally-isolated structures lose organic carbon along their way to the open ocean via sinking, while rejunevating the organic carbon reservoir through local recycling and through the use of the trapped nutrients. Anticyclonic eddies, instead, are high in organic carbon at their rim; new production in this kind of eddies is allowed by the lateral stirring of the large scale regional gradient of nutrients combined with substantial vertical mixing. Overall, the results of this work demonstrate the importance of accounting for the three-dimensionality of the marine biological pump, and highlight the essential role of the coastal ocean in the global carbon cycle. Moreover, they clarify the role of mesoscale activity in the lateral redistribution of the organic carbon from the coast, identifying the diversity of underlying mechanisms that regulate the organic carbon budget of mesoscale eddies.