Mesoscale contribution to the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

Elisa Lovecchio,Matthias Münnich,Nicolas Gruber

doi:10.5194/bg-15-5061-2018

Abstract

Abstract. Several studies in upwelling regions have suggested that mesoscale structures, such as eddies and filaments, contribute substantially to the long-range transport of the organic carbon from the nearshore region of production to the offshore region of remineralization. Yet a comprehensive analysis of this mesoscale flux and of its impact across the Canary Upwelling System (CanUS) has not been provided. Here, we fill this gap using simulations with the Regional Oceanic Modeling System (ROMS) coupled to a Nutrient, Phytoplankton, Zooplankton and Detritus (NPZD) ecosystem model. We run climatological simulations on an Atlantic telescopic grid with an eddy-resolving resolution in the CanUS. Using both a Reynolds flux decomposition and structure-identification algorithms, we quantify and characterize the organic carbon fluxes driven by filaments and eddies within the upper 100 m and put them in relationship to the total offshore transport. Our analysis reveals that both coastal filaments and eddies enhance the offshore flux of organic carbon, but that their contribution is very different. Upwelling filaments, with their high speeds and high concentrations, transport the organic carbon offshore in a very intense, but coastally confined manner, contributing nearly 80 % to the total flux of organic carbon at 100 km offshore. The filament contribution tapers off quickly to near zero values at 1000 km off the coast, leading to a strong offshore flux divergence that is the main lateral source of organic carbon in the coastal waters up to 1000 km offshore. Some of this divergence is also due to the filaments inducing a substantial vertical subduction of the organic carbon below 100 m. Owing to the temporal persistence and spatial recurrence of filaments, the filament transport largely constitutes a time-mean flux, while the time-varying component, i.e., the turbulent flux, is comparatively small. At distances beyond 500 km from the coast, eddies dominate the mesoscale offshore transport. Although their contribution represents only 20 % of the total offshore flux and its divergence, eddies, especially cyclones, transport organic carbon offshore to distances as great as 2000 km from the coast. The eddy transport largely represents a turbulent flux, but striations in this transport highlight the existence of typical formation spots and recurrent offshore propagation pathways. While they propagate slowly, eddies are an important organic carbon reservoir for the open waters, as they contain, on average, a third of the organic carbon in this region, two thirds of which is found in cyclones. Our analysis confirms the importance of mesoscale processes for the offshore organic carbon transport and the fueling of the heterotrophic activity in the eastern subtropical North Atlantic, and highlights the need to consider the mesoscale flux in order to fully resolve the three-dimensionality of the marine organic carbon cycle.

Highlights

The Canary Upwelling System (CanUS) is one of the four major Eastern Boundary Upwelling Systems (EBUS), i.e., coastal regions along the western boundaries of the continents characterized by equatorward winds inducing an offshore Ekman transport
The possible repercussions of the lack of modeling of dissolved organic carbon (DOC) in our model were already discussed by Lovecchio et al (2017), where we presented the results of a sensitivity study, in which we tested the implications of modeling a purely suspended particulate organic carbon (POC) pool for our lateral export and the fueling of the biological activity
Our study in the CanUS confirms that mesoscale processes contribute substantially to the long-range offshore transport of Corg through a combination of mean and turbulent transport

Summary

Introduction

The Canary Upwelling System (CanUS) is one of the four major Eastern Boundary Upwelling Systems (EBUS), i.e., coastal regions along the western boundaries of the continents characterized by equatorward winds inducing an offshore Ekman transport This causes the upwelling of cold, nutrient-rich water in the nearshore region, fueling intense biological activity near the coast (Chavez and Messié, 2009; Carr, 2002). Coastal shear and irregular topography, obstacles such as islands, and the density gradient generated by the upwelling of cold waters at the coast produce substantial variability in the flow of the coastal currents, generating instabilities that give rise to mesoscale fronts, filaments, eddies and other forms of turbulent structures (Barton et al, 1998; Capet et al, 2013) This variability, especially that on the oceanic mesoscale (scales of ∼ 20 to ∼ 200 km), modulates the spatial distribution of tracers with an important impact on the biological activity (McGillicuddy, 2016; Mahadevan, 2014; Rossi et al, 2008; Gruber et al, 2011). It is expected to have an important role in the offshore transport of the coastally produced organic matter, fueling the biological activity of the oligotrophic open waters (Álvarez-Salgado, 2007; Sangrà, 2015; Pelegrí et al, 2005)

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