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

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 this has not been demonstrated in a quantitative manner for the entire Canary Upwelling System (CanUS). Here, we fill this gap using 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 5 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 analyses reveal 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 organic carbon concentrations, transport this carbon offshore in a very intense, but coastally-confined, manner, contributing nearly 10 80% to the total flux at 100 km offshore distance. The filament contribution tapers off quickly to near zero values at 1000 km distance, leading to a strong offshore flux divergence that is the main lateral source of organic carbon in the first 500 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 and only to a limited degree represents a turbulent flux. At distances beyond 500 km from the coast, eddies 15 dominate the mesoscale offshore transport. Although their contribution represents only 20 % of the total offshore flux and of 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, since they contain on average a third of the offshore organic carbon, two third of which is found 20 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 account for the three-dimensionality of the marine biological pump.

Origin, Transformation, and Fate: The Three‐Dimensional Biological Pump in the California Current System

“…The eddies that propagate along these conduits, that is, eddy corridors, tend to dominate the far‐reaching transport beyond a few hundred kilometers. In contrast, the transport for the first few hundred kilometers tends to be dominated by filaments superimposed on the mean Ekman transport (e.g., Combes et al, ; Lovecchio et al, ; Nagai et al, ).…”

Changes in the Dust‐Influenced Biological Carbon Pump in the Canary Current System: Implications From a Coastal and an Offshore Sediment Trap Record Off Cape Blanc, Mauritania

“…However, remote sensing cannot quantify carbon export and sequestration to the deep ocean. Export fluxes at continental margins are complicated by significant cross‐shelf export of coastal chlorophyll and resuspended biogenic and nonbiogenic shelf material (e.g., Arístegui et al, ; Barton et al, ; Helmke et al, ; Karakas et al, ; Lovecchio et al, ; Pelegrí et al, ). As persistent upwelling occurs off Cape Blanc, this is a major hotspot for offshore advection of particles (Karakas et al, ; Lovecchio et al, ) but the fate of organic matter and the transport processes remain largely unresolved on regional and temporal scales.…”

Drivers and impact of the seasonal variability of the organic carbon offshore transport in the Canary Upwelling System

“…Our analysis of the seasonality of the offshore transport in the CanUS is based on the results from a CanUS model simulation undertaken and described by Lovecchio et al (2018). This simulation used the UCLA-ETH version of the Regional Ocean Modeling System (ROMS) (Shchepetkin and McWilliams, 2005) coupled with a Nutrient Phytoplankton Zooplankton Detritus (NPZD) ecosystem model (Gruber et al, 2006).…”

“…a mesoscale-resolving resolution in the region of study, achieved through a strong grid refinement towards the north-western African coast. We refer to Lovecchio et al (2017) and Lovecchio et al (2018) for further details on the model, on its forcing, and for providing a careful evaluation against a large range of observational constraints.…”

“…This enhanced supply of C org to the North Atlantic Subtropical Gyre increases respiration there, pushing vertically integrated net community production to become negative. Over a year, narrow coastal filaments are, on average, responsible for about 80 % of this offshore flux at 100 km from the coast, while coastally generated mesoscale eddies dominate this transport further offshore (Lovecchio et al, 2018).…”