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

Abstract

Abstract. A compilation of measurements of net community production (NCP) in the upper waters of the eastern subtropical North Atlantic had suggested net heterotrophic conditions, purportedly supported by the lateral export of organic carbon from the adjacent, highly productive Canary Upwelling System (CanUS). Here, we quantify and assess this lateral export using the Regional Ocean Modeling System (ROMS) coupled to a nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model. We employ a new Atlantic telescopic grid with a strong refinement towards the northwestern African shelf to combine an eddy-resolving resolution in the CanUS with a full Atlantic basin perspective. Our climatologically forced simulation reveals an intense offshore flux of organic carbon that transports about 19 Tg C yr −1 away from the nearshore 100 km over the whole CanUS, amounting to more than a third of the NCP in this region. The offshore transport extends beyond 1500 km into the subtropical North Atlantic, adding organic carbon along the way to the upper 100 m at rates of between 8 and 34 % of the alongshore average NCP as a function of offshore distance. Although the divergence of this lateral export of organic carbon enhances local respiration, the upper 100 m layer in our model remains net autotrophic in the entire eastern subtropical North Atlantic. However, the vertical export of this organic carbon and its subsequent remineralization at depth makes the vertically integrated NCP strongly negative throughout this region, with the exception of a narrow band along the northwestern African shelf. The magnitude and efficiency of the lateral export varies substantially between the different subregions. In particular, the central coast near Cape Blanc is particularly efficient in collecting organic carbon on the shelf and subsequently transporting it offshore. In this central subregion, the offshore transport adds as much organic carbon as nearly 60 % of the local NCP to the upper 100 m, giving rise to a sharp peak of offshore respiration that extends to the middle of the gyre. Our modeled offshore transport of organic carbon is likely a lower-bound estimate due to our lack of full consideration of the contribution of dissolved organic carbon and that of particulate organic carbon stemming from the resuspension of sediments. But even in the absence of these contributions, our results emphasize the fundamental role of the lateral redistribution of the organic carbon for the maintenance of the heterotrophic activity in the open sea.

Fronts at the Surface Ocean Can Shape Distinct Regions of Microbial Activity and Community Assemblages Down to the Bathypelagic Zone: The Azores Front as a Case Study

“…Recent and past studies support the view that the eastern boundary Canary Current, south of the AF, is a region affected by intense coastal-offshore transport of suspended particles (see Lovecchio et al, 2017, for a review and references therein). The combined effect of strong upwelling filaments, like those stretching from Cape Sao Vicente (Relvas and Barton, 2002) and Cape Guir (Sangrà et al, 2015), and mesoscale eddies ) may extend the transport hundreds of miles to the open ocean.…”

“…Particularly, the Cape Guir filament is known to export a large fraction of the coastal primary production to the open ocean (García-Muñoz et al, 2004;Pelegrí et al, 2005;Santana-Falcón et al, 2016). Lovecchio et al (2017), used a Regional Ocean Modeling System (ROMS) coupled to a Nutrient, Phytoplankton, Zooplankton and Detritus (NPZD) ecosystem model to quantify and assess the lateral export of particulate organic carbon (POC) from the Canary Upwelling System to the open waters of the subtropical Northeast Atlantic. They estimated that the offshore transport of coastal production may extend beyond 1,500 km, contributing as nearly 60% of the local net community production in the open ocean.…”

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

“…Since the model setup used for the present study is the same as the one employed in Lovecchio et al (2017), we refer to this previous publication for a detailed description of the model performance. We first summarize here the most important findings, and then extend the evaluation to include aspects that are particular to this study, namely the model's representation of 10 mesoscale processes.…”

“…The Reynolds decomposition of the advective fluxes of C org in the top 100 m reveals that, while the time-mean flux dominates the total flux (see also : Lovecchio et al (2017), Figure 11, Total organic carbon fluxes), the turbulent flux contributes substantially to the total fluxes and to their divergences in both lateral and vertical direction ( Figure 6). In the zonal direction, the turbulent transport strengthens the offshore transport of the C org at every latitude with its persistently negative signature, The contribution of the turbulent offshore flux is particularly important in the northern and southern CanUS.…”

“…The coupled model was run with the same grid, boundary conditions and climatological mean atmospheric forcings as those presented in Lovecchio et al (2017). The employed Atlantic telescopic grid is curvilinear, covers the entire Atlantic, and has a strong grid refinement towards the north-western African coast (Figure 1).…”