A Lagrangian study of the contribution of the Canary coastal upwelling to the open North Atlantic nitrogen budget
Authors: Hailegeorgis Derara , Lachkar Zouhair , Rieper Christoph , Gruber Nicolas 
Published: February 2020

Abstract. The Canary Current System (CanCS) is a major Eastern Boundary Upwelling System (EBUS), known for its high nearshore productivity and for sustaining large fisheries. Only a part of the inorganic nutrients that upwell along Northwest Africa are being used to fuel the high nearshore productivity. The remainder together with some of the newly formed organic nutrients are exported offshore into the adjacent oligotrophic subtropical gyre of the North Atlantic. Yet, the offshore reach of these nutrients and their importance for the biogeochemistry of the open North Atlantic is not yet fully quantified. Here, we determine the lateral transport of both organic and inorganic nitrogen from the Canary upwelling and investigate the timescales, reach, and structure of offshore transport using a Lagrangian modelling approach. To this end, we track all water parcels entering the coastal ocean and upwelling along the Northwest African coast between 14° N and 35° N, as simulated by an eddy-resolving configuration of the Regional Ocean Modeling System (ROMS). Our model analysis suggests that the vast majority of the upwelled waters originate from offshore and below the euphotic zone (70 m depth), and once upwelled remain in the top 100 m. The offshore transport is intense, yet it varies greatly along the coast. The central CanCS (21° N–28° N) transports the largest amount of water offshore, thanks to a larger upwelling volume and a faster offshore transport. In contrast, the southern CanCS (14° N–21° N) exports more nitrogen from the nearshore, primarily because of the higher nitrogen-content of its upwelling waters. Beyond 200 km, this nitrogen offshore transport declines rapidly because the shallow depth of most water parcels supports high organic matter formation and subsequent export of the organic nitrogen to depth. The horizontal pattern of offshore transport is characterized by latitudinally alternating offshore-onshore corridors indicating a strong contribution of mesoscale eddies and filaments to the mean transport. Around 1/3 of the total offshore transport of water occurs around major capes along the CanCS. The persistent filaments associated with these capes are responsible for an up to four-fold enhancement of the offshore transport of water and nitrogen in the first 400 km. Much of this water and nitrogen stems from upwelling at quite some distance from the capes, confirming the capes' role in collecting water from along the coast. North of Cape Blanc and within the first 500 km from the coast, water recirculation is a dominant feature of offshore transport. This process, likely associated with mesoscale eddies, tends to reduce the efficiency of offshore transport. This process is less important in the southern CanCS along the Mauritanian coast. The Canary upwelling is modelled to supply around 44 mmol N m−2 yr−1 and 7 mmol N m−2 yr−1 to the North Atlantic Tropical Gyral (NATR) and the North Atlantic Subtropical Gyral East (NASE) Longhurst provinces, respectively. In the NATR, this represents nearly half (45 ± 15 %) of the estimated total new production, while in the NASE, this fraction is small (3.5 ± 1.5 %). Our results highlight the importance of the CanCS upwelling as a key source of nutrient to the open North Atlantic and stress the need for improving the representation of EBUS in global coarse resolution models.