On the Processes Sustaining Biological Production in the Offshore Propagating Eddies of the Northern Canary Upwelling System. Issue 2 (14th February 2022)

• Record Type: Journal Article
• Authors: Lovecchio, Elisa; Gruber, Nicolas; Münnich, Matthias; Frenger, Ivy
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-14
• Subjects: mesoscale eddy -- Canary Upwelling System -- biophysical interactions -- eddy tracking -- organic matter -- productivity; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC017691
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Oceanic mesoscale eddies constitute ephemeral hotspots for marine life and are pivotal for the lateral transport of nutrients and organic matter. Here, we use a high‐resolution coupled physical‐biogeochemical model to study the processes sustaining biological production and export in long‐living cyclonic (CE) and anticyclonic (AE) eddies of the northern Canary Upwelling System (CanUS). We track the eddies for 18 months as they propagate offshore, and study their composite properties in time in a Lagrangian manner. Our model shows that long‐living CEs sustain their production with the nitrogen that they initially trap in the nearshore nutrient‐rich waters and keep isolated in their cores. The vertical input of nitrate from below tends to be comparatively small, and is mostly driven by mixing. In contrast, AEs tend to start with low nutrient concentrations in their core as they do not trap coastal waters, but have elevated concentrations at their periphery. In AEs, stirring is responsible for both the building up of the positive nitrate anomaly at depth and the enhanced lateral input of organic nitrogen in the near‐surface. Compared to CEs, the input of nitrate into the euphotic zone by vertical mixing is substantially more important. Though regenerated production dominates in both types of eddies, new production is higher than the regional average in CE cores and at the rim of AEs, partially compensating for the intense losses due to sinking. Both cyclonic trapping and transport and anticyclonic stirring shape the regional pattern of organic matter and nutrients in the northern CanUS. Plain Language Summary: Mesoscale eddies are ubiquitous rotating flow structures in the ocean with a diameter of 20–200 km. They are hot‐spots of productivity and contribute to the lateral transport of nutrients. Through the use of ocean simulations, we studied the nutrient dynamics in long‐living cyclonic (counter‐clockwise rotating) and anticyclonic (clockwise rotating) mesoscale eddies that are formed near the north‐western African coast and propagate westward into the subtropical North Atlantic. We find that biological activity in both types of eddies rely mostly on the local recycling of the already present organic matter, and that the supply of new nutrients is sustained by small‐scale vertical mixing of nutrients from below. High nutrient concentrations in the center of cyclonic eddies are in most part trapped during the eddy formation and preserved for a long time thanks to their lateral isolation from the surrounding water. Anticyclones, which are characterized by lower nutrients in the center compared to the eddy periphery, are strongly influenced by lateral stirring of the high nutrient levels at depth and by the subsequent vertical mixing of nutrients into the sun‐lit ocean layer. Key Points: The pattern of biogeochemical tracers and production is determined by trapping and isolation in cyclones and by stirring in anticyclones Vertical small‐scale mixing drives the nutrient supply in mature eddies while the vertical advective flux downwells nutrients Eddy regenerated production dominates over new production highlighting the intense recycling of the organic material exported offshore