Drivers and impact of the seasonal variability of the organic carbon offshore transport in the Canary upwelling system

Abstract

Abstract. The Canary upwelling system (CanUS) is a productive coastal region characterized by strong seasonality and an intense offshore transport of organic carbon (Corg) to the adjacent oligotrophic offshore waters. There, the respiration of this Corg substantially modifies net community production (NCP). While this transport and the resulting coupling of the biogeochemistry between the coastal and open ocean has been well studied in the annual mean, the temporal variability, and especially its seasonality, has not yet been investigated. Here, we determine the seasonal variability of the offshore transport of Corg, its mesoscale component, latitudinal differences, and the underlying physical and biological drivers. To this end, we employ the Regional Ocean Modeling System (ROMS) coupled to a nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model. Our results reveal the importance of the mesoscale fluxes and of the upwelling processes (coastal upwelling and Ekman pumping) in modulating the seasonal variation of the offshore Corg transport. We find that the region surrounding Cape Blanc (21∘ N) hosts the most intense Corg offshore flux in every season, linked to the persistent, and far reaching Cape Blanc filament and its interaction with the Cape Verde Front. Coastal upwelling filaments dominate the seasonality of the total offshore flux up to 100 km from the coast, contributing in every season at least 80 % to the total flux. The seasonality of the upwelling modulates the offshore Corg seasonality hundreds of kilometers from the CanUS coast via lateral redistribution of nearshore production. North of 24.5∘ N, the sharp summer–fall peak of coastal upwelling results in an export of more than 30 % of the coastal Corg at 100 km offshore due to a combination of intensified nearshore production and offshore fluxes. To the south, the less pronounced upwelling seasonality regulates an overall larger but farther-reaching and less seasonally varying lateral flux, which exports between 60 % and 90 % of the coastal production more than 100 km offshore. Overall, we show that the temporal variability of nearshore processes modulates the variability of Corg and NCP hundreds of kilometers offshore from the CanUS coast via the offshore transport of the nearshore production.

On the Processes Sustaining Biological Production in the Offshore Propagating Eddies of the Northern Canary Upwelling System

“…In the case of the vertical advective fluxes, their smaller absolute magnitude in the core of AEs is largely due to the depletion of DIN in the euphotic zone of AEs. This is possibly partially associated with a southward drift of the AE (Figure 2) towards regions of less negative wind stress curl (Bonino et al., 2021). In fact, the net contribution of vertical advection in AE cores is not negative at all times, showing a substantially more noisy and vacillating contribution to the budget at most stages of the eddy life, compared to CEs.…”

“…An Eulerian analysis of the same model run adopted in this study highlighted seasonal variations in the rates of production in the core of the eddies of the CanUS (Bonino et al., 2021). These result from a combination of factors among which large scale temporal variations in vertical fluxes of nutrients, the seasonality of upwelling and the eddy distance from the coast.…”

“…Tracer patterns in the AEs of the northern CanUS, instead, are strongly influenced by the lateral stirring of the large scale regional gradient, especially due to the southern drift of AE tracks, which approach the biophysical boundary of the Cape Verde front. This front, in fact, separates the nutrient‐depleted offshore waters of the open North Atlantic Subtropical Gyre from the region of the North Atlantic Tropical circulation, characterized by the DIN rich South Atlantic Central Water and by a positive wind‐stress curl fueling organic matter production also offshore (Arístegui et al., 2009; Bonino et al., 2021). Through this lateral stirring, AEs have therefore an important role in shaping the large scale DIN and PON distribution of the region, laterally coupling the subtropical and tropical regions of the CanUS located respectively north and south of the Cape Verde front.…”

“…On the other side, the large scale negative signature of the wind stress curl in the CanUS leads to a downward Ekman pumping (Pelegrí & Benazzouz, 2015). This large scale downwelling flux is especially important in CEs, which tend to drift northwards (Figure 2) toward the areas with the most negative wind stress curl (Bonino et al, 2021).…”

Climate Change Impacts on Eastern Boundary Upwelling Systems

“…Uncertainties in the future of productivity, combined with uncertainties in ecosystem shifts induced by changes in temperature, acidity, and oxygenation, imply crucial uncertainties in the future of organic carbon export and sequestration in EBUS regions. This uncertainty further extends to the adjacent open basins, which receive and export significant amounts of coastally produced carbon (Bonino et al 2021), highlighting how understanding the future of EBUSs is key to refining projections of the organic carbon cycle on larger scales.…”