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

The Canary upwelling system (CanUS) is a productive coastal region characterized by strong seasonality and an intense offshore transport of organic carbon (C-org) to the adjacent oligotrophic offshore waters. There, the respiration of this C-org 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 C-org, 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 C-org transport. We find that the region surrounding Cape Blanc (21 degrees N) hosts the most intense C-org 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 C-org seasonality hundreds of kilometers from the CanUS coast via lateral redistribution of nearshore production. North of 24.5 degrees N, the sharp summer-fall peak of coastal upwelling results in an export of more than 30% of the coastal C-org 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 C-org and NCP hundreds of kilometers offshore from the CanUS coast via the offshore transport of the nearshore production.