Impact of the North Equatorial Current meandering on a pelagic ecosystem: A modeling approach

A modeling study was conducted to investigate the effects of time-dependent mesoscale meandering of the North Equatorial Current on a pelagic ecosystem in the southwestern Canary Basin. The North Equatorial Current jet was represented as a quasi-geostrophic flow using a two-layer model; a standard bulk mixed layer model is included. Two cases for the biological/physical coupled model were examined: (a) a nutrient-phytoplankton-zooplankton (APZ) model and (b) the addition of a sinking detritus pool (APZD) in the ecosystem. The horizontal length scale of simulated eddies is 100 to 200 km. The surface eddy kinetic energy has a peak value of 110 cm*/s* and a mean value of 26 cm*/s* in the simulated North Equatorial Current. Maximum vertical velocity is of the order of 1.5 m/day at 100 m depth, the base of the mixed layer. The additional nutrients due to eddy upwelling lead to a maximum increase of phytoplankton biomass up to 26% located at the edge of eddies. This trend is even more pronounced when introducing a detritus pool with a 1 m/day sinking velocity into the ecosystem (33%). When upwelling events are seldom present at “mooring” sites, it is the particulate organic carbon input by horizontal advection which feeds the carbon loss by detritus sinking. At “mooring” sites undergoing upwelling events, the upwelled carbon flux largely dominates losses by sedimentation and leads to a 10% enhancement of the sinking exported carbon flux. When the eddies are resolved, the mean values of the primary and exported productions in the jet zone are doubled. The results suggest that the vertical motion due to eddies and eddy-eddy interactions in a weak (10 cm/s) horizontal current such as the North Equatorial Current can be a non-negligible source of nitrogen-nutrients for oceanic plankton production in the mixed layer.