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-depe ndent mesoscale meandering of the North Equatorial Current on a pelagi c ecosystem in the southwestern Canary Basin. The North Equatorial Cur rent jet was represented as a quasi-geostrophic flow using a two-layer model; a standard bulk mixed layer model is included. Two cases for t he biological/physical coupled model were examined: (a) a nutrient-phy toplankton-zooplankton (NPZ) model and (b) the addition of a sinking d etritus pool (NPZD) in the ecosystem. The horizontal length scale of s imulated eddies is 100 to 200 km. The surface eddy kinetic energy has a peak value of 110 cm(2)/s(2) and a mean value of 26 cm(2)/s(2) in th e 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. Th e additional nutrients due to eddy upwelling lead to a maximum increas e of phytoplankton biomass up to 26% located at the edge of eddies. Th is trend is even more pronounced when introducing a detritus pool with a 1 m/day sinking velocity into the ecosystem (33%). When upwelling e vents are seldom present at ''mooring'' sites, it is the particulate o rganic carbon input by horizontal advection which feeds the carbon los s by detritus sinking. At ''mooring'' sites undergoing upwelling event s, the upwelled carbon flux largely dominates losses by sedimentation and leads to a 10% enhancement of the sinking exported carbon flux. Wh en the eddies are resolved, the mean values of the primary and exporte d productions in the jet zone are doubled. The results suggest that th e vertical motion due to eddies and eddy-eddy interactions in a weak ( 10 cm/s) horizontal current such as the North Equatorial Current can b e a non-negligible source of nitrogen-nutrients for oceanic plankton p roduction in the mixed layer.