Upstream control of the frontal jet regulating plankton production in the Alboran Sea (Western Mediterranean)

Using a coupled physical-biological model, we document that a 30 km wide meandering jet constitutes a major source of biological enrichment in the Alboran Sea (Western Mediterranean) even in the absence of wind forcing and tidal dynamics. The level of enrichment is shown to vary markedly during the year depending on the upstream characteristics of the jet as it exits from the Gibraltar Strait. When its intensity is sufficiently low and characterized by weaker cross-frontal density gradients during winter-spring, the jet is weakly nonlinear and may not fulfill the necessary conditions for frontogenesis. It then remains weakly productive. In the case of stronger jet intensity (>1.1 Sv) accompanied by stronger cross-frontal density and velocity gradients within the Alboran Sea during summer-autumn, the frontal jet becomes strongly nonlinear and ageostrophic with large cross-frontal vorticity changes on the order of planetary vorticity. Under these conditions, upward vertical velocities in the range 10–50 m d supply nutrients into the euphotic layer more effectively and support high-level frontogenesis-induced phytoplankton production on the anticyclonic side of the main jet axis. The strong eddy pumping mechanism also provides a comparable level of plankton production within strongly nonlinear elongated cyclonic eddies along the outer periphery of the frontal jet. The plankton biomass is advected partially by the jet along its trajectory and dispersed within the basin by mesoscale eddies and meanders.