Ageostrophic Frontal Processes Controlling Phytoplankton Production in the Catalano-Balearic Sea (Western Mediterranean).

Temel Oguz,Joaquin Tintore,Diego Macias,Moncho Gomez-Gesteira

doi:10.1371/journal.pone.0129045

Abstract

Buoyancy-induced unstable boundary currents and the accompanying retrograde density fronts are often the sites of pronounced mesoscale activity, ageostrophic frontal processes, and associated high biological production in marginal seas. Biophysical model simulations of the Catalano-Balearic Sea (Western Mediterranean) illustrated that the unstable and nonlinear southward frontal boundary current along the Spanish coast resulted in a strain-driven frontogenesis mechanism. High upwelling velocities of up to 80 m d-1 injected nutrients into the photic layer and promoted enhanced production on the less dense, onshore side of the front characterized by negative relative vorticity. Additional down-front wind stress and heat flux (cooling) intensified boundary current instabilities and thus ageostrophic cross-frontal circulation and augmented production. Specifically, entrainment of nutrients by relatively strong buoyancy-induced vertical mixing gave rise to a more widespread phytoplankton biomass distribution within the onshore side of the front. Mesoscale cyclonic eddies contributed to production through an eddy pumping mechanism, but it was less effective and more limited regionally than the frontal processes. The model was configured for the Catalano-Balearic Sea, but the mechanisms and model findings apply to other marginal seas with similar unstable frontal boundary current systems.

Highlights

The boundary current along the coast of the Catalano-Balearic (CB) Sea (Fig 1) constitutes a part of the Western Mediterranean cyclonic circulation system
The link between frontogenesis mechanisms and plankton production is substantiated here by a coupled physical-biological model describing how an initially stable frontal boundary current changed structurally in response to growing frontal instabilities and how the phytoplankton biomass distribution responded to these changes (1) in the absence of any atmospheric forcing and (2) in the presence of down-front wind stress and buoyancy-induced cooling
Numerous studies and field observations have noted the favorable roles of mesoscale features in quasi-geostrophic flow dynamics on the production and distribution of biological properties

Summary

Introduction

The boundary current along the coast of the Catalano-Balearic (CB) Sea (Fig 1) constitutes a part of the Western Mediterranean cyclonic circulation system. Strain generates frontogenesis and associated ageostrophic cross-frontal circulation [27,28,29] This mechanism gives rise to high upward vertical velocities within anticyclonic eddies on the onshore side of the front along the Ligurian boundary current of the Western Mediterranean [30]. Oguz et al [31] described frontogenesis-induced enhanced autotrophic production along the meandering path of the narrow (15–20 km wide) Atlantic jet around the western and eastern anticyclonic gyres in the Alboran Sea. The link between frontogenesis mechanisms and plankton production is substantiated here by a coupled physical-biological model describing how an initially stable frontal boundary current changed structurally in response to growing frontal instabilities and how the phytoplankton biomass distribution responded to these changes (1) in the absence of any atmospheric forcing and (2) in the presence of down-front wind stress and buoyancy-induced cooling. Wind stress and buoyancy flux at the surface was or was not prescribed depending on the type of simulations performed as specified below

Design of model simulations

Findings

Discussion and Conclusions