Biologically induced circulation at fronts

Abstract

Consider a frontal region that has high phytoplankton biomass on one side and low biomass on the other. Irradiance penetrates deeply through the water column on the low-biomass side but is attenuated nearer the surface on the biomass-rich side because of absorption by phytoplankton. Thus the near-surface water is heated more on the biomass-rich side than on the clearer side, resulting in lower-density surface water on the biomass-rich side. At greater depths the situation is reversed, with lower-density water occurring on the biomass-poor side. We model this situation and examine the resulting perturbations to the frontal circulation. Our aim is to provide an order-of-magnitude estimate of the feedbacks from the biological component of the ecosystem to the current field. The model consists of the steady state momentum equations, including Coriolis, pressure gradient, and viscous effects. We compute induced vertical velocities of up to 0.2 mm s-1, commensurate with field measurements and previous modeling estimates of vertical velocities at fronts. The horizontal along-frontal velocities are of order 2 cm s-1 or less and so will not represent a major contribution to the overall flow field; however, such values are certainly not insignificant. Copyright 2001 by the American Geophysical Union.