Phytoplankton patches at fronts: A model of formation and response to wind events

Abstract

A subsurface patch of chlorophyll is a feature common to many fronts. The dynamics underlying the patch formation are not well understood; however, there appears to be a strong link between the biological and physical dynamics. Here we test the hypothesis that patch formation is a result of wind-forced motions of the front. Frontal responses to transient wind events include acceleration of the surface layers, changes in mixed-layer depth, and excitation of nonlinear oscillations at the front. Such dynamics can affect nutrient fluxes across the pycnocline and the coupling between trophic levels. To test this hypothesis, we developed a two-dimensional coupled mixed-layer/primitive-equation/ecosystem model, which we forced with transient wind events. We explored a range of initial conditions, including the cross-frontal nutrient gradient, the sinking of phytoplankton, the depth of the euphotic zone, and the depth of the front. The model showed the subsurface chlorophyll patch to be dependent on all these factors. The cross-frontal scale of the patch was found to be a historical artifact of past wind events, and so was not strongly related to the scale of the front (given by the Rossby radius of deformation). Winds aligned with the frontal jet weakened the cross-frontal density gradient, causing a flux of nutrients into the warmer waters of the front, and eroding the chlorophyll patch through deep vertical mixing. Winds aligned against the frontal jet enhanced the cross-frontal density gradient and isolated the subsurface patch from vertical mixing. The transient forcing of the fronts led to relatively high f-ratios in the subsurface patches; this enhanced biomass would be invisible to most satellite sensors.