NPZ response to eddy-induced upwelling in a Brazil Current ring: A theoretical approach

Abstract

This work explores the effect of a cyclonic mesoscale feature of the Brazil Current (BC) moving northward off SE Brazil (20° S–23° S) in the Nutrients-Phytoplankton-Zooplankton (NPZ) dynamics. We employ the contour dynamics technique in a quasi-geostrophic, inviscid, 1½-layer model set as a meridional jet flowing southward along a rigid western boundary. This hydrodynamical model is coupled to a conventional NPZ model. We evaluate two distinct scenarios: (1) Plankton in a mixed layer (ML) with time-variable depth and no entrainment and; (2) Plankton in an ML held at a fixed depth, which allows entrainment. We perform simulations with different ML depths representing summer (42 m) and winter (86 m) for the region of interest. The presence of the western boundary allows the cyclonic ring to propagate northward due to image effect. The eddy motion produces upwelling (downwelling) in the leading (trailing) edge resulting in different responses in the two scenarios examined. In the variable ML depth simulations, Z is favored relative to P because there is no injection of N. On the other hand, in the fixed ML depth simulations, P is favored relative to Z due to dilution. All simulations show P enrichment within the eddy, but winter simulations show a greater response to the physical forcing. We conclude that the cyclonic eddy shed from BC provides favorable conditions for planktonic growth. Winter-like conditions seem to be more favorable than summer-like ones and, therefore, present more intense blooms associated with the ring.