On the Vertical Velocity and Nutrient Delivery in Warm Core Rings

Mesoscale eddies, such as those generated by the Gulf Stream, excite vertical motions that can result in both the upwelling of nutrients and the downward flux of nutrients and phytoplankton. Here we examine multiple solutions to an idealized numerical model of a representative anticyclonic warm core Gulf Stream Ring. Initial conditions were constructed to reproduce observed density and nutrient profiles collected during the Warm Core Rings Program. The contributions to vertical fluxes diagnosed from the numerical simulations were compared against a divergence-based semi-diagnostic equation and a generalized omega equation to better understand the dynamics of the vertical velocity field. Frictional decay alone was found to be ineffective in raising isopycnals and transporting nutrients to the upper ocean. Under representative wind forcing, the magnitude of vorticity gradient-induced Ekman pumping is not necessarily larger than current-induced counterpart on a time scale relevant to ecosystem response. Under realistic forcing conditions, strain deformation can perturb the ring to be non-circular and induce vertical motions much larger than the Ekman vertical velocities. Nutrient budgets together with analysis of the relative magnitudes of the various types of vertical fluxes allow us to describe the time scale dependence of nutrient delivery. At time scales that are relevant to individual phytoplankton (hours to days), the magnitudes of nutrient flux by Ekman velocities and deformation-induced velocities are comparable. Over the life span of a typical warm core ring, which can span multiple seasons, surface current-induced linear Ekman pumping is the most effective mechanism in upper ocean nutrient enrichment because of its persistence in the center of anticyclones regardless of the direction of the wind forcing.