CoAuthors

Dennis McGillicuddy (Woods Hole Oceanographic Institution, United States)

Meanders of the Gulf Stream can be identified as eastward propagating anomalies in sea surface height that generate local perturbations of the ambient near-surface chlorophyll field (CHL). These meanders can become unstable and pinch-off to form nonlinear mesoscale eddies that trap large parcels of water. Following formation, ecosystems trapped within these eddies are subjected to temporally varying vertical velocities throughout their lifetime. As a result of this interplay of horizontal advection and vertical fluxes, multiple physical-biological mechanisms can simultaneously influence phytoplankton communities trapped in eddies. In this study we examine how CHL evolves in meanders and eddies by comparing satellite observations with an eddy-resolving ocean circulation model.

During the formation of nonlinear Gulf Stream eddies, water with elevated CHL is trapped in cyclones and water with suppressed CHL is trapped in anticyclones. Following formation, CHL is observed to increase in the cores of anticyclones and decrease in cyclones. We suggest that the observed trends in CHL following eddy formation are a result of eddy-induced Ekman pumping generated by the influence of ocean surface currents on the surface stress.

To test this hypothesis we compare two separate eddy-resolving physical-biological simulations. The first simulation is forced with a realistic surface stress that includes the influence of ocean surface currents. The second simulation neglects this process and therefore does not include eddy-induced Ekman pumping. The time evolution of CHL within eddies is very different in these two simulations; the model including eddy-induced Ekman pumping reproduces observed trends in CHL within eddies, whereas the other simulation does not.