The influence of Gulf Stream eddies and meanders on near-surface chlorophyll

Abstract

The Gulf Stream region contains strong mesoscale variability that significantly influences planktonic ecosystems residing therein. Meanders of the Gulf Stream can be identified as eastward propagating features in maps of sea level anomaly. These meanders can become unstable and pinch off to form nonlinear mesoscale eddies (rings) 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 both horizontal advection and vertical fluxes, multiple physical-biological mechanisms can simultaneously influence phytoplankton communities trapped in eddies. In this study we examine how the near-surface chlorophyll field (CHL) evolves in meanders and eddies by comparing satellite observations with an eddy-resolving ocean model.Prior in situ and satellite observations have revealed that during the formation of cyclonic Gulf Stream meanders, water with elevated CHL is transported southward. In anticyclonic meanders, water with reduced CHL is transported northward. Alternating submesoscale patches of upwelling and downwelling occur along the meandering front; however, evidence of a biological response to meander-induced vertical motion was not observed in meander-centric composite averages. During the formation of nonlinear Gulf Stream eddies, elevated and suppressed CHL is trapped and subsequently transported westward in cyclones and anticyclones, respectively. Following formation, CHL is observed to increase in the cores of anticyclones. The observed positive trend in CHL in anticyclones is consistent with the influence of eddy-induced Ekman pumping (eddy/wind interaction) that generates upwelling in anticyclones and downwelling in cyclones.To substantiate the influence of eddy-induced Ekman pumping on CHL in Gulf Stream eddies, two separate eddy-resolving physical-biological simulations are compared. The first simulation is forced with a realistic surface stress that includes the influence of ocean surface currents. The second simulation neglects this process. The time evolution of CHL within eddies is very different in these two simulations. The model that includes eddy-induced Ekman pumping generates temporal trends in CHL that are similar to the observations.