Abstract

Vertical motions associated with mesoscale ocean eddies modulate the light and nutrient environment, stimulating anomalies in phytoplankton biomass and chlorophyll. Phytoplankton populations can be subsequently trapped by the horizontal circulation or laterally diluted. In a time-varying flow, Lagrangian methods can be used to quantify eddy trapping, also known as Lagrangian coherency. From two decades of remote sensing observations in the North Pacific Subtropical Gyre, we compared coincident Eulerian and Lagrangian eddy atlases to assess the impact of eddy trapping on chlorophyll concentration. We found higher chlorophyll within Lagrangian coherent boundaries than in Eulerianeddies and outside-eddy waters. Yet, there are differences regionally and seasonally. For example, chlorophyll is most enriched within coherent boundaries of the Hawaiian Lee eddies and to the south of 23N in fall and winter. Our results suggest that by not accounting for lateral dilution, Eulerian analyses may underestimate the role of mesoscale eddies in enhancing chlorophyll.