McGillicuddy Dennis (Woods Hole Oceanographic Institution , USA); Long Matthew (National Center for Atmospheric Research , USA); Bryan Frank (National Center for Atmospheric Research , USA)

The influence of mesoscale ocean eddies and meanders on phytoplankton varies regionally; cyclonic features enhance near-surface chlorophyll concentration (CHL) whereas anticyclonic features reduce CHL in most boundary current regions. In some regions, however, the opposite is observed; for instance, in the South Indian Ocean (SIO) in the vicinity of the Leeuwin Current anticyclonic eddies tend to contain elevated CHL and cyclones suppressed CHL. Two primary mechanisms have been identified that could result in enhanced (reduced) CHL in SIO anticyclones (cyclones): (1) Eddy-induced Ekman upwelling in the interiors of anticyclones and downwelling in cyclones, and (2) deeper vertical mixing in anticyclonic eddies enhances mixed-layer nutrient supply relative to cyclonic eddies. The relative impact of these two mechanisms in generating the observed CHL response in eddies, however, cannot be determined from satellite observations alone.

We present a detailed analysis of the available satellite and in situ observations in the SIO and compare these to a global eddy-resolving, physical/biological model, which simulates both of the mechanisms described above. To determine the relative contribution of these two mechanisms to observed enhanced (reduced) CHL in SIO anticyclones (cyclones), we derive a nutrient budget for the mixed layer which is evaluated in an eddy-centric reference frame allowing for the various contributions to mixed layer nutrient fluxes to be determined.