Influence on phytoplankton of different developmental stages of mesoscale eddies off eastern Australia

The influence of oceanic mesoscale eddies on their internal phytoplankton levels varies at different developmental stages. Based on 12 years of satellite ocean color observations, we investigated the variability of phytoplankton inside 4564 anticyclonic eddies and 3675 cyclonic eddies off eastern Australia in different development stages using the method of composited average analysis. The results indicated that the lowest level of chlorophyll was observed in the forth (decay) development stage for anticyclonic eddies, which was associated with the warmest SST, largest eddies amplitude, rotation speeds, angular velocities and surface water convergence. It is indicated that downwelling induced by anticyclonic eddies dominates the chlorophyll variations. Because the convergence induced by the ageostrophic velocity components occurred throughout the eddy's development stage, relaxation of the density perturbations (upwelling) associated with eddy decay was not observed in this study. Chlorophyll concentration near the center of cyclonic eddies decreased from the first to the middle stage, and then increased to the largest levels at the last stage. Although vertical motions induced by the ageostrophic velocity components varied from divergence to convergence during the development of an eddy, a higher eddy-ambient water exchange occurred and dominated the increases in total divergence and chlorophyll concentration in the last stage.