The asymmetric distribution of phytoplankton in anticyclonic eddies in the western South China Sea

- An asymmetric phytoplankton distribution at the periphery of anticyclonic eddies in the western South China Sea.
- Non-uniform surface velocity field generates flow divergence/convergence.
- Ageostrophic secondary circulation occurs at the northwestern periphery of anticyclonic eddies.

An anticyclonic eddy's periphery is characterized by large horizontal density gradients, strain and vertical velocity. In this paper we document the asymmetric distribution of phytoplankton around the periphery of anticyclonic eddies in the western South China Sea based on 432 eddies detected from satellite data. The high level of phytoplankton occurs consistently at the northwestern periphery of eddies, with a maximum positive chlorophyll anomaly greater than 0.01 mg m−3. The asymmetric distribution of phytoplankton primarily tags the non-uniform surface velocity field varying from 0.15 m s−1 to 0.3 m s−1 along the eddy's periphery. The coastal boundary and off-coast jet may be the primary cause of the non-uniform flow. Associated with the non-uniform flow speed, the combined effects of the velocity convergence and steepened northwestern part of the eddies act to sharpen the density fronts and tend to result in an ageostrophic secondary circulation at the northwestern edge of the eddy. The upward component of the ageostrophic secondary circulation can enhance the nutrient flux into the euphotic layer, thereby increasing phytoplankton productivity. Anticyclonic eddy-induced ageostrophic secondary circulation appears to be an important mechanism for increasing phytoplankton productivity in the oligotrophic waters of the South China Sea.