Impact of nonlinear mesoscale eddy on phytoplankton distribution in the northern South China Sea

Approximately thirteen years of mean sea level anomaly (MSLA) and chlorophyll (Chl) data in the northern South China Sea (NSCS) were analysed to investigate the influence of a nonlinear, mesoscale eddy on ecological and biogeochemical processes. Over two thousand mesoscale eddies were identified and tracked using a new sea surface height-based method and approximately 40% of these eddies were nonlinear. A westward co-propagation of Chl and MSLA signals at a speed of approximately 0.07 m s− 1 was found to correspond well with the variability of Chl within cyclonic and anticyclonic eddies. The covariability relationships were different depending on the season. The results of a lagged cross correlation between MSLA and Chl, as well as Chl composite averaging within mesoscale eddies, showed that eddy advection dominates the Chl dipole structure within mesoscale eddies. This mechanism was further confirmed by the significant correlations of the west-to-east Chl difference with background Chl gradient (R = 0.32 for cyclonic eddies and R = − 0.20 for anticyclonic eddies) and eddy scale (R = 0.33 for cyclonic eddies and R = − 0.21 for anticyclonic eddies). Moreover, the strong correlation (0.44) between the in-to-out Chl difference and amplitude for the cyclonic eddy implied that eddy pumping contributes to the high Chl levels near the centre of cyclonic eddy.