Eddy-induced chlorophyll profile characteristics and underlying dynamic mechanisms in the South Pacific Ocean

Abstract. Many studies have consistently demonstrated that the near-surface phytoplankton chlorophyll (Chl) levels in anticyclonic eddies (AEs) are higher than in cyclonic eddies (CEs) in the South Pacific Ocean (SPO) using remote sensing data, which is attributed to higher phytoplankton biomass or physiological adjustments in AEs. However, the characteristics of the Chl profile induced by mesoscale eddies and their underlying dynamic mechanism have not been comprehensively studied by means of field measurement. In this study, we mainly utilized BGC-Argo data to investigate the relationships between Chl levels and environmental factors (C_Phyto, Nitrate, Temperature and Light) and the underlying dynamic mechanisms of mesoscale eddies in SPO. Our findings showed that, the elevated Chl levels in AEs primarily result from increased phytoplankton biomass within the Mixed Layer Depth (MLD), which is induced by enhanced nutrient availability due to the deepening MLD in AEs. At depths ranging from 50 m to 110 m (the depth between the bottom of the mixed layer and pycnocline), the dominant factor affecting higher Chl levels in CEs is the physiological adaptation of phytoplankton, driven by reduced temperature and light availability. Between 110 m and 150 m (near the depth of pycnocline or bottom of the euphotic zone), both phytoplankton biomass induced by eddy pumping and physiological adjustments for lower light and temperature contributed to higher Chl levels in CEs. At depths exceeding 150 m (beyond the euphotic zone), higher Chl in AEs is primarily influenced by phytoplankton biomass as a result of the downwelling by eddy pumping. To a certain extent, this work would advance our comprehensive understanding of the physical-biological interactions of mesoscale eddies and their impacts on primary productivity throughout the water column, which has important implications for accurately assessing the biogeochemical processes and ocean carbon cycle.