Submesoscale ocean dynamic process contributions to diurnal subsurface chlorophyll variation along Lagrangian recirculation inside mesoscale eddies: A case study in the Southern Ocean

Studies regarding oceanic mesoscale and submesoscale processes and their impact on chlorophyll are mainly confined to weeks to decadal time scales. Based on biogeochemical-Argo float observations and altimeter data in the Southern Ocean in summer of 2016, we show the day-night chlorophyll difference inside a cyclonic eddy (ΔChl_TCE) and an anticyclonic eddy (ΔChl_TACE) associated with submesoscale processes. A diurnal cycle of chlorophyll is observed in the upper 50 m, with ΔChl_TCE (1.5 mg m⁻³) as much as ten times that of ΔChl_TACE (0.15 mg m⁻³). However, there are similar ratios of day-night chlorophyll difference to the maximum chlorophyll concentration in a day for the cyclonic and anticyclonic eddies (∼67%). Submesoscale processes present different impacts on the subsurface chlorophyll between the cyclonic and anticyclonic eddies on the diurnal scale. More significant submesoscale processes in the cyclonic eddy dominate the subsurface negative ΔChl_TCE. It causes the phytoplankton to penetrate the bottom of the mixed layer and extend ∼50 m below the mixed layer. In contrast, submesoscale processes and their associated with vertical buoyancy flux only generate weak negative subsurface ΔChl_TACE. The strong vertical gradient of ΔChl_TACE is mainly dominated by the vertical displacement of the deep chlorophyll maximum.