Submesoscale variability of subsurface chlorophyll-a across eddy-driven fronts by glider observations

Abstract

Reflecting the local dynamic state of the pelagic system, subsurface chlorophyll maxima (SCM) are ubiquitous in the global ocean. Here, a high-resolution glider survey was conducted in the South China Sea to study the physical and biogeochemical change across the periphery of a mesoscale cyclonic eddy. We found intense vertical dispersion of SCM associated with enhanced diapycnal mixing at the frontal zones on the edge of the eddy. Submesoscale processes were found to play important roles within the eddy-driven fronts despite the general impact of the mesoscale Ekman upwelling and downwelling. Our results also suggested that wind-driven overturns at the frontal zone could result in a stronger gravitational instability at the northern edge of the eddy leading to an enhanced vertical mixing of subsurface Chl-a. However, turbulent mixing induced by symmetric instability with energy extracted from the lateral buoyancy gradient at the northeastern edge of the eddy had caused a divergence of the subsurface chlorophyll-a at the frontal zone. Our results provide direct evidence of the interaction between mesoscale and submesoscale processes at the oceanic frontal zone. These findings also call for tight cooperation between multidisciplinary observational and modeling approaches for a better understanding of the SCM variability and associated biogeochemical transformations at transient oceanic fronts.