Estimation of 234Th-based export fluxes in mesoscale eddies with submesoscale variability

Abstract:

Mesoscale eddies are one of the major conduit for nutrient transport into the upper ocean which may subsequently induce significant biogeochemical responses. It has been hypothesized that cyclonic eddies may stimulate export productivity whilst anticyclonic eddies would not induce any significant biological responses. Here, we showed complex submesoscale circulations associated with the eddy in one case study that examined ²³⁴Th-based particle fluxes associated with three coherent anticyclonic eddies in the largest marginal sea of the Pacific, northern South China Sea (SCS). Notably, there occurred dynamic interactions at submesoscales which might induce advection into the eddy core from the edge where the deficit of ²³⁴Th was elevated owing to higher particle production and export likely stimulated by the upwelling at the edges. Indeed, by integrating the entire anticyclonic eddy system, the export level was 1.6-fold higher compared to that from the reference sites. Submescale variability was also shown to be important in another case study associated with two cyclonic eddies in western SCS. We concluded thus that consideration of submesoscale interactions within eddy systems are essential in estimating export fluxes using ²³⁴Th deficit, and resolving the 3-D circulation structure of an eddy system is ideal.