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Vertical Fluxes in the Upper Ocean

Mara Freilich, Ph.D., 2021
Amala Mahadevan, Advisor
Oceanic fronts at the mesoscale and submesoscale are associated with enhanced vertical motion, which strengthens their role in promoting both primary production and subduction of carbon from the surface. Using process study models, theory, and observations of biogeochemical tracers, we improve understanding of submesoscale vertical tracer fluxes and their influence on carbon cycling. By decomposing the vertical velocity below the mixed layer into two components: vertical velocity along sloping isopycnal surfaces and adiabatic vertical velocity of isopycnal surfaces we show that vertical velocity is increasingly along isopycnal at submesoscales. We find that vertical nutrient flux, and consequently primary production, depends nonlinearly on the rate of nutrient utilization and is maximum when the biological and physical timescales are similar. Using Lagrangian analysis, we show that subduction across the mixed layer base, which contributes to carbon export, is driven by geostrophic and ageostrophic frontogenesis. We demonstrate observationally that intrusions of high chlorophyll and particulate organic carbon are advected below the euphotic zone at Mediterranean Sea fronts on timescales of days to weeks. The community composition changes systematically in the intrusions. We conclude that advective fluxes could make a contribution to carbon export in oligotrophic gyres that is equal to the sinking flux.