Vertical Transport and Transformation of Biogeochemical Properties in the Upper Ocean

Abstract

In the upper ocean, biogeochemical properties including oxygen, carbon and nutrients change rapidly with depth. Vertical gradients are shaped by air-sea exchange and the attenuation of light below the surface and are maintained by density stratification. Intermittent vertical exchange driven by wind and buoyancy forcing transports tracer anomalies across this gradient, which perturb ecosystems in the upper ocean. The importance of local vertical exchange for global ocean biogeochemical cycles is still not fully understood. Moreover, this vertical transport occurs on timescales that overlap with planktonic growth and mortality, so it is difficult to tease apart how physical and biological processes affect tracer fluxes. In this thesis, we synthesize observations and models to study transient vertical transport between the surface and interior ocean and its role in regulating vertical fluxes of carbon and oxygen. We begin by exploring how microbial community respiration modulates the eddy subduction pump, which supplies small, non-sinking organic carbon to food webs below the sunlit layer. During a field campaign in the northwestern Mediterranean Sea, we measured respiration rates in water that had recently come from the mixed layer. We combine these data with biogeochemical observations to show that rates are shaped by how much alteration water has experienced since leaving the surface. Next, we develop a method to constrain the age and mixing rates of subducted water using commonly measured tracers like chlorophyll, oxygen and temperature-salinity. The age of water estimated via this method can be used to infer the mean vertical velocity and the rate of advective tracer supply at submesoscales. We validate our method using idealized modeling and then apply it to field observations in the Mediterranean Sea. Lastly, we turn our focus to coastal upwelling processes during the Southwest Monsoon along the western coastlines of the Arabian Sea and the Bay of Bengal. We show that differences in the surface signature of upwelling between these two coastlines can be explained by regional differences in both density stratification and intraseasonal wind variability. Collectively, the results of this thesis shed light on the fluxes and fate of transient vertical exchange in the upper ocean.