UC San Diego

The Southern Ocean plays an outsized role in the global overturning circulation and climate system by transporting mass, heat, and tracers between basins, as well as between the surface and abyssal oceans. Consequently, the Southern Ocean accounts for a disproportionately large percentage of the total oceanic carbon uptake and helps set global nutrient inventories. Therefore, understanding the coupling between physical and biogeochemical processes in this region is crucial to reducing uncertainty in future climate projections. Historically, studying the Southern Ocean has been limited by the paucity of observational data from this remote environment. However, recent advances in autonomous observing technology have provided unprecedented spatial coverage of subsurface biogeochemical measurements. This thesis uses data from an array of more than 200 autonomous profiling floats—in conjunction with satellite data, numerical models, and theory—to investigate the fundamental question: How do physical processes in the Southern Ocean drive variability of phytoplankton biomass and carbon system parameters? Naturally, the answer to this question will depend on the spatial and temporal scales of interest. Our approach is to consider multiple scales, with the central motivation of better understanding the carbon cycle on climatic timescales.

First, we investigate regional patterns of phytoplankton seasonality in the Southern Ocean (Chapter 2). Results show that enhanced mixing at topographic features contributes to spatial variability in bloom magnitude and timing. Looking to smaller scales, we examine the generation of phytoplankton patchiness by turbulent stirring (Chapter 3). We find that parameterizing eddy transport as an enhanced diffusion requires timescale separation between the physical and biological processes, which raises concerns for the representation of subgrid scale primary productivity in coarse resolution climate models. Next, we turn to air-sea carbon fluxes. We show that carbon outgassing occurs preferentially in the Indo-Pacific sector of the Southern Ocean due to regional differences in the mixed-layer entrainment of upwelled carbon-rich deep water (Chapter 4). Finally, we quantify the relative importance of different frequency bands in driving year-to-year variations of Southern Ocean primary productivity. We find that changes in annual mean phytoplankton biomass are driven by intermittent sub-seasonal events associated with storms and eddies, rather than low frequency climate variability (Chapter 5). Together, these chapters use a novel combination of in situ measurements, satellite data, and model output to elucidate physical mechanisms that control Southern Ocean biogeochemistry. Understanding these drivers is necessary to improve climate models and predict the response of the ocean to climate change.