Role of Mixed Layer Depth and Subduction Processes for the Southern Ocean Carbon and Nutrient Cycles

Abstract

Changes in wind forcing in the Southern Ocean exert a large impact on the dynamics of the surface mixed layer and subduction processes. Over the last two decades, the index of the Southern Annular Mode (SAM) has experienced a trend towards its positive phase, which is characterized by stronger westerly winds. The positive trend in the SAM index results from the complex interaction between the steady increase of atmospheric CO2 concentration due to anthropogenic emissions and the stratospheric ozone depletion. Co-occurring with the wind signal is the global warming effect driven by the increase in atmospheric CO2. Increased wind forcing alone would lead to a deepening of the mixed layer and enhance the supply of carbon and nutrients to the euphotic zone. In contrast, the surface ocean warming alone would lead to more surface stratification, and therefore to a shoaling of the mixed layer. The main objective of this PhD thesis is to answer the question: How did the combined changes in atmospheric forcing affect the surface mixed layer and the carbon and nutrient subduction rates on the timescale of interannual to decadal variability? In the first part of my thesis, I assessed the impact of the recent changes in atmospheric temperature and zonal wind speed on the summer mixed-layer depth (MLD) in the SO (south of 30AAA S) from observations and a set of model sensitivity experiments over the period of 2002-2011. The study showed that summer MLD changes in response to recent atmospheric forcing were zonally asymmetric. Summer MLD increased in the Antarctic Zone of the Atlantic and the Indian Ocean sectors. Overall, the effect of recent changes in wind forcing dominated over temperature-induced changes in summer MLD. In the second part of this thesis, I examined the decadal variability in nutrient and dissolved inorganic carbon (DIC) concentrations in the Antarctic Intermediate Water of the Atlantic sector of the Southern Ocean between 1990 and 2014 using cruise data sampled along the Prime Meridian. The results showed a positive trend in DIC and nitrate concentrations along with a negative trend in temperature and salinity. These observations support a scenario of an increase in the upper-ocean overturning circulation probably linked to the positive trend in the SAM index. The third part of this thesis focused on the SAM impact on the inter-annual variability of carbon and nutrient subduction rates across the base of the winter mixed layer between 1958 and 2016 using a coupled physical-biogeochemical general circulation model. The study showed that the variations in SAM led to large-scale anomalies in carbon and nutrient subduction and obduction rates that are zonally symmetric. More obduction occured south of the Antarctic Polar Front (APF) and more subduction occurred where the MLD gradient is strongest in response to the positive trend in the SAM index. Also, I found that the annual mean carbon and nutrient subduction rates varied by around 10% around the long-term mean on interannual to decadal time scales with a stronger positive trend since 1990 leading to an approximately 20% increase in DIC and nitrate subduction rates between 1990 and 2016. My findings (parts I, II and III) suggest that the positive trend of the SAM index (wind intensification) has profoundly affected the surface mixed layer, and increased upwelling of carbon and nutrient-rich deep water. The increased upwelling is driven by the Ekman divergence and is balanced by the stronger northward Ekman transport across the APF. North of the APF these water masses subduct as mode and intermediate waters. While today changes in the wind forcing play a larger role than atmospheric temperature changes, this might reverse in the future.