Variability of the contemporary Southern Ocean carbon fluxes and storage

• Main Author: Keppler, L.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Hamburg 2020
• Subjects: Southern Ocean
• Online Access: http://hdl.handle.net/21.11116/0000-0006-B3C8-E; http://hdl.handle.net/21.11116/0000-0006-B3CA-C

Around half of the ocean’s uptake of anthropogenic carbon from the atmosphere currently takes place in the Southern Ocean. However, the variability of this important carbon sink, as well as the drivers behind this variability, are still debated and it is unclear if the Southern Ocean will remain a carbon sink in the future. Until this PhD project, the development of the Southern Ocean carbon uptake at the air-sea interface was unknown based on observations beyond 2011. Furthermore, the seasonal to interannual variability of dissolved inorganic carbon (DIC) in the interior Southern Ocean had not been analyzed based on observations at regional scale. This dissertation closes these research gaps. In the first part of my dissertation (Appendix A), I investigate the Southern Ocean carbon flux and its drivers until 2016 using an updated observation-based air-sea carbon flux estimate. After a stagnation period in the 1990s, and a reinvigoration in the 2000s, I find that the Southern Ocean carbon uptake weakened again since about 2011. My study reveals that the Southern Annular Mode, the dominant mode of climate variability in the southern high latitudes, is not the driver behind this weakening due to opposing effects that cancel each other out. Instead, regional shifts in surface wind velocity modulate the recent evolution of the carbon uptake in the Southern Ocean. In the second part (Appendix B), I develop a monthly climatology of global mapped interior DIC fields using a neural-network mapping approach. Using this new data product, I describe the seasonal carbon dynamics at global scale, including the phase and amplitude of the surface seasonal cycle, how deep seasonal signals are detectable, and I estimate the net community production. In the third part (Appendix C), I increase the temporal resolution of my new data product to resolve monthly fields from 2004 through 2017. I then re-focus on the Southern Ocean to investigate the interannual variability of DIC in the water column and determine the potential drivers behind ...