News & Events

PhD thesis defense: Natalie Freeman

Friday, May 05, 2017, 1:00PM - 3:00PM


Natalie Freeman



SEEC S228 (Sievers Conference Room)


Physical and biogeochemical features of the Southern Ocean: Their variability and change over the recent past and coming century


While the Southern Ocean has experienced substantial changes in atmospheric forcing over the past few decades, the subsequent impacts on basin circulation and biogeochemical cycling is not well understood. Using satellite and hydrographic observations and model output, this dissertation investigates the variable and changing large-scale physical and biogeochemical features of the Southern Ocean. From 1998 to 2014, austral summer surface phytoplankton calcification and calcite concentration are found to decline by 9% and 24%, respectively, in large portions of the Southern Ocean's Great Calcite Belt, concurrent with a reduction in surface ocean carbonate ion concentration. A regional increase in biocalcification and calcite near the Antarctic Polar Front (PF) in the Atlantic sector is attributed to a physical southward shift in the location of the PF, altering local temperature and nutrient availability. Using methods that avoid cloud contamination and steric sea level change, a novel, comprehensive frontal mapping scheme was developed to create a high-resolution data set of weekly PF locations from 2002-2014. The spatio-temporal variability in the location and strength of the PF and its associated Silicate Front (SF) is largely influenced by the underlying bathymetry: over shallow bathymetry, Southern Ocean fronts are strong and interannual variability in position is low, while long-term meridional shifts in front position are found over the deeper regions of the Indian and Pacific sectors. From 2002 to 2014, the PF is found to have intensified both in zonal average and at nearly all longitudes; a more zonally asymmetric atmospheric circulation drives changes in wind forcing and, through modifications in SST, the strength of the PF. A state-of-the-art coupled climate model predicts a more poleward SF by the end of the 21st century under a high emission scenario, due to large-scale reductions in surface silicate as a consequence of a warmer, more stratified Southern Ocean, with implications for local biology and the interpretation of paleoclimate records from deep sea sediments.