The impact of submesoscales on the stratification dynamics in the Southern Ocean

• Main Author: du Plessis, Marcel David
• Other Authors: Swart, Sebastiaan, Ansorge, Isabel, Mahadevan, Amala
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Cape Town 2018
• Subjects: Oceanography; Southern Ocean
• Online Access: http://hdl.handle.net/11427/29624; https://open.uct.ac.za/bitstream/11427/29624/1/thesis_sci_2018_du_plessis_marcel_david.pdf

Submesoscale dynamics O(1-10 km, hours to days) are considered to strongly affect the stratification of the upper ocean. In the Southern Ocean, studies of submesoscale dynamics are biased to regions preconditioned for strong frontal activity and topographical influence. This dissertation considers the role of submesoscales on the evolution of mixed layer depth and upper ocean stratification in the open-ocean Subantarctic Ocean. First, we present autonomous ocean glider measurements from spring to late-summer to show that transient increases in stratification within the mixed layer during spring result in rapid mixed layer shoaling events. A realistically-forced simulation using a one-dimensional mixed layer model fails to explain these observed stratification events. We show that during this time, baroclinic mixed layer instabilities periodically induce a restratification flux of over 1000 W. m2, suggesting that the unexplained restratification is likely a result of submesoscale flows. Second, we study four separate years of seasonal-length (mid-winter to latesummer) glider experiments to define how submesoscale flows may induce interannual variations in the onset of spring/summer mixed layer restratification. Sustained temporal increases of stratification above the winter mixed layer, which defines the onset of seasonal restratification, can differ by up to 28 days between the four years studied. To explain this discrepancy, equivalent heat fluxes of baroclinic mixed layer instabilities (restratification) and Ekman buoyancy flux (restratification or mixing) are parameterized into a one-dimensional mixed layer model. Simulations including the parameterizations reveal a seasonal evolution of mixed layer stratification which is significantly more comparable to the glider observations than model simulations using heat and freshwater fluxes alone. Furthermore, the parameterization dramatically improves the sub-seasonal variability of mixed layer stratification, particularly during the onset of seasonal ...