Presenter: Kelly Ogden (Western University, Mechanical Engineering)

Description:
The deep branches of the ocean's meridional overturning circulation are associated with rising abyssal waters. Recent work (Ferrari et al., JPO, 2016) has suggested that the rising waters are confined to boundary layers along the ocean ridges and seamounts, where mixing is most intense. To assess this theory, simulations of the Brazil Basin are conducted using the MITgcm, including the release of a numerical tracer, and the results are compared to observational data from the Brazil Basin Tracer Release Experiment (BBTRE) to validate the simulation. Simulations are conducted using MITgcm with a specified, bottom intensified, diffusivity profile, whose vertical profile is assumed to be self-similar as a function of height above the bottom and based on microstructure estimates. However, the overall magnitude of the diffusivity must be doubled, compared to the microstructure estimates, in order for numerical tracers to better match the diapcynal dispersion of the tracer released during BBTRE.  The simulation results show upwelling occurs along the boundaries, while interior waters sink slowly. The magnitude, distribution, and peak volume transport are consistent with previously published calculations and observations (de Lavergne et al. Nature, 2017; Toole et al., JPO 2001; Lumpkin and Speer, JPO, 2007), supporting the recent theory that upwelling in the ocean occurs primarily along the boundaries. This agreement suggests that the specified bottom intensified diffusivity profile is a reasonable mixing model and confirms the recent theory of deep ocean mixing.