Enhanced abyssal mixing in the equatorial Ocean associated with non-traditional effects

The ocean plays a key role in regulating the Earth's climate by storing vast quantities of heat and carbon and distributing them all over the world. Schematically, the ocean circulation can be represented by two meridional overturning cells stacked in the vertical, which form together the so-called Meridional Overturning Circulation (MOC). The upper cell is associated with sinking of polar water to mid-depth in the North Atlantic, which then upwells back to the surface along constant density surfaces under the pull exerted by strong winds blowing over the Southern Ocean. This upwelling process therefore requires very little mixing. However, the lower cell, associated with sources of abyssal water around Antarctica, inherently requires mixing processes to lift bottom waters towards mid-depth and eventually close the MOC. Although the polar source regions for these overturning cells have been identified, the energy sources driving mixing and their spatial distribution are still debated. My research brings new elements to this debate by exploring an hypothesis according to which mixing can be energized in the tropics by Equatorial Waves (EWs) whose physics has been modified by the so-called Non-Traditional (NT) effects (i.e., the effects associated with the horizontal component of the Coriolis parameter, which is traditionally ignored in most theories and models of the ocean circulation). I use both analytical theory and idealized numerical simulations to show that the reflection of EWs off the bottom leads to seafloor-intensified mixing near the equator when NT effects are taken into account through the mechanism of critical reflection. In these idealized simulations, it is estimated that this process can result in order 10 Sv of diapycnal upwelling in the abyss, and thus could play an important role in closing the abyssal MOC. These results manifest also in realistic numerical simulations of the Eastern Equatorial Pacific that are nested into a Pacific-wide parent solution forced with climatological data and realistic bathymetry, resulting in an EW field and a deep circulation consistent with observations. The enhanced mixing in the realistic simulations drives diapycnal upwelling, in agreement with the predictions from the idealized simulations. Since these results were derived using a parameterization for mixing that has not been optimized for the application of critical reflection of EWs modified by NT effects, I run high-resolution, non-hydrostatic simulations of the critical reflection mechanism to resolve the turbulence that develops and investigate the amount of mixing that results from this mechanism. These simulations allow me to characterize the nonlinear dynamics to smaller scales involved during the critical reflection of the wave and thus to provide elements that lead to a parameterization of the diapycnal mixing induced by this mechanism. Integrating these results into climate models could yield a better representation of the ocean circulation and thus could lead to better predictions of how our climate will evolve over the next century

Keywords: Earth System Science.
Advisor: Fringer Oliver B. (Oliver Bartlett)
O'Neill Morgan (Morgan E.)
Thomas Leif N.