Abyssal Mixing in the Equatorial Pacific from Realistic Simulations

Recent theoretical work and numerical simulations have shown that, when the full Coriolis force and the so-called non-traditional effects are taken into account, the reflection of Equatorially Trapped Waves (ETWs) off the seafloor generates beams of short inertia-gravity waves with strong vertical shear and low Richardson numbers that result in bottom-intensified, persistent, zonally-invariant mixing at the inertial latitude of the ETW through the mechanism of critical reflection. It has been estimated that this process can result in order 10 Sv of diapycnal upwelling per wavelength of ETW in the abyss, and thus could play an important role in closing the Abyssal Meridional Overturning Circulation and connecting the deep and abyssal ocean. However, these results have been derived under an idealized configuration with a background state at rest, a flat seafloor, a periodic zonal domain, and an idealized, monochromatic ETW field. To test the theory in a flow that is more representative of the ocean, we use realistic numerical simulations of the Equatorial Pacific where the traditional approximation of neglecting the horizontal component of the Coriolis parameter has been relaxed (i.e. we use a quasi-hydrostatic version of the model). Our simulations are nested into a Pacific-wide hydrostatic parent solution forced with climatological forcing and realistic bathymetry, resulting in an ETW field and a deep mean circulation consistent with observations. Using these realistic simulations, we observe enhanced mixing at the equator, even over smooth topography, in the quasi-hydrostatic case, which is consistent with theory. Implications of strong abyssal mixing in the equatorial regions on vertical transport of water masses and the closure of the Abyssal Meridional Overturning Circulation will be discussed.