Abyssal Upwelling and Downwelling Driven by Near-Boundary Mixing

Abstract

A buoyancy and volume budget analysis of bottom-intensified mixing in the abyssal ocean reveals simple expressions for the strong upwelling in very thin continental boundary layers and the interior near-boundary downwelling in the stratified ocean interior. For a given amount of Antarctic Bottom Water that is upwelled through neutral density surfaces in the abyssal ocean (between 2000 and 5000 m), up to 5 times this volume flux is upwelled in narrow, turbulent, sloping bottom boundary layers, while up to 4 times the net upward volume transport of Bottom Water flows downward across isopycnals in the near-boundary stratified ocean interior. These ratios are a direct result of a buoyancy budget with respect to buoyancy surfaces, and these ratios are calculated from knowledge of the stratification in the abyss along with the assumed e-folding height that characterizes the decrease of the magnitude of the turbulent diapycnal buoyancy flux away from the seafloor. These strong diapycnal upward and downward volume transports are confined to a few hundred kilometers of the continental boundaries, with no appreciable diapycnal motion in the bulk of the interior ocean.

Hydrothermal Heat Enhances Abyssal Mixing in the Antarctic Circumpolar Current

“…Coherent stratification and mixing anomalies are found at several locations where the ACC crosses mid-ocean ridges. These anomalies extend as much as 500 m above the sea floor, in contrast to several recent studies (Ferrari et al, 2016;McDougall & Ferrari, 2017) that assume geothermal heating only impacts diapycnal upwelling in the bottom boundary layer, a layer tens of meters thick where the turbulent density flux is transported into the ocean floor, and not the stratified mixing layer above that is hundreds of meters thick. However, these studies do not take into account the effect of convective mixing by hydrothermal heat.…”

“…Here we show that hydrothermal heating increases upwelling by up to an order of magnitude within 300 m, or even 500 m, of the seabed. While the diapycnal diffusivity above the bottom boundary layer due to the conductive geothermal heat flux is estimated as ∼ O(10 −4 ) m/s 2 (Emile-Gaey & Madec, 2009;McDougall & Ferrari, 2017), we find diapycnal diffusivities of approximately 5 ×10 −3 m/s 2 within 500 m of the seabed associated with hydrothermal heating and the associated changes in stratification (Figure 3).…”

Mixed up at the sea floor

“…Bottom-intensified mixing, combined with geothermal heating through the ocean bed, implies that water-mass transformation should be concentrated close to the sea floor 11 . In turn, this suggests a pattern of circulation that involves deep waters rising near the sea floor, in contrast to Munk's model of waters rising throughout the ocean interior 10,12 (Fig. 1).…”

Deep Sea Currents Driven by Breaking Internal Tides on the Continental Slope

“…While a fraction of tidal energy goes into high-mode internal tides that locally dissipate and lead to near-bottom enhancement of mixing near rough topography (Polzin et al, 1997;Waterhouse et al, 2014), the major fraction is converted into low-mode internal tides, which can propagate for a long distance away from the generation sources. Understanding the ultimate fate of these low-mode internal tides, especially for their potential dissipation at a sloping bottom boundary, is important for improving the estimation of the largescale meridional overturning circulation (de Lavergne et al, 2016;Ferrari et al, 2016;McDougall & Ferrari, 2017).…”