Topographic enhancement of vertical turbulent mixing in the Southern Ocean

Abstract

It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

Transformation of Deep Water Masses Along Lagrangian Upwelling Pathways in the Southern Ocean

“…However, most of this bottom boundary layer mixing occurs within 1,000 m of the bottom (Garabato et al, ; St. Laurent et al, ; Waterhouse et al, ), so away from the continental slope and shallow topographic features, it is not clear how important mixing is in upwelling of deep water in the 1,000–2,000 m depth range (Toggweiler & Samuels, ). Recent analysis of repeat observations across Drake Passage indicates that diapycnal mixing sustains the overturning in the upper 1,000 m and in the 1,000–2,000 m above the seafloor, while in between, isopycnal stirring dominates in the Antarctic Intermediate Water and Upper Circumpolar Deep Water (UCDW) density classes (Mashayek et al, ; Naveira Garabato et al, ).…”

“…While there are many observations characterizing local episodes of intense turbulent mixing at rough topography, these measurements are sparse and infrequent in time, making it difficult to scale this to the integrated diapycnal mixing that affects the large‐scale overturning. Tracer release experiments are suited for addressing this question, but to date these efforts have been confined to regional studies, which are then extrapolated to the circumpolar Southern Ocean (Mashayek et al, ; Watson et al, ). Additionally, while there is evidence that diapycnal mixing in the interior of the Southern Ocean is elevated in hot spots associated with topography (Naveira Garabato et al, ; Ruan et al, ; Watson et al, ), these hot spots make up a relatively small fraction of the total volume of the deep Southern Ocean.…”

Does Southern Ocean Surface Forcing Shape the Global Ocean Overturning Circulation?

“…Their approximation that the isopycnal slope in the Southern Ocean is insensitive to surface forcing perturbations is only qualitatively supported in observations (Böning et al, ) and models (Gent & Danabasoglu, ; Viebahn & Eden, ). More importantly, observations suggest that there is substantial diapycnal mixing over rough topography in the Southern Ocean (e.g., Mashayek et al, ; Naveira Garabato et al, ; Whalen et al, ; Wu et al, ), which is at odds with the adiabatic approximation for the Southern Ocean circulation. In the present study, three simulations that were carried out with the ocean component of a state‐of‐the‐art climate model are analyzed to investigate the extent to which changes in Southern Ocean surface buoyancy forcing alone can explain the shoaling of the AMOC at the LGM.…”

Squeeze Dispersion and the Effective Diapycnal Diffusivity of Oceanic Tracers

“…In section , we implement this method for calculating κ e in in an analysis of microstructure observations from the Samoan Passage and find that the effective diapycnal diffusivity of tracers advected through the Samoan passage differs from the local diffusivity averaged on isopycnals by factors of 0.5–3. This difference between bulk effective diffusivity and average local diffusivity in the Samoan passage suggests that realistic variations in diffusivity and squeezing can modulate the dispersion of oceanic tracers and may contribute to differences between tracer‐based and microstructure‐based estimates of diapycnal diffusivity inferred from observations as, for example, in the Brazil Basin (Ledwell et al, ), the East Pacific sector of the Antarctic Circumpolar Current (Ledwell et al, ), and Drake Passage (Mashayek et al, ; St. Laurent et al, ; Watson et al, ).…”

“…differences between tracer-based and microstructure-based estimates of diapycnal diffusivity inferred from observations as, for example, in the Brazil Basin (Ledwell et al, 2000), the East Pacific sector of the Antarctic Circumpolar Current (Ledwell et al, 2011), and Drake Passage (Mashayek et al, 2017;St. Laurent et al, 2012;Watson et al, 2013).…”