Eddy-induced variability in Southern Ocean abyssal mixing on climatic timescales

Abstract

The Southern Ocean plays a pivotal role in global ocean circulation and climate [1][2][3] . It is there that the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange large amounts of heat and carbon with the atmosphere [4][5][6] . While the climatic drivers of changes in the upper cell are relatively well established 7 , little is known about how the lower cell responds to changes in climatic forcing. Here, we show the first observational evidence that 1 small-scale mixing in the abyssal Southern Ocean, a major driver of the lower overturning cell [8][9][10] , exhibits variability on time scales of months to decades, consistent with a significant modulation by oceanic eddies impinging on seafloor topography. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerlies 11,12 , our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing.The Southern Ocean limb of the global overturning circulation consists of two cells 4, 5,13 . The upper cell involves the upwelling and southward flow of mid-depth waters of North Atlantic origin, their transformation into lighter waters within the upper layers of the Antarctic Circumpolar Current (ACC), and their subsequent return northward as mode and intermediate waters. This vertical circulation is underpinned by a combination of wind-driven Ekman motions, eddy-induced flows, and air-sea interaction, which sustains the diabatic near-surface water mass transformation 4, 7,14 . In the lower cell, the southward shoaling of mid-depth waters is balanced by the production of dense abyssal waters by intense oceanic heat loss along the Antarctic margin. These abyssal waters are exported northward into and across the ACC and, in the process, are transformed into mid-depth waters by small-scale, turbulent diabatic mixing. Ultimately, it is the intensity of this mixing that sets the rate at which the abyssal ocean overturns 8, 9,15 .Observations of the spatial distribution of turbulent mixing [16][17][18][19][20] and idealised modelling studies 15,21 link the occurrence of Southern Ocean abyssal mixing to the breaking of internal lee waves, generated as the ACC's vigorous mesoscale eddy flows impinge on seafloor topography. 2The radiation and breaking of lee waves is estimated to account for the bulk of the dissipation of the Southern Ocean eddy field 21,22 , and to support a major fraction of the diabatic water mass transformation closing the lower overturning cell in the abyssal ocean 23 . This prompts the hypothesis that Southern Ocean abyssal mixing and overturning are sensitive to the intensity of the regional eddy field and, since the eddy field is primarily energised by instabilities of the windforced circulation 8,24,25 , to climatic perturbations in atmospheric forcing.We address this hypothesis by analysing the temporal variability of Southern Ocean abyssal mixing and inte...

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…Both Sheen et al (2014) and Waterman et al (2014) showed that the shear based parameterization overestimates observed mixing rates in near-bottom regions of strong internal wave generation. The parameterization relies on the assumption that the background internal-wave field is well characterized by the Garrett-Munk (GM) spectrum.…”

“…Ledwell et al (2010), St. Laurent et al (2012), and Sheen et al (2014) showed that enhanced internal wave fine-structure and turbulence levels were not widespread, but limited to frontal zones where strong bottom currents collide with steep large-amplitude topography. This thesis uses data primarily from two field expeditions of the DIMES study, hereafter referred to as the US2 and US5 expeditions (Fig 1-10 and 1-12).…”

“…The direct measurements of dissipation made as part of the DIMES study are described by Ledwell et al (2010), St. Laurent et al (2012) and Sheen et al (2014). These studies showed that enhanced internal wave fine structure and turbulence levels were not widespread, but limited to frontal zones where strong bottom currents collide with significant topography.…”

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

“…The direct measurements of dissipation made as part of the DIMES study are described by Ledwell et al (2011), St. Laurent et al (2012, and Sheen et al (2014). These studies showed that enhanced internal wave fine structure and turbulence levels were not widespread but were limited to frontal zones where strong bottom currents collide with significant topography.…”

In situ measurement of the biogeochemical properties of Southern Ocean mesoscale eddies in the Southwest Indian Ocean, April 2014

“…This scarcity of in situ data is particularly pronounced in the Southern Ocean. Despite the significance of the Southern Ocean to ocean-atmosphere CO 2 exchange and global climate, and the important role of ocean eddies to these pro- cesses (Frenger et al, 2013;Sheen et al, 2014;Morrow et al, 1994), it remains a remote, hostile and under-sampled ocean environment.…”