A Microscale View of Mixing and Overturning across the Antarctic Circumpolar Current

Garabato, Alberto C. Naveira; Polzin, Kurt L.; Ferrari, Raffaele; Zika, Jan D.; Forryan, Alexander

doi:10.1175/jpo-d-15-0025.1

Cited by 29 publications

(41 citation statements)

References 56 publications

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“…To assess the relative role of eddy stirring relative to vertical mixing, we consider previous estimates of K e from modeling studies. High-resolution modeling of the ACC system has been considered by a number of investigators (Abernathey et al 2010;Ferrari and Nikurashin 2010;APRIL 2016Klocker and Data from the US5 expedition used in this study was taken in the Subantarctic Front and Polar Front, such that suppressed levels of eddy mixing described by Ferrari and Nikurashin (2010) and Naveira Garabato et al (2016) are expected. Thus, our estimates from (7) are seemingly O(2-3) times larger than these studies suggest for the eddies in this region.…”

Section: Discussionmentioning

confidence: 99%

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

Merrifield

Laurent

Owens

et al. 2016

Journal of Physical Oceanography

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Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate « estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance x is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn-Cox method are larger than ensemble statistics based on « and the buoyancy frequency.

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Section: Discussionmentioning

confidence: 99%

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

Merrifield

Laurent

Owens

et al. 2016

Journal of Physical Oceanography

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“…However, observational studies suggest that the interior ocean mixing is an important component of the Southern Ocean meridional overturning circulation (Garabato et al, ; Katsumata et al, ; Naveira Garabato et al, ; Sloyan & Rintoul, ). Near‐bottom mixing in hot spots over rough seafloor topography (Ledwell et al, ; Polzin, ) can accomplish upwelling of the denser Lower Circumpolar Deep Water (LCDW) to lighter Upper Circumpolar Deep Water (UCDW).…”

Section: Introductionmentioning

confidence: 99%

“…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, ).…”

Section: Introductionmentioning

confidence: 99%

“…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. Therefore, for an upwelling deep water parcel, it is not well understood how mixing at these hot spots contributes to the integrated density change along the path from the deep ocean to the surface.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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Upwelling of northern deep waters in the Southern Ocean is fundamentally important for the closure of the global meridional overturning circulation and delivers carbon and nutrient‐rich deep waters to the sea surface. We quantify water mass transformation along upwelling pathways originating in the Atlantic, Indian, and Pacific and ending at the surface of the Southern Ocean using Lagrangian trajectories in an eddy‐permitting ocean state estimate. Recent related work shows that upwelling in the interior below about 400 m depth is localized at hot spots associated with major topographic features in the path of the Antarctic Circumpolar Current, while upwelling through the surface layer is more broadly distributed. In the ocean interior upwelling is largely isopycnal; Atlantic and to a lesser extent Indian Deep Waters cool and freshen while Pacific deep waters are more stable, leading to a homogenization of water mass properties. As upwelling water approaches the mixed layer, there is net strong transformation toward lighter densities due to mixing of freshwater, but there is a divergence in the density distribution as Upper Circumpolar Deep Water tends become lighter and dense Lower Circumpolar Deep Water tends to become denser. The spatial distribution of transformation shows more rapid transformation at eddy hot spots associated with major topography where density gradients are enhanced; however, the majority of cumulative density change along trajectories is achieved by background mixing. We compare the Lagrangian analysis to diagnosed Eulerian water mass transformation to attribute the mechanisms leading to the observed transformation.

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“…The tracer had been released midway between the two fronts, and so when compromises in sampling were needed, sampling was curtailed beyond the fronts, especially north of the Subantarctic front in the Pacific, guided by the notion that the fronts are barriers to cross-ACC transport (e.g. Naveira Garabato et al, 2016) and also by an altimetry-based prediction of tracer patch spreading. It Figure 2. a) The peak concentration measured (log10 mol L −1 ) at each section as a function of time since release (months).…”

Section: Tracer Datamentioning

confidence: 99%

Tracking the spread of a passive tracer through Southern Ocean water masses

Zika¹,

Sallée²,

Meijers³

et al. 2019

Preprint

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A dynamically passive inert tracer was released in the interior South Pacific Ocean at latitudes of the Antarctic Circumpolar Current. Observational cross sections of the tracer were taken over four consecutive years as it drifted through Drake Passage and into the Atlantic Ocean. The tracer was released within a region of high salinity relative to surrounding waters at the same density. In the absence of irreversible mixing a tracer remains at constant salinity and temperature on an isopycnal surface. To investigate the process of irreversible mixing we analysed the tracer in potential density versus salinity-5 anomaly coordinates. Observations of high tracer concentration tended to be collocated with isopycnal salinity anomalies. With time an initially narrow peak in tracer concentration as a function of salinity at constant density, broadened with the tracer being found at ever fresher salinities, consistent with diffusion-like behaviour in that coordinate system. The second moment of the tracer as a function of salinity suggested an initial period of slow spreading for approximately 2 years in the Pacific, followed by more rapid spreading as the tracer entered Drake Passage and the Scotia Sea. Analysis of isopycnal salinity gradients based 10 on the Argo programme suggests that part of this apparent change can be explained by changes in background salinity gradients while part of the change may be explained by geographical changes in background mixing.

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A Microscale View of Mixing and Overturning across the Antarctic Circumpolar Current

Cited by 29 publications

References 56 publications

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

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

Tracking the spread of a passive tracer through Southern Ocean water masses

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