Abstract
In situ observations obtained over the last several decades have shown that the intensity of turbulent mixing in the abyssal ocean is enhanced toward the seafloor. Consequently, a new ...paradigm has emerged whereby dianeutral downwelling dominates in the ocean interior and dianeutral upwelling only occurs within thin bottom boundary layers. This study shows that when mixing is bottom intensified the net abyssal dianeutral transports and the stratification can depend on subtle features of the seafloor geometry. Under an assumption of depth-independent net dianeutral upwelling, small changes in the curvature of the seafloor can result in interior stratification that is bottom intensified, uniform, or surface intensified. Further, when the net dianeutral transport is allowed to vary in the vertical, changes in the seafloor slope and bathymetric contour length with height can drive lateral exchange between the boundary layer and interior, with particularly strong lateral outflows predicted at the crests of midocean ridges. Finally, using a realistic neutral density climatology the authors suggest that the increase in the perimeter of abyssal neutral density surfaces with height drives much of the dianeutral upwelling at depths greater than 4 km, while the increase in the slope of the seafloor at shallower depths acts to oppose upwelling. These results add to a growing body of literature highlighting the key control of seafloor geometry on the abyssal overturning circulation.
Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy‐constrained parameterization of mixing by internal ocean tides. ...Here, we present an energy‐conserving mixing scheme which accounts for the local breaking of high‐mode internal tides and the distant dissipation of low‐mode internal tides. The scheme relies on four static two‐dimensional maps of internal tide dissipation, constructed using mode‐by‐mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three‐dimensional map of dissipation which compares well with available microstructure observations and with upper‐ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing.
Plain Language Summary
When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers—a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three‐dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three‐dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models.
Key Points
A global three‐dimensional map of mixing induced by internal tides is presented
The map can serve as a comprehensive and energy‐constrained tidal mixing parameterization in global ocean models
The map compares well to available microstructure and upper‐ocean finestructure mixing estimates
Tracer Transport within Abyssal Mixing Layers Holmes, R. M.; de Lavergne, Casimir; McDougall, Trevor J.
Journal of physical oceanography,
10/2019, Volume:
49, Issue:
10
Journal Article
Peer reviewed
Open access
Abstract
Mixing layers near sloped topography in the abyss are thought to play a critical role in the global overturning circulation. Yet the behavior of passive tracers within sloping boundary layer ...systems has received little attention, despite the extensive use of tracer observations to understand abyssal circulation. Here, we investigate the behavior of a passive tracer released near a sloping boundary within a flow governed by one-dimensional boundary layer theory. The spreading rate of the tracer across isopycnals is influenced by factors such as the bottom-intensification of mixing, the dipole of upwelling (in the boundary layer) and downwelling (in the outer mixing layer), and along-isopycnal diffusion. For isolated near-boundary tracer releases, the bulk diffusivity, proportional to the rate of increase of the variance of the tracer distribution in buoyancy space, is much less than what would be expected from averaging the diapycnal diffusivity over the tracer patch. This stems from the presence of the bottom boundary that prevents tracer diffusion through it. Furthermore, when along-isopycnal diffusion is weak, the boundary tends to drive the tracer up the slope toward less dense fluid on average due to asymmetries between boundary layer and interior flows. With strong along-isopycnal diffusion this upslope movement is reduced, while at the same time the average diapycnal spreading rate is increased due to a reduced influence of the bottom boundary. These results have implications for what can be learned about the characteristics of mixing near sloping boundaries from past and future tracer-release experiments.
Abstract
Horizontal fluxes of heat and other scalar quantities in the ocean are due to correlations between the horizontal velocity and tracer fields. However, the limited spatial resolution of ocean ...models means that these correlations are not fully resolved using the velocity and temperature evaluated on the model grid, due to the limited spatial resolution and the boxcar-averaged nature of the velocity and the scalar field. In this article, a method of estimating the horizontal flux due to unresolved spatial correlations is proposed, based on the depth-integrated horizontal transport from the seafloor to the density surface whose spatially averaged height is the height of the calculation. This depth-integrated horizontal transport takes into account the subgrid velocity and density variations to compensate the standard estimate of horizontal transport based on staircase-like velocity and density. It is not a parameterization of unresolved eddies, since it utilizes data available in ocean models without relying on any presumed parameter such as diffusivity. The method is termed the horizontal residual mean (HRM). The method is capable of estimating the spatial-correlation-induced water transport in a 1/4° global ocean model, using model data smoothed to 3/4°. The HRM extra overturning has a peak in the Southern Ocean of about 1.5 Sv (1 Sv ≡ 10
6
m
3
s
−1
). This indicates an extra heat transport of 0.015 PW on average in the same area. It is expected that implementing the scheme in a coarse-resolution ocean model will improve its representation of lateral heat fluxes.
The world's largest ice shelves are found in the Antarctic Weddell Sea
and Ross Sea where complex interactions between the atmosphere, sea ice,
ice shelves and ocean transform shelf waters into High ...Salinity Shelf Water
(HSSW) and Ice Shelf Water (ISW), the parent waters of Antarctic Bottom
Water (AABW). This process feeds the lower limb of the global overturning
circulation as AABW, the world's densest and deepest water mass, spreads
outwards from Antarctica. None of the coupled climate models contributing to
CMIP6 directly simulated ocean–ice shelf interactions, thereby omitting a
potentially critical piece of the climate puzzle. As a first step towards
better representing these processes in a global ocean model, we run a
1∘ resolution Nucleus for European Modelling of the Ocean (NEMO; eORCA1) forced configuration to explicitly
simulate circulation beneath the Filchner-Ronne Ice Shelf (FRIS), Larsen C Ice Shelf (LCIS) and
Ross Ice Shelf (RIS). These locations are thought to supply the majority
of the source waters for AABW, and so melt in all other cavities is
provisionally prescribed. Results show that the grid resolution of
1∘ is sufficient to produce melt rate patterns and total melt
fluxes of FRIS (117 ± 21 Gt yr−1), LCIS (36 ± 7 Gt yr−1) and RIS
(112 ± 22 Gt yr−1) that agree well with both high-resolution models and
satellite measurements. Most notably, allowing sub-ice shelf circulation
reduces salinity biases (0.1 psu), produces the previously unresolved water
mass ISW and re-organizes the shelf circulation to bring the regional model
hydrography closer to observations. A change in AABW within the Weddell Sea and
the Ross Sea towards colder, fresher values is identified, but the magnitude is
limited by the absence of a realistic overflow. This study presents a NEMO
configuration that can be used for climate applications with improved
realism of the Antarctic continental shelf circulation and a better
representation of the precursors of AABW.
Abstract
The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO
2
over ...glacial‐interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ
18
O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid‐Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO
2
(
R
2
= 0.98). Experiments with a 0.25° resolution ocean‐sea‐ice‐carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO
2
outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO
2
rise, and suggest outgassing of natural CO
2
from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.
Plain Language Summary
The mid‐latitudes of the Southern Hemisphere are characterized by a band of strong westerly winds. These winds play an important role in driving the circulation of the deep ocean and are thought to influence the oceans' ability to store carbon. Understanding how the westerlies have varied in the past is challenging as we have few methods to track the winds directly. Here we use oxygen isotopes in foraminiferal shells to track changes in the broad‐scale pattern of sea surface temperature across the Southern Ocean, which we link to changes in the winds using climate models. We find the westerly winds were displaced around 5° equatorward during the cold climate of the last ice age, and that the poleward shift in the winds we observe as the earth warmed out of the ice age bears an uncanny resemblance to the increase in atmospheric CO
2
. We then force the winds in a climate model toward the equator in a similar manner to the shift we observe in the ice age, and find the model stores more carbon in the ocean. Our results support a link between shifts in the Southern Hemisphere westerly winds and atmospheric CO
2
.
Key Points
We use planktic foraminiferal δ
18
O and climate models to infer deglacial changes in the Southern Hemisphere surface westerlies
We estimate the westerlies were ∼5° equatorward and ∼25% weaker at the LGM; their poleward shift over deglaciation mirrors the rise in CO
2
Experiments with a 1/4° ocean‐sea‐ice‐carbon model indicate increased oceanic carbon storage with equatorward shifted westerlies
The ocean's internal pycnocline is a layer of elevated stratification that separates the well‐ventilated upper ocean from the more slowly renewed deep ocean. Despite its pivotal role in organizing ...ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high‐latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanisms generating the internal pycnocline at southern high latitudes through the analysis of a high‐resolution, realistic, global sea ice–ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with sea ice‐ocean interactions in two distinct ice‐covered regions, fringing the Antarctic continental slope and the winter sea‐ice edge. In both areas, winter‐persistent sea‐ice melt creates strong, salinity‐based stratification at the base of the winter mixed layer. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice‐ocean interactions in configuring the vertical structure of the Southern Ocean.
Plain Language Summary
Satellite observations have revealed significant trends in Antarctic sea‐ice concentration over recent decades. While the science community is starting to unravel the causes of the observed changes in sea‐ice extent, our understanding of how these ice changes are influencing ocean circulation remains rudimentary. Here we take a step toward addressing this important gap by analyzing relationships between sea ice and ocean density structure in a state‐of‐the‐art, realistic sea ice‐ocean model. We find that localized sea ice‐ocean interactions in the Southern Ocean, in particular the counter‐intuitive melting of sea ice in winter, contribute to shape the vertical structure of the Southern Hemisphere oceans.
Key Points
The internal pycnocline in the high‐latitude Southern Ocean is generated by winter‐persistent sea ice melting
Sea‐ice melt persists in winter due to ice drift and warm‐water entrainment, thus maintaining salinity‐based stratification at the base of the winter mixed layer
The subpolar internal pycnocline descends into the ocean interior at fronts of the Antarctic Circumpolar Current
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