Formation Generation

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The labels W and E indicate whether the jets are directed westward or eastward. In section 3c , we showed that the lateral eddy momentum flux and vertical eddy form stress are the dominant contributions to the net momentum flux vector indicated via arrows over the continental slope. Figures 9a and 9b reveal that these fluxes are in fact strongly tied to the structure of the along-slope jets; the eddy form stress is concentrated in the core of the eastward jets, while the lateral eddy momentum flux is concentrated at the southern flanks of the eastward jets.

This differs from the behavior of quasigeostrophic jets, based on which one might expect the lateral momentum fluxes occur at all jet flanks, transferring eastward momentum out of the westward jets and into the eastward jets Lee The asymmetry in the momentum transfer between the eastward and westward jets may be expected to result in asymmetries in the residual circulation, which approximately balances the net momentum flux convergence [cf. Despite these interjet variations of the mean and eddy streamfunctions, Fig. However, this analysis reveals that the cross-slope transport is accomplished via distinct mechanisms in the westward and eastward jets.

Finally, we note that in Fig. However, this term is typically orders of magnitude smaller than the other terms in 6. To understand the asymmetry in the momentum fluxes between the eastward and westward jets, we now consider the eddy energy budget at the jet scale. Figure 10a shows that the EKE is concentrated in the eastward jets, with a bias toward the southern jet flanks, consistent with the locations of enhanced eddy momentum fluxes in Fig.

As in Fig. However, Fig. Intuitively, this is because the eastward CDW jets oppose the direction of the flow in the AASW and AABW layers and are therefore necessarily associated with stronger vertical shear and thus baroclinicity. Crucially, in the eastward jets the isopycnals are more steeply sloped than the topography; the inset in Fig.

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Linear baroclinic instability theory therefore predicts that the eastward jets should be unstable Mechoso ; Isachsen , whereas the westward jets may be expected to be stable. This energy conversion makes a relatively minor contribution to the MKE budget, shown in Fig. However, it provides a necessary route to dissipation for EKE in the CDW layer; after being converted to MKE at the southern flanks for the eastward jets, the energy is transferred downward and removed by bottom friction at the ocean bed.

This vertical MKE flux is indicated by arrows in Fig. Some mismatch is to be expected because the jets are translating down the continental slope and thus the time derivatives of the EKE and MKE are nonzero. However, as suggested by Fig. This temporal variability also explains the apparent unbalanced convergence of energy in the CDW layer in Fig. The key insight lies in the relation between the cross-slope CDW mass flux, which determines the heat flux in most of the simulations examined here, and the EKE in the CDW layer over the continental slope, which serves as an input parameter for the mixing length theory described below.

Though in principle it may also be possible to constrain the EKE based on the model parameters, it is not clear that such a theory would have utility beyond the particular idealized model configuration used in this paper. The key ingredients of our theory are illustrated schematically in Fig. This conceptual decomposition of the water masses and overturning circulation is similar to that introduced by Ou Comparison between our conceptual model prediction for b , c the midslope shoreward CDW transport and d , e the shoreward eddy heat flux.

In b and d , we have set the mixing length equal to the topographic Rhines scale Rhines , while in c and e we have set the mixing length equal to the slope width. Motivated by our diagnosis of the overturning circulation in section 3a , we assume that the shoreward transport of AASW is accomplished entirely by wind-driven Ekman transport see also Zhou et al. Using the actual CDW layer thickness gradient diagnosed from our simulations does not qualitatively change the results. Note that in 21 we have used the barotropic Rhines scale because the along-slope jets in Fig.

For our reference simulation, 21 predicts a jet width of 26 km, which is within the range of jet widths diagnosed in section 4. Visbeck et al. Note that 25 results in a net shoreward CDW transport that scales only with EKE, that is, , a result that does not have a clear interpretation based on our analyses of the momentum and energy budgets in sections 3 — 4.

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As a further test of the predictive power of our mixing length—based theory, we draw on our finding in Fig. Finally, recall that the heat flux associated with shoreward transport of AASW is due to the wind-driven mean shoreward Ekman transport, while the heat flux associated with shoreward transport of CDW is due to eddy transport Figs. We therefore assume that the full, cross-slope heat flux [ 26 ] may be decomposed into mean and eddy components defined positive offshore as. In these panels the agreement between the theory and the simulations is less satisfactory than for the CDW mass transport in Figs.

The most notable source of discrepancy lies in the experiments with varying surface wind stress and varying slope width square and triangular symbols in Figs. When the wind stress is reduced, the wind extracts less energy from the deep stratification at the CDW—AABW interface, leading to a greater baroclinic release of potential energy.

A broader continental slope produces a similar effect because the shallower topographic slope is less effective at suppressing baroclinic instability. In both cases, the more energetic eddies over the continental slope—almost an order of magnitude larger in the case of zero surface wind stress than in our reference simulation—more efficiently stir tracers across the continental slope.

As a result, our assumption of a purely advective cross-slope heat exchange leads to an underestimate of the eddy heat flux. The ASF occupies a critical location in the near-Antarctic and global ocean circulation because it supports a range of salient water mass exchanges between the open ocean and the Antarctic continental shelf, many of which are mediated by the genesis of mesoscale eddies e.

In this study, we have used an idealized, eddy-resolving process model to elucidate the dynamics of these eddies along bottom water—forming stretches of the Antarctic coastline. We partly motivated this work in section 1 by the prospect that AABW outflow might passively facilitate shoreward heat transport by creating an isopycnal connection between the continental shelf waters and the CDW layer offshore.

In sections 3a and 3b , we verified this mechanism; in our model, the shoreward transports of heat and mass are supported by eddy thickness fluxes along isopycnals. The vertical momentum flux takes the form of an eddy form stress and coincides with release of available potential energy at the CDW—AABW interface. This injects energy into the CDW layer, producing a local maximum in EKE that is ultimately sourced from buoyancy loss on the continental shelf. The enhancement of EKE in the subsurface CDW layer also poses a challenge to future observational campaigns around the Antarctic margins, as it has a limited surface signature.

In section 4 , we showed that the continental slope hosts a series of alternating along-slope jets, qualitatively similar to those found in recent observations Thompson and Heywood ; Thompson et al. These jets continually drift down the continental slope, vanishing in a sufficiently long time average, suggesting that the fronts in the in situ measurements may in fact be transient features.

The jet-scale momentum and energy balances demonstrate that the eddy fluxes of momentum and energy to the CDW layer occur almost entirely in the eastward jets, which oppose the mean flow of the ASF and therefore have greater baroclinicity. We interpreted this asymmetry from the standpoint of linear baroclinic instability theory Mechoso ; Isachsen , which suggests that only the eastward jets should be baroclinically unstable because there the isopycnals dividing the CDW and AABW layers are more steeply sloped than the bottom topography. The formation of multiple along-slope jets has been reported previously in a conceptually similar model configuration with no surface wind forcing Spall The down-slope drift of the jets is consistent with the quasigeostrophic and primitive equation model results of Stern et al.

In section 5 , we combined previous theory Visbeck et al. Both parameterizations have skill in predicting the cross-slope CDW mass flux and the simulated shoreward eddy heat fluxes, though the assumptions used to predict the latter break down as the surface wind stress weakens. We found the predictive skill of the parameterization based on slope width—constrained mixing to be superior to that of the parameterization based on Rhines scale—constrained mixing, despite the stronger physical motivation for the latter.

However, the slope width—based parameterization is more ad hoc in that the constant of proportionality C has been chosen simply to obtain a fit. The Rhines scale—based parameterization has no undetermined constants because it is directly modified from Visbeck et al. As outlined in section 2 , we employed an idealized model configuration because it allowed us to resolve eddies across a range of model parameters while preserving realistic near-Antarctic ocean stratification Stewart and Thompson a.

However, this idealization carries various caveats that we regard as avenues for further work. For example, our model does not include a representation of tides, which contribute to AABW production via water mass transport by tidal residual flows and via diapycnal mixing due to breaking internal waves Padman et al. Flexas et al. Our model configuration also cannot accurately represent down-slope gravity currents, by which newly formed dense shelf water spills out onto the continental slope e.

Though our model cannot represent the initial gravity-driven flow and resulting entrainment in these currents, it can capture the later stages of their evolution when they adjust to form geostrophic, bottom-trapped, along-isobath flows Gordon et al. While we purposefully minimized the complexity of the surface forcing in our model, in reality AABW formation depends on a series of processes that are not represented here. In particular, we exclude the formation of Ice Shelf Water ISW , which is the lightest water mass along some stretches of the coastline e.

The interaction of these shelf processes and seasonal surface buoyancy fluxes with mesoscale turbulence over the continental slope remains unexplored. Additionally, the strong seasonal variations in surface buoyancy forcing at these latitudes lead to a pronounced annual cycle in the formation of high-salinity shelf water HSSW; e. Our representations of the surface wind forcing and ice—ocean thermodynamic exchanges are also highly idealized and neglect the subtleties associated with transmission of stress through sea ice Uotila et al.

The cyclonic winds that circulate around the Weddell Gyre, for example, also have a pronounced seasonal cycle that modulates the export of AABW e. We performed a simulation using a simple annual cycle with an amplitude of 0. It is also possible that higher-frequency variability in the winds might directly influence the EKE and jet formation in the CDW layer. A key element to be addressed in future work is the role of alongshore asymmetries in modulating cross-slope eddy transport of CDW.

In analogy with ACC e. As part of this work we performed several simulations that i included a topographic anomaly, for example, trough or ridge, on the shelf and slope, ii localized the brine input over the continental shelf, and iii shifted the spatial displacement of the brine injection and topographic anomaly.

In all cases we found that there was little impact on the eddy transport of CDW across the center of the continental slope. This is consistent with the findings of Spall , who introduced troughs and ridges into a simulation of dense water formation over an island and found that, in along-isobath coordinates, the system behaved identically to a case without troughs and ridges. Importantly, as reported in previous modeling studies and in situ observations e. However, our exploration of the influence of alongshore topographic variations on cross-slope eddy transport is too preliminary to draw firm conclusions and warrants further study.

It is conceivable that the processes described in this paper could apply in a three-dimensional sense, with the rate of AABW export upstream of the Ronne polynya modulating the rate of shoreward CDW transport around the polynya itself.

Some of the simulations presented herein were conducted using the CITerra computing cluster in the Division of Geological and Planetary Sciences at the California Institute of Technology, and the authors thank the CITerra technicians for facilitating this work. The data presented in this article are available from the authors on request. The authors gratefully acknowledge the modeling efforts of the MITgcm team. The authors also thank Tore Hattermann and an anonymous reviewer for many constructive comments that improved the manuscript.

In sections 3 and 4 , we use instantaneous daily snapshots to calculate all mean and eddy quantities.

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Image of typeset table. Role of eddies in cross-slope exchange. Cross-slope mass and tracer fluxes. View larger version 39K Fig. View larger version 19K Fig. View larger version 53K Fig. Alongshore flow and momentum transfer. View larger version 50K Fig. Dynamics of along-slope jets. Jet circulation and momentum balance. View larger version 44K Fig. View larger version 54K Fig. View larger version 59K Fig. Mixing length theory for cross-slope eddy transfer. View larger version 38K Fig. Discussion and conclusions. Caveats and outlook for further research. December Share this Article Share.

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Burke, A. Stewart, J. Adkins, R. Ferrari, M. Jansen, and A. Thompson, : The glacial mid-depth radiocarbon bulge and its implications for the overturning circulation. Chavanne, C. Heywood, K. Nicholls, and I. Fer, : Observations of the Antarctic slope undercurrent in the southeastern Weddell Sea. Favier, L. Ferrari, R. Nikurashin, : Suppression of eddy diffusivity across jets in the Southern Ocean.

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Karl Polanyi and the formation of this generation's new Left

From the journal: RSC Advances. Kinetic study of hydroxyl radical formation in a continuous hydroxyl generation system. This article is Open Access. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Article type: Paper.

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  7. DOI: Kinetic study of hydroxyl radical formation in a continuous hydroxyl generation system X. Wang and L. Zhang, RSC Adv. Search articles by author Xin Wang. Long Zhang. Currently, Lutheran campus ministries are present on state and private campuses across the nation with over congregations cooperating in these efforts.

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