Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya

Ohshima, Κay I.; Fukamachi, Yasushi; Williams, Guy D.; Nihashi, Sohey; Roquet, Fabien; Kitade, Yujiro; Tamura, Takeshi; Hirano, Daisuke; Herráiz‐Borreguero, Laura; Field, Iain C.; Hindell, Mark A.; Aoki, Shigeru; Wakatsuchi, Masaaki

doi:10.1038/ngeo1738

Cited by 293 publications

(371 citation statements)

References 41 publications

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“…So far, we have prescribed a transformation of NADW into AABW exclusively in the Atlantic sector of our box model, reflecting the predominance of AABW generated in the Weddell Sea. However, AABW forms at a number of sites around the Antarctic margins (Jacobs et al 1970;Aoki et al 2005;Ohshima et al 2013). Zonal exchange in the ACC, which is largely confined to layers n 5 3, 4, is more sensitive to changes in T AABW A than to T AABW P (Fig.…”

Section: September 2016 T H O M P S O N E T a Lmentioning

confidence: 99%

A Multibasin Residual-Mean Model for the Global Overturning Circulation

Thompson

Stewart

Bischoff

2016

Journal of Physical Oceanography

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The ocean's overturning circulation is inherently three-dimensional, yet modern quantitative estimates of the overturning typically represent the subsurface circulation as a two-dimensional, two-cell streamfunction that varies with latitude and depth only. This approach suppresses information about zonal mass and tracer transport. In this article, the authors extend earlier, zonally averaged overturning theory to explore the dynamics of a ''figure-eight'' circulation that cycles through multiple basins. A three-dimensional residual-mean model of the overturning circulation is derived and then simplified to a multibasin isopycnal box model to explore how stratification and diabatic water mass transformations in each basin depend on the basin widths and on deep and bottom-water formation in both hemispheres. The idealization to multiple, two-dimensional basins permits zonal mass transport along isopycnals in a Southern Ocean-like channel, while retaining the dynamical framework of residual-mean theory. The model qualitatively reproduces the deeper isopycnal surfaces in the Pacific Basin relative to the Atlantic. This supports a transfer of Antarctic Bottom Water from the Atlantic sector to the Pacific sector via the Southern Ocean, which subsequently upwells in the northern Pacific Basin. A solution for the full isopycnal structure in the Southern Ocean reproduces observed stratification differences between Atlantic and Pacific Basins and provides a scaling for the diffusive boundary layer in which the zonal mass transport occurs. These results are consistent with observational indications that North Atlantic Deep Water is preferentially transformed into Antarctic Bottom Water, which undermines the importance of an adiabatic, upper overturning cell in the modern ocean.

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Section: September 2016 T H O M P S O N E T a Lmentioning

confidence: 99%

A Multibasin Residual-Mean Model for the Global Overturning Circulation

Thompson

Stewart

Bischoff

2016

Journal of Physical Oceanography

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“…In the present work, we focus on sections of the ASF that admit AABW export, such as the Weddell and Ross Seas (Muench and Gordon 1995;Gordon 2009) and parts of East Antarctica (Ohshima et al 2013), in which we expect all of the physical processes outlined in the previous paragraph to be simultaneously active. We illustrate the configuration of interest in Fig.…”

Section: Introductionmentioning

confidence: 99%

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Stewart

Thompson

2016

Journal of Physical Oceanography

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Along various stretches of the Antarctic margins, dense Antarctic Bottom Water (AABW) escapes its formation sites and descends the continental slope. This export necessarily raises the isopycnals associated with lighter density classes over the continental slope, resulting in density surfaces that connect the near-freezing waters of the continental shelf to the much warmer circumpolar deep water (CDW) at middepth offshore. In this article, an eddy-resolving process model is used to explore the possibility that AABW export enhances shoreward heat transport by creating a pathway for CDW to access the continental shelf without doing any work against buoyancy forces. In the absence of a net alongshore pressure gradient, the shoreward CDW transport is effected entirely by mesoscale and submesoscale eddy transfer. Eddies are generated partly by instabilities at the pycnocline, sourcing their energy from the alongshore wind stress, but primarily by instabilities at the CDW-AABW interface, sourcing their energy from buoyancy loss on the continental shelf. This combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth CDW layer, sustaining a local maximum in the eddy kinetic energy over the slope and balancing the Coriolis force associated with the shoreward CDW transport. The resulting slope turbulence self-organizes into a series of alternating along-slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets. Cross-shore variations in the potential vorticity gradient cause the jets to drift continuously offshore, suggesting that fronts observed in regions of AABW down-slope flow may in fact be transient features.

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“…Estimates of the relative importance of these sites have evolved with time and are still subject to debate [Jacobs, 2004]. Additional sites like the Prydz Bay sector have been recently mentioned as potentially large contributors [Orsi, 2010;Ohshima et al, 2013]. The Australian Antarctic Basin (AAB) collects bottom water from two of these sources, the Ad elie Land Bottom Water (ALBW), considered as a local source formed on the shelf off the Ad elie-George V Land and probably the larger contributor [Rintoul, 1998;Orsi et al, 1999], and the Ross Sea Bottom Water (RSBW) which is advected from the Ross Sea to the east.…”

Section: Introductionmentioning

confidence: 99%

Dense shelf water production in the Adélie Depression, East Antarctica, 2004–2012: Impact of the Mertz Glacier calving

et al. 2014

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Summer repeated hydrographic surveys and 4 years of mooring observations are used to characterize for the first time the interannual variability of the bottom water in the Mertz Glacier Polynya (MGP) on the East Antarctic shelf (142 E-146 E). Until 2010, large interannual variability was observed in the summer bottom salinity with year-to-year changes reaching 0.12 in Commonwealth Bay, the region with the highest sea ice production. The summer variability was shown to be linked to the efficiency of the convection during the preceding winter. The recent freshening of the bottom waters subsequent to the Mertz Glacier calving was well beyond the range of the precalving interannual variability. Within 2 years after the event, the bottom water of the shelf became too light to possibly contribute to renewal of the Antarctic Bottom Water. Rough estimates of the freshwater budget of the Ad elie Depression indicate that the freshening necessary to compensate for net sea ice production in the MGP did not change drastically after the Mertz calving. The year-to-year salinity changes appeared to respond to the MGP activity. Yet, prior to the calving, the convective system in the polynya was also partly controlled by the late winter bottom salinity through a mechanism leading to a sequence of alternatively fresher and more saline bottom waters over the period [2007][2008][2009][2010]. Exceptional events like the Mertz calving seem to be able to switch over the system into a less stratified state where convection responds more directly to changes in the surface forcing. IntroductionAntarctic Bottom Water (AABW), the densest water mass of the world ocean, occupies the abyssal layer of the Southern Ocean as a relatively well-oxygenated, cold and fresh product, which spreads north into the Atlantic, Indian, and Pacific oceans. Surface water transformation on the continental margins around Antarctica and subsequent sinking of the dense shelf waters along the slope supply the bottom limb of the ocean thermohaline circulation. It has been argued that changes in the properties or formation rate of the AABW could affect the strength of the thermohaline circulation, and therefore the global climate [Stouffer et al., 2007;Purkey and Johnson, 2012].The production of AABW is considered to originate on the Antarctic shelf in coastal polynyas where nearfreezing water (the so-called High-Salinity Shelf Water, HSSW) is formed through ocean surface cooling and brine drainage from growing sea ice. Upon mixing with ambient waters on the shelf and subsequent sinking and entrainment of Circumpolar Deep Water down the continental slope, these waters eventually acquire final AABW properties when reaching the abyssal layer of the Southern Ocean. While this process is suspected to occur at many locations around Antarctica, observations have unambiguously identified some primary formation sites: the Weddell Sea, the Ross Sea [Carmack, 1977], and the Ad elie-George V Land [Gordon and Tchernia, 1972;Rintoul, 1998]. Estimates of the relative im...

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Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya

Cited by 293 publications

References 41 publications

A Multibasin Residual-Mean Model for the Global Overturning Circulation

A Multibasin Residual-Mean Model for the Global Overturning Circulation

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Dense shelf water production in the Adélie Depression, East Antarctica, 2004–2012: Impact of the Mertz Glacier calving

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