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Antarctic Bottom Water Production by Intense Sea-Ice Formation in the Cape Darnley Polynya

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Antarctic Bottom Water Production by Intense Sea-Ice Formation in the Cape Darnley Polynya ARTICLES PUBLISHED ONLINE: 24 FEBRUARY 2013 | DOI: 10.1038/NGEO1738 Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya Kay I. Ohshima1*(, Yasushi Fukamachi1(, Guy D. Williams2(, Sohey Nihashi3, Fabien Roquet4, Yujiro Kitade5, Takeshi Tamura6, Daisuke Hirano5, Laura Herraiz-Borreguero2, Iain Field7, Mark Hindell8, Shigeru Aoki1 and Masaaki Wakatsuchi1 The formation of Antarctic Bottom Water—the cold, dense water that occupies the abyssal layer of the global ocean—is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65◦–69◦ E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0:3–0:7×106 m3 s−1 of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic Bottom Water. The transformation of this water mass, which we term Cape Darnley Bottom Water, accounts for 6–13% of the circumpolar total. ntarctic bottom water (AABW) is the cold, dense water with its large continental embayment and the presence of the in the abyssal layer, accounting for 30–40% of the global Amery Ice Shelf resembling the Weddell Sea and Ross Sea AABW Aocean mass1. AABW production is a major contributor to regions. However, the results from several ship-based studies21 were the global overturning circulation1–3 and represents an important inconclusive and the export of DSW and its downslope transport sink for heat and possibly CO2 (ref. 4). AABW originates as were never observed from this region. dense shelf water (DSW), which forms on the continental shelf Most recently, satellite-derived estimates of sea-ice production by regionally varying combinations of brine rejection from sea-ice suggest that the Cape Darnley polynya (CDP), located northwest growth and ocean/ice-shelf interactions. With sufficient negative of the Amery Ice Shelf (Fig. 1), has the second highest ice buoyancy and an export pathway across the shelf break, the production around Antarctica after the Ross Sea Polynya25. This DSW can mix down the continental slope with ambient water satellite analysis provided the motivation for an extensive Japanese masses to produce AABW (ref. 5). At present, AABW production mooring programme conducted in 2008–2009 as part of the is recognized in three main regions: the Weddell Sea6–9, the International Polar Year, to prove the production of AABW Ross Sea10–12 and off the Adélie Coast13–15 (Fig. 1, inset map). from the CDP. Here we present the successful results of the In the Weddell and Ross seas, large continental embayments moorings and observations of new AABW on the continental associated with major continental ice shelves were considered slope off the CDP. Furthermore, we confirm that the CDP is the necessary mechanisms to generate sufficient negative buoyancy DSW source for this AABW based on instrumented seal data, in local DSW for the production of AABW (refs 9,10). This which has recently become an important source of hydrographic paradigm was broken when Adélie Land Bottom Water was profiles in logistically challenging regions/seasons around the directly linked to coastal polynyas, thin ice areas of enhanced Antarctic margin26–28. Finally, the contribution of this AABW to sea-ice production and associated brine rejection. Although total AABW is discussed. many polynya regions exist, particularly in East Antarctica, the storage capacity of the continental shelf in this region was Intense sea-ice production in the polynya proposed as the reason this seemed to be the only polynya A new high-resolution satellite data set is used here to enhance region capable of forming sufficiently saline DSW for producing understanding of the CDP from previous studies25,29,30. A typical AABW (refs 14,15). synthetic aperture radar image of the CDP (Fig. 1) clearly reveals Another independent variety of AABW, previously identified the extent of the CDP (>104 km2), in which the newly formed sea in the Weddell–Enderby Basin (Fig. 1, inset map) from offshore ice appears as white streaks (high radar backscatter). The annual properties detailed in hydrographic and tracer studies16–21, has ice-production estimate from the Advanced Microwave Scanning presented a puzzle in terms of its existence and DSW source. The Radiometer-EOS (AMSR-E) data (Supplementary Section S1) Prydz Bay region (71◦–80◦ E) seemed the most likely candidate22–24, details the extremely high ice production of 195 ± 71 km3, with 1Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan, 2Antarctic Climate and Ecosystem Cooperative Research Centre, University of Tasmania, Hobart 7001, Australia, 3Tomakomai National College of Technology, Tomakomai 059-1275, Japan, 4Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden, 5Tokyo University of Marine Science and Technology, Tokyo 108-8477, Japan, 6National Institute of Polar Research, Tachikawa 190-8518, Japan, 7Graduate School of the Environment, Macquarie University, 2109, Australia, 8Institute for Marine and Antarctic Studies, University of Tasmania, 7001, Australia. (These authors contributed equally to this work. *e-mail: [email protected]. NATURE GEOSCIENCE j VOL 6 j MARCH 2013 j www.nature.com/naturegeoscience 235 © 2013 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE GEOSCIENCE DOI: 10.1038/NGEO1738 Wild Canyon 3,000 ¬20 ¬15 ¬10 ¬5 (dB) B1 M4 1,000 0.1 m s¬1 500 67° S 50 km 3,000 Prydz Daly Canyon 2,500 M3 Bay 66° S 2,500 B5 M1 M2 Weddell¬Enderby Basin Weddell Sea 68° S B. B. Amery Ice Shelf N. B. Antarctica Cape Darnley Ross Sea Antarctica Adélie Coast 66 oE 68 oE 135° W 135° E64° E66° E68° E70° E Figure 1 j Intense sea-ice production in the CDP, revealed from satellite data. The background ENVISAT ASAR image is from 7 August 2008. The black–white graded scale bar in the top left indicates the backscatter coefficient. Annual sea-ice production in 2008, estimated from the AMSR-E data, is indicated by red (>10 m yr−1) and pink (>5 m yr−1) contours. Fast ice areas, as detected by the AMSR-E algorithm, are highlighted in green. The bathymetry is indicated by blue contours. Mooring locations are indicated by orange symbols, with the mean velocity at the bottom and top of the mooring shown as orange and yellow vectors, respectively. CTD station lines A, B and C are denoted by squares, circles and crosses, respectively. The Nielsen (N.B.) and Burton (B.B.) basins are as shown. The inset map shows the location of the study area (red rectangle). a 1.5 ) 3 (km Ice prod. 0.0 08/F MAM J J A S OND09/J b 0.0 C) ° ( θ M3 (bottom depth: 2,608 m) ¬1.5 08/F MAM J J A S OND09/J c 34.7 28.5 γ n Salinity 34.6 28.3 08/F MAM J J A S OND09/J d 0.5 ) ¬1 (m s Velocity 0.0 08/F MAM J J A S OND09/J e 0.15 500 Layer (m) ) ¬1 (3) (Sv km Transport 0.00 0 08/F MAM J J A S OND09/J Month (2008¬09) Figure 2 j Observations of new AABW production in Wild Canyon offshore from the CDP. a, Daily sea-ice production in the CDP estimated from AMSR-E data and a heat-flux calculation. b–e, M3 mooring time-series of potential temperature (θ) at depths of 26 m (blue) and 224 m (red) from the bottom (b), salinity (orange) and neutral density (γ n, green) at 26 m from the bottom (c), the velocity component of the mean (dominant) flow direction at 20 m (blue) and 226 m (red) from the bottom (d) and the estimated layer thickness (red, 15-day running mean) and volume transport per unit width (light blue, daily mean; dark blue, 15-day running mean) of new AABW (e). The value in parentheses on the left axis shows the volume transport (Sv D 106 m3 s−1) for an assumed current width of 20 km in e. 236 NATURE GEOSCIENCE j VOL 6 j MARCH 2013 j www.nature.com/naturegeoscience © 2013 Macmillan Publishers Limited. All rights reserved. NATURE GEOSCIENCE DOI: 10.1038/NGEO1738 ARTICLES a ° 3,000 mean flow was directed downslope, as determined by multi- 66 S 2,500 00 M1 3,0 2,000 2,500 M4 2,500 1,000 beam bathymetry measurements during deployment, and had M3 2,500 2,0001,500 500 1,500 0 M2 2,000 1,00 prominent bottom intensification (Fig. 1). In May, two months 500 67° S 200 after the onset of active sea-ice production (Fig. 2a), a colder, 500 less saline, and denser signal abruptly appeared at all instruments 0 200 500 51,0000 0 50 (up to 224 m from the bottom) and became dominant after June 500 (Fig. 2b,c). The current speeds across this layer intensified in 68° S 200 conjunction with the cold and dense signal (Fig. 2d). Thereafter, −1 500 downslope current speeds of up to 0:5 m s fluctuated coherently with these water properties.
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