Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

Aoki, Shigeru; Yamazaki, Kenta; Hirano, Daisuke; Katsumata, Katsuro; Shimada, Koji; Kitade, Yujiro; Sasaki, Hidetada; Murase, Hiroto

doi:10.1038/s41598-020-71290-6

Cited by 35 publications

(35 citation statements)

References 33 publications

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“…Nevertheless from 2010 to 2018, AABW regained its salinity (Figure 2B). This is consistent with the observations by Aoki et al (2020b) who reported reversal of freshening of AABW during 2018-19 in comparison with the previous decades. The observed freshening is pronounced at 80 • E, in contrast to the increase in salinity to the east of 115 • E (Aoki et al, 2020b).…”

Section: Observations On Freshening and Warming Of Antarctic Bottom Watersupporting

confidence: 93%

“…This is consistent with the observations by Aoki et al (2020b) who reported reversal of freshening of AABW during 2018-19 in comparison with the previous decades. The observed freshening is pronounced at 80 • E, in contrast to the increase in salinity to the east of 115 • E (Aoki et al, 2020b). Furthermore from 2018 to 2020, the AABW lost its salinity by ∼0.002 and became warmer by ∼0.04 • C suggesting re-freshening and warming from 2018 to 2020, though this freshening was not as intense as that in 2010.…”

Section: Observations On Freshening and Warming Of Antarctic Bottom Watersupporting

confidence: 93%

“…Enhanced advection of CDW toward the ice shelf also leads to the decline of the shelf water density and subsequently influences the formation of AABW (Guo et al, 2019). The modifications of the AABW in the AAB in a decadal time scale showed asymmetrical variations (Aoki et al, 2020b).…”

Section: Introductionmentioning

confidence: 99%

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Recent Freshening, Warming, and Contraction of the Antarctic Bottom Water in the Indian Sector of the Southern Ocean

et al. 2021

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High saline and cold Antarctic Bottom Water (AABW) forms around the continental margin of Antarctica that ventilates into the global ocean. To study the recent changes in AABW, we have analyzed the in situ observations collected from Indian Ocean expeditions to the Southern Ocean during 2010, 2011, 2017, 2018, and 2020. A comprehensive analysis of these observations indicated recent freshening, warming, and contraction in the layer thickness of the AABW. Even though the AABW depicted inter-annual variability, it changed to moderately fresher and lighter water mass at the end of the recent decade. The characteristics of AABW exhibited a contraction in its layer thickness (∼50–120 m) during recent years. The water mass showed its freshening (∼0.002) and warming (∼0.04°C) tendency from 2018 to 2020. The recent warming (∼0.3°C) of Circumpolar Deep Water (CDW) near the Prydz Bay suggests enhanced melting of ice shelves. It is hypothesized that the combined influences of onshore intrusion of warm CDW, upper ocean warming, sea ice decline, wind forcing, polynya, and calving events possibly caused the freshening and reduction in the thickness of AABW. The continued changes in the ocean-atmospheric environmental conditions and the subsequent changes in the bottom water characteristics likely influence the global climate, overturning circulation, and the biogeochemical cycle.

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Section: Observations On Freshening and Warming Of Antarctic Bottom Watersupporting

confidence: 93%

Section: Observations On Freshening and Warming Of Antarctic Bottom Watersupporting

confidence: 93%

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Recent Freshening, Warming, and Contraction of the Antarctic Bottom Water in the Indian Sector of the Southern Ocean

et al. 2021

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“…A relatively small temperature increase in the water transformation term (up to 0.1°C) is observed in a few sections (e.g., 150°E) below the 28.1 kg m −3 isopycnal, where the upper overturning circulation is unlikely to affect the temperature. This change may be related to a modified mixing ratio of the locally produced AABW (22,32) and salinification of the ambient Ross Sea Bottom Water (34). By the five-section average (Table 1), the warming of Upper CDW associated with the poleward shift of the SB (0.31°C) is about 70% (0.21°C) due to the frontal shift term and about 30% (0.10°C) due to the water transformation term.…”

Section: Isopycnal Changes Due To Frontal Shifts and Enhanced Overturning Circulationmentioning

confidence: 99%

Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica

et al. 2021

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The southern boundary (SB) of the Antarctic Circumpolar Current, the southernmost extent of the upper overturning circulation, regulates the Antarctic thermal conditions. The SB’s behavior remains unconstrained because it does not have a clear surface signature. Revisited hydrographic data from off East Antarctica indicate full-depth warming from 1996 to 2019, concurrent with an extensive poleward shift of the SB subsurface isotherms (>50 km), which is most prominent at 120°E off the Sabrina Coast. The SB shift is attributable to enhanced upper overturning circulation and a depth-independent frontal shift, generally accounting for 30 and 70%, respectively. Thirty years of oceanographic data corroborate the overall and localized poleward shifts that are likely controlled by continental slope topography. Numerical experiments successfully reproduce this locality and demonstrate its sensitivity to mesoscale processes and wind forcing. The poleward SB shift under intensified westerlies potentially induces multidecadal warming of Antarctic shelf water.

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“…AABW is the densest water mass found globally. Made of cold and saline water, it occupies a large portion of the World Ocean's sea floor (Johnson, that the deep Southern Ocean has been warming and freshening over the past decades, in particular in the East Antarctic basins (Aoki et al, 2005;Rintoul, 2007), while Aoki et al (2020) revealed a reversal of the trend after 2010. As modifications of AABW are expected to have far reaching consequences, such changes illustrate the need for trustworthy projections of future polar climate.…”

Section: Introductionmentioning

confidence: 99%

Influence of ocean tides and ice shelves on ocean–ice interactions and dense shelf water formation in the D’Urville Sea, Antarctica

et al. 2021

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The D'Urville Sea, East Antarctica, is a major source of Dense Shelf Water (DSW), a precursor of Antarctic Bottom Water (AABW). AABW is a key water mass involved in the worldwide ocean circulation and longterm climate variability. The properties of AABW in global climate models suffer from several biases, making climate projections uncertain. These models are potentially omitting or misrepresenting important mechanisms involved in the formation of DSW, such as tides and ocean-ice shelf interactions. Recent studies pointed out that tides and ice shelves significantly influence the coastal seas of Antarctica, where AABW originates from. Yet, the implications of these two processes in the formation and evolution of DSW are poorly understood, in particular in the D'Urville Sea. Using a series of NEMO-LIM numerical simulations, we assess the sensitivity of dense water formation in the D'Urville Sea to the representation of tides and ocean-ice shelf interactions during the years 2010-2015. We show that the ice shelves off Adélie Land are highly sensitive to tidal forcing, with a significant basal melt increase in the presence of tides. Ice shelf basal melt freshens and cools the ocean over significant portions of the coastal seas at the depth of the ice shelf draft. An opposite warming and increase in salinity are found in the upper layers. The influence of ice shelf basal melt on the ocean is largely increased in the presence of tides. However, the production of sea ice is found to be mostly unaffected by these two processes. Water mass transport out of polynyas and ice shelf cavities are then investigated, together with their sensitivity to tides and ocean-ice shelf interactions. Ice shelf basal melt impacts the volume of dense waters in two ways: (1) Dense Shelf Water and Modified Shelf Water are consumed to form water masses of intermediate density inside the ice shelf cavities, and (2) the freshening of the ocean subsurface makes its transformation into dense water by sea ice formation more difficult. These results suggest that ice shelf basal melt variability can explain part of the observed changes of dense water properties, and may also affect the production of dense water in a future climate.

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Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

Cited by 35 publications

References 33 publications

Recent Freshening, Warming, and Contraction of the Antarctic Bottom Water in the Indian Sector of the Southern Ocean

Recent Freshening, Warming, and Contraction of the Antarctic Bottom Water in the Indian Sector of the Southern Ocean

Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica

Influence of ocean tides and ice shelves on ocean–ice interactions and dense shelf water formation in the D’Urville Sea, Antarctica

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