![]() ![]() The 1-D TTD, which is a solution of the 1-D advection–diffusion equation 28, is not applicable in the Southern Ocean where distinct water masses are mixing, especially when one endmember has essentially no tracers (e.g., ref. Other studies of CFC and SF 6 over the past couple decades have used the concept of Transit Time Distribution (TTD) to estimate the water mass age (e.g., ref. 18), their ratio age might be able to estimate the AABW age, especially if the CDW is essentially CFC and SF 6-free. As the pSF 6/pCFC ratio is still increasing in the atmosphere (e.g., ref. Although the partial pressure of SF 6 (pSF 6) is still increasing in the atmosphere, the pSF 6 age (as well as the pCFC age) cannot realistically estimate the mean age when waters of different ages mix. However, because the atmospheric pCFC and its ratio reached a maximum during the 1980s–2000s, after which their temporal changes are small 18, 19, 21, it is difficult to uniquely determine the age in the region off CD by applying these previous methods. The AABW found off CD should exhibit a broad range of spreading timescales, from a few years to decades due to inflows of AABW from different source regions. However, water mass ages and the transport timescale for CDBW have not been investigated. In the downstream Weddell Sea, an intrusion of high CFC water from the east suggested inflow of new AABW from the CD region 25, 26. In the region off CD, the CFC maxima in the bottom water suggest that the AABW formation occurs around 60°–70☎ 24. The pCFC ratio age is also used in a similar way as the pCFC age. The pCFC age is determined by comparing the pCFC of the water (defined as the observed tracer concentration divided by the solubility function 22, 23) with the atmospheric time history (e.g., ref. The age means the time elapsed since the water was in contact with the atmosphere. In the Weddell and Ross Seas, previous studies have estimated the timescale of water mass spread using the partial pressure of CFC (pCFC) age or its ratio (pCFC-11/pCFC-12) age (e.g., refs. As atmospheric time histories for these gases are well established 19, their concentrations in a water parcel can be used to estimate the year in which the water was last in contact with the atmosphere. In the interior of the ocean, their concentrations change only by mixing with other water masses, as they are chemically and biologically inert (e.g., ref. In the real ocean, concentrations in surface water are also affected by physical processes, such as the rate and degree of gas exchange, which are altered by cooling, warming, vertical mixing, as well sea ice cover in polar oceans. Their equilibrium concentrations in seawater are determined by their atmospheric concentrations and solubility at the surface. Although the regions in which each AABW type is produced and their transport pathways are roughly known, the timescale of AABW transport is not clearly understood.Īnthropogenic gases such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF 6) have been used as transient tracers to detect newly ventilated water masses and to quantitatively understand water mass spreading pathways and their timescales. From the west, AABW produced in the Weddell Sea (Weddell Sea Deep Water WSDW) is transported into the region by the Weddell Gyre 16, 17. From the east, warmer and more saline AABW (a mixture of Ross Sea Bottom Water and Adélie Land Bottom Water) flows into the region through the Princess Elizabeth Trough (PET) 3, 15. In addition to the CDBW, two other types of AABW are present in the region off CD. The CDBW mainly descends the canyons in a northwestward direction 10, 12, 14. The main source of this AABW (Cape Darnley Bottom Water CDBW) is DSW that formed in CD polynya 10, 12, with a contribution of DSW outflow from Prydz Bay 13. 10 found that AABW production off Cape Darnley (CD) occurs due to intense sea ice production 11. The Weddell and Ross Seas, and the Adélie Land coast are known as the three major regions of AABW production 6– 9. Thus, a quantitative assessment of the production and spread of AABW is critical for understanding global ocean circulation and climate. The production of AABW drives global thermohaline circulation, delivering oxygen and carbon to the global abyssal ocean (e.g., refs. AABW continues to mix with overlying and adjacent waters as it is advected. With strong buoyancy loss, the DSW flows down the continental slope and mixes with ambient Circumpolar Deep Water (CDW) to produce Antarctic Bottom Water (AABW) 1. On the Antarctic continental shelf, cold Dense Shelf Water (DSW) forms through cooling and brine rejection during ice formation in coastal polynyas.
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