Sources of deep and bottom water in the Weddell Sea and their variability
Final Report Abstract
In our project “Sources of deep and bottom water in the Weddell Sea and their variability” we participated in 4 Polarstern expeditions between Nov. 2004 and Oct. 2006 and analysed successfully 2000 chlorofluorocarbon (CFC) samples and 750 noble gas (3He, 4He, Ne) samples from deep ocean CTD casts. From our stable noble-gas and transient CFC observations from a drift station close to the Larsen Ice Shelf (ISPOL) we could determine the composition of recently formed Weddell Sea Bottom Water (WSBW) that was formed most likely by interaction with the nearby shelf ice. We estimated a formation rate of WSBW 1.1 ± 0.5 Sv (1 Sv = 1x106 m3/s) and a basal melt rate for Larsen C of 35 ± 19 Gt/year. These findings emphasize the role of the western Weddell Sea in deep and bottom water formation, particularly in view of changing environmental conditions due to climate variability, e.g., decay or enhanced melting of ice shelves, changing temperatures or circulation patterns of ambient water or sea ice conditions. Surprising high CFC values were observed in the southern part of a section across Drake Passage (DRAKE I). This recently ventilated water mass can only originate from the Weddell Sea. That finding is a clear evidence, that deep water formed in the Weddell Gyre’s margin escapes the Weddell basin and turns west against the Antarctic Circumpolar Current with an transport of roughly 2.2 Sv.. The location of the bottom CFC maximum and comparison with later observations on the same section and additional observations from the area of the South Scotia Ridge system (WWOS) indicate a complex temporal and spatial flow pattern of WSDW escaping the Weddell Basin to the west against the dominating ACC circulation. The helium isotope ratio distribution in Drake Passage (DRAKE I) reveals a strong inflow of distinct deep Pacific water mass. Comparison with later observations indicate a temporally (and spatially) highly variable flow of this water mass and its associated import of primordial helium into the South Atlantic. On the shelf and slope of northwestern Antarctic Peninsula a freshening of 0.1 was observed within 17 years. The causes of that freshening might be enhanced basal melting of the nearby ice shelves, southward retreat of the summer sea ice edge, or enhanced precipitation. Noble-gas observations indicate large fractions of glacial melt water that can only originate from the nearby Larsen C Ice Shelf. Hence, a contribution of enhanced basal melting can not be excluded. The extended time series of transient CFC observations along the Prime Meridian in the Southern Ocean allowed determining the parameters of transit time distributions of a water mass, which is formed at easterly sources and enters the Weddell Sea as a deep boundary following the Antarctic continental slope. From the TTDs we could estimate an inflow of 2.2 Sv of that deep water. Furthermore, we were able to assess the related concentrations of anthropogenic carbon and deduced a value if roughly 11 µmol/kg for the reference year 2006. We could demonstrate the ability of a more complex approach than other transient tracer methods to calculate anthropogenic carbon, which reproduces the tracer observations far better and which accounts for an exchange of the boundary current with an “older” inner reservoir and a time dependent tracer and carbon saturation source water in the source water.
Publications
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(2008). Evidence of deep and bottom water formation in the western Weddell Sea. Deep-Sea Research II, 55, pp. 1098-1116
Huhn, O., H. H. Hellmer, M. Rhein, W. Roether, C. Rodehacke, M. Schodlok, M. Schröder
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(2008): Chlorofluorocarbons, helium, and neon measured on water bottle samples during POLARSTERN cruise ANT-XXII/2 (ISPOL)
Huhn, O. et al.
