On the total carbon dioxide and oxygen signature of the Circumpolar Deep Water in the Weddell Gyre

efahrbach [ at ] awi-bremerhaven.de


Sections from two 'Polarstern' cruises in austral winter 1992 and summer 1992/93 were used to track the course of Circumpolar Deep Water (CDW) in the Weddell Sea. Total inorganic carbon (TCO2) is a valuable tracer for that water mass because it allows to distinguish features that cannot be seen in the distributions of temperature and salinity. Upon entrance into the eastern Weddell Gyre, a shallow maximum in TCO2 at about 200 m (likewise a temperature maximum and oxygen minimum) indicates the depth level to which vertical mixing with Winter Water penetrates the CDW layer in the Weddell Gyre. The lower boundary of this CDW layer, which is not apparent in the temperature and salinity profiles, is a TCO2 maximum at 1000-1500 m (sq27.835), originating through the superposition of the recently advected CDW from the Antarctic Circumpolar Current and the Weddell Sea Deep Water (WSDW) with opposite vertical gradients. A coinciding, weak oxygen minimum is only present on the prime meridian and is probably caused bydifferent biological histories of the CDW and the WSDW underneath. Using this TCO2 maximum, the newly injected CDW can be traced as a well-defined band around the Weddell Sea up to south of the South Orkney plateau. Downstream in the northern limb of the Weddell Gyre at the prime meridian its trace has disappeared.The band of 'new' CDW, as part of the boundary current, envelopes a central area where currents are significantly smaller. In this interior a special modification of CDW, the Central Intermediate Water (CIW), can be distinguished. This water mass is characterised by a secondary TCO2 maximum and oxygen minimum, with no comparable structures in the temperature and salinitiy fields. CIW is enriched in CO2 compared to the CDW that enters the Weddell Gyre and is most pronounced in the western part of the Weddell basin. Data in the west suggest that the CIW is related to the lower part of the 'new'-CDW layer. Thus, the central Weddell basin is replenished from the west side rather than from the east. Within the interior the CDW is further modified by mixing with the WSDW below and by entrainment into the surface layer above. Part of it is also advected out of the Weddell Sea into the bottom layer of the ACC, conveying water that has been biologically enriched in CO2 to the abyssal oceans.

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Hoppema, M. , Fahrbach, E. and Schröder, M. (1997): On the total carbon dioxide and oxygen signature of the Circumpolar Deep Water in the Weddell Gyre , Oceanologica Acta, 20 , pp. 783-798 .

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