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Uptake and transport of CO2 in the Weddell Sea, Antarctica

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Citation:
Hoppema, M. (2007): Uptake and transport of CO2 in the Weddell Sea, Antarctica , "Polar Dynamics: Monitoring, understanding and prediction", 29-31 August 2007, Bergen, Norway. .
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Abstract:

Several ways of exchange and transport of CO2 within the subpolar cyclonic Weddell Gyre will be discussed. Different analyses come to different conclusions as to the sink function of the Weddell Sea region. Some budget studies found the region to be a significant sink, whereas the new climatology of Takahashi (in prep.) shows it to be about neutral with respect to CO2 exchange with the atmosphere. All analyses are based on measurements but also work with assumptions that may have considerable impact on the outcome of the analysis. Some aspects of these different approaches will be shown.Notwithstanding the open question whether the region is a present-day sink for atmospheric CO2 or not, the Weddell Gyre definitely is a sink for anthropogenic CO2. The reason for this is that in the divergent gyre, upwelling of CO2-charged intermediate water occurs, which gives the region a natural tendency for CO2 supersaturation. Thus, in the case that even if the present-day Weddell Sea is only neutral with respect to atmospheric exchange of CO2, it must have absorbed anthropogenic CO2. By using contemporary CO2 data it appears to be possible to get a view the changing source-sink function of the area during the transition from pre-industrial time to present.Uptake of anthropogenic CO2 takes place at the air-sea interface, so it is obvious that most anthropogenic CO2 resides in the oceanic surface layer. Knowing additionally that the Weddell Sea is one of the regions in the global ocean where ventilated deep and bottom waters are generated, by implication a significant route for sequestration of anthropogenic CO2 is suspected. There are several routes that come into question. First, anthropogenic CO2 charged surface water may move northwards and participate in deep-mixing events in the subantarctic zone where intermediate and mode waters are produced. Second, surface water may be involved in deep and bottom water production along the continental margins of the Weddell Sea. And third, anthropogenic CO2-rich surface water may be involved in open-ocean deep convection, like during the time of the great Weddell Polynya in the mid-1970s. The second and third routes will be discussed to some extent.Finally, non- or slightly ventilated waters of the Weddell Sea play a pivotal role in the carbon cycle as well. A mechanism is active that transfers natural CO2 from the subsurface central Weddell Sea to the abyssal global ocean. It is estimated that it has global significance for the lower limb of the global oceanic carbon cycle. Although this is a natural process, changes invoked by anthropogenic activity (e.g. changed upwelling activity) may feedback on it, thus modifying the oceanic and atmospheric carbon content.

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