On The Distribution of Bomb Carbon-14 in the Southern Ocean

In an earlier WOCE newsletter (Ribbe & Tomczak, 1995) we reported on the development of an off-line radiocarbon validated tracer model for the Southern Ocean based on the Fine Resolution Antarctic Model (FRAM). During the initial stages of the project computational experiment were carried out with the model for an idealised oceanic tracer. We intended to verify the physical mechanisms that operate in the model and are responsible for removing atmospheric tracer and surface water. The results of these experiments have recently been reported in a series of papers (Ribbe & Tomczak, 1996a, 1996b, 1996c) and some very early results were published in this newsletter previously. In this note we report on the first experiments to investigate the uptake of bomb carbon-14 within the model. The effect of convection has been quantified in one of our experiments shown here. The integrated bomb carbon-14 flux due to convection alone results in an oceanic uptake of approximately 15 per cent. We are presently investigating the sensitivity of the total uptake to effects such as changes in fluxes at the northern model boundary, the distribution of convection, and sea-ice coverage. Although the model covers the ocean area south of 24o S, the discussion in this note is limited to bomb carbon-14 in the Indian Ocean. This ocean has been the focus of our earlier communications, which dealt with the formation of Subantarctic Mode Water (SAMW) in the southeast Indian Ocean. Our intention with the model is to quantify the role of this process for bomb carbon-14 uptake. During the last few months we were able to analyse samples collected from the Great Australian Bight (GAB) for carbon-14 using, for the first time in Australia, the Accelerator Mass Spectrometry (AMS) technique (Ribbe et al., 1996). The model will eventually be applied to interpret the distribution of carbon14 within the Australian sector of the Southern Ocean where a radiocarbon sampling program is being carried out along WOCE Sections SR3 and SR4.