Long-term observations of carbon, nutrients and oxygen in upper thermocline waters, such as subtropical and subpolar mode waters, have revealed substantial interannual to decadal variations. While part of this variability can be ascribed to internal ocean and ecosystem dynamics as well as large-scale climate phenomena (like ENSO, NAO or the PDO), we presently do not know to which extent this variability is influenced by anthropogenic climate change. As a first step to answer this detection question, the impact of natural variability on biogeochemical properties in thermocline waters must be understood and quantified. This permits us then to accurately describe the natural "noise" against which an anthropogenic change needs to be detected. Subtropical and subpolar mode waters may be ideally suited to look at this task since they tend to respond sensitively to climate variations, integrate short-time scale variations over time, and hence exhibit maximum signal to noise ratio. We investigate the role of mode water formation and spreading on interannual to decadal accumulation and release of nutrients and carbon by analyzing results from model runs with the Upper Ocean Model [Danabasoglu and McWilliams 2000] coupled to the ecosystem model of Moore et al. [2002]. We compare results from a run forced with NCEP reanalysis data for the period from 1948 to present with a climatological control run. To better isolate the mechanisms forcing these biogeochemical changes, we compare our results also to a set of experiments in which we manipulate the wind stress forcing and sea surface temperature fields of the model locally.

Ice cores are unique archives of past climatic and atmospheric conditions through the isotopic composition of the ice and the analysis of the air bubbles trapped. In 1999 Petit et al published the reconstruction of the Antarctic climate and atmospheric composition over the last 420 000 years from the Vostok ice core. This record covered the last four glacial inter glacial cycles back to the end of the marine interstadial 11 (MIS 11). It has revealed the close relationship between the atmospheric part of the carbon cycle and the climate. With CO₂ concentration oscillating between 180 and 280 ppmv during the last 4 climatic cycles. In a similar way the methane concentration followed closely temperature on glacial interglacial time scales, with millennial-scale structures during glacial times which appear out of phased with Antarctic temperature but, at least for the last glaciation, in phase with the Greenland rapid climatic oscillations, as revealed by the GISP and GRIP ice cores.