Longitudinal and vertical CO2 gradients over the subtropical/subantarctic oceanic sink

The North-South gradient of atmospheric CO2 is commonly used to infer the latitudinal distribution of sources and sinks at the earth surface. Here we analyze the East-West and vertical gradients occurring at regional scale over the subtropical/subantarctic ocean (around 30 degrees S-45 degrees S), which is known to be the major sink of the southern hemisphere. Using French and Australian inter calibrated datasets, we find a significant depletion of atmospheric CO2 near Tasmania (Cape Grim, 40 degrees 41'S, 144 degrees 41'E, CGO) compared to the open Indian Ocean (Amsterdam Is., 37 degrees 48'S, 77 degrees 32'E, AMS). This depletion was about -0.4 +/- 0.2 ppmv for the 1988-1992 period. For the same period, CGO values were also depleted by -0.85 +/- 0.25 ppmv and -1.1 +/- 0.4 ppmv relative to the mid- and high-troposphere. Using a 3-D atmospheric transport model based on meteorological analysis, and a diagnostic CO2 flux scenario updated for the year 1990, we investigate the respective role of industrial, oceanic and biospheric fluxes. The main component which can explain such longitudinal and vertical oceanic sink appears to be the subtropical/subantarctic oceanic sink and its regional patterns. Using the oceanographic datasets in the Atlantic, Pacific, and West Indian oceans, we can reconstruct more than half of vertical gradient observed over CGO, but not the CGO depletion relative to AMS. Finally, we discuss the hypothesis of an extra oceanic CO2 sink south of the Australian mainland.