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An estimate of the upwelling rate in the equatorial Atlantic based on the distribution of bomb radiocarbon and quasi-geostrophic dynamics


  • Carl Wunsch


Estimates of the near-surface Upwelling in the tropical Atlantic ocean (15°S to 15°N), made by Broecker et al. (1978) from a radiocarbon box model and by Wunsch (1984) from a quasi-geostrophic inversion, differ considerably (in the former case the range was estimated as about 17–20 Sv, while the latter obtained an absolute maximum of about 10 Sv). To understand these two apparently contradictory results, Δ14C constraints were added to the inverse model with realistic estimates of the observational errors in all fields (including the radiocarbon). It is found that the radiocarbon constraints are consistent with the quasi-geostrophic model and with an upwelling estimated as 7–10 Sv. The near-surface circulation in the tropical Atlantic is dominated by a northward flux of warm water across the equator necessary to feed the heat losses at high northern latitudes. The Δ14C balance is maintained by the advection of southern hemisphere waters of comparatively low radiocarbon concentration into the tropical zone by upwelling from below of very low radiocarbon water and by its removal to the north, also by advection. The major uncertainty in the radiocarbon calculations derives from the sparcity of sampling so that very great extrapolations are required to estimate the zonal mean Δ14C concentrations and the time histories between the prebomb period and 1972.

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