On the basis of the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO) data set of the monthly tritium concentration in precipitation, the temporal and spatial pattern of the tritium concentration in marine precipitation of the southern hemisphere has been reconstructed. A numerical model that couples the tritium concentrations in the atmosphere and in the Weddell Sea's surface layer by parameterizing the relevant flux and exchange mechanisms, as well as the surface layer dynamics, has been used to calculate the time history of tritium in near-surface waters of the Weddell Sea. The model also simulates the gas transfer of the chlorofluorocarbons CFC 11 and CFC 12 and of helium across the ocean-atmosphere interface. The model accounts for the pronounced seasonality of the climatological and oceanographic conditions of the Weddell Sea. The calculated input functions of tritium, CFC 11 and CFC 12 provide the surface boundary conditions for numerical studies of the distributions of these transient tracers in the sub-surface water masses of the Weddell Sea. ¿ 1998 American Geophysical Union