The bomb 14C transient in the Pacific Ocean

A modeling study of the bomb C-14 transient is presented for the Pacific Ocean. A primitive equation ocean circulation model has been configured for a high resolution domain that accounts for the Indonesian Throughflow (ITF). Four separate runs were performed: (1) seasonal forcing with 20 Sv of ITF transport, (2) seasonal forcing with 10 Sv of ITF transport, (3) seasonal forcing with no ITF transport, and (4) interannual forcing with 15 Sv of ITF transport. This study has two main objectives. First, it is intended to describe the time evolution of the bomb C-14 transient. This serves as a tool with which one can identify the physical processes controlling the evolving bomb C-14 distribution in the Pacific thermocline and thus provides an interpretive framework for the database of Delta(14)C measurements in the Pacific. Second, transient tracers are applied to the physical oceanographic problem of intergyre exchange. This is of importance in furthering our understanding of the potential role of the upper Pacific Ocean in climate variability. We use bomb C-14 as a dye tracer of intergyre exchange between the subtropical gyres and the equatorial upwelling regions of the equatorial Pacific. Observations show that while the atmospheric Delta(14)C signal peaked in the early to mid-1960s, the Delta(14)C levels in the surface water waters of the subtropical gyres peaked near 1970, and the Delta(14)C of surface waters in the equatorial Pacific continued to rise through the 1980s. It is shown that the model exhibits skill in representing the large-scale observed features observed for the bomb C-14 transient in the Pacific Ocean. The model successfully captures the basin-scale inventories of bomb C-14 in the tropics as well as in the extratropics of the North Pacific. For the equatorial Pacific this is attributed to the model's high meridional resolution. The discrepancies in the three-dimensional distribution of bomb C-14 between the model and data are discussed within the context of the dynamical controls on the Delta(14)C distribution of bomb C-14 in the Pacific.