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A high resolution simulation of the bomb radiocarbon transient in the Pacific Ocean is being conducted using the primitive equation ocean circulation model of Gent and Cane. The results are being validated against WOCE, GEOSECS, and NORPAX data, as well as against bimonthly radiocarbon measurements on corals from Galapagos, Christmas Island, and the Solomon Islands in the Equatorial Pacific. The modeling study has two objectives. The first involves the identification of the physical processes controlling the distribution and variability of radiocarbon on seasonal to interannual timescales in the surface waters of the Pacific Ocean. The second objective is the identification of the pathways and timescales responsible for the exchange of thermocline waters between the subtropical gyres of the Pacific and the equatorial Pacific.
The second objective will have bearing on the way in which the ocean redistributes heat, and therefore implications for climate variability on decadal and longer timescales. Changing climate conditions in the mid latitudes can cause changes in tropical SSTs via thermocline adjustment, since the thermocline controls the temperature of the water which upwells along the equator. If this process of thermocline adjustment were to operate, modeling studies conducted with the coupled models of Zebiak and Cane have shown that it would change the decadal variability of ENSO. Of all the parameter sensitivity studies which have been conducted with the Zebiak and Cane model, changing the temperature structure of the thermocline has the strongest influence on model behavior.
The radiocarbon simulation uses a pan-Pacific domain in order to account for the decadal scale process of equatorial thermocline ventilation. The tracer integration begins in 1955, and the physical circulation model is forced with interannually varying winds. Both the atmospheric radiocarbon time series of Peng and the windspeed-dependent gas exchange formulation of Wanninkhopf are used to represent fluxes at the sea surface.
The radiocarbon simulation builds on a previous study of radiocarbon variability in the Equatorial Pacific (Rodgers et al., 1996). That work was motivated by a desire to understand the dynamical processes responsible for the large seasonal radiocarbon variability (30 to 50 per mil for the 1970s and early 1980s) measured in corals from Guam, Galapagos, Fanning, and Canton. In that study, it was shown that seasonally varying lateral advection in conjunction with the large zonal gradients in sea surface radiocarbon associated with the bomb transient can account for the large seasonal radiocarbon variability in the Equatorial Pacific. Upwelling and gas exchange were shown to play essential supporting roles in establishing the basin-scale gradients of sea surface radiocarbon.
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