Prediction of the fate of radioactive material in the South Pacific Ocean using a global high-resolution ocean model

We investigate the release of radioactive contaminants from Moruroa Atoll in a global high-resolution off-line model. The spread of tracer is studied in a series of simulations with varying release depths and time-scales, and into ocean velocity fields corresponding to long-term annual mean, seasonal, and interannually varying scenarios. In the instantaneous surface release scenarios we find that the incorporation of a seasonal cycle greatly influences tracer advection, with maximum concentrations still found within the French Polynesia region after 10 years. In contrast, the maximum trace is located in the southeast Pacific when long-term annual mean fields are used. This emphasizes the importance of the seasonal cycle in models of pollution dispersion on large scales. We further find that during an El Niño/Southern Oscillation (ENSO) event reduced currents in the region of Moruroa Atoll result in increased concentrations of radioactive material in French Polynesia, as direct flushing from the source is reduced. In terms of the sensitivity to tracer release time-rates, we find that a gradual input results in maximum concentrations in the near vicinity of French Polynesia. This contrasts the instantaneous-release scenarios, which see maximum concentrations and tracer spread across much of the South Pacific Ocean. For example, in as little as seven years radioactive contamination can reach the east coast of Australia diluted by only a factor of 1000 of the initial concentration. A comparison of results is made with previous studies. Overall, we find much higher concentrations of radionuclides in the South Pacific than has previously been predicted using coarser-resolution models.