Abstract

¹³⁷Cs originating from global fallout is transported into the ocean interior by advection and diffusion, and the ¹³⁷Cs concentration is reduced by radioactive decay. ¹³⁷Cs concentrations in the global ocean can be simulated by global integration of the coarse-resolution Parallel Ocean Program to ... Show more¹³⁷Cs originating from global fallout is transported into the ocean interior by advection and diffusion, and the ¹³⁷Cs concentration is reduced by radioactive decay. ¹³⁷Cs concentrations in the global ocean can be simulated by global integration of the coarse-resolution Parallel Ocean Program to understand the mechanism of material transport in the ocean. We investigated the transport mechanism of ¹³⁷Cs to the Southern Hemisphere using an ocean general circulation model (OGCM) and compared the simulated results with observations of ¹³⁷Cs concentrations in the Southern Hemisphere. ¹³⁷Cs was deposited on the ocean surface mainly as global fallout originating from atmospheric nuclear weapons testing since 1945, and the global distribution of cumulative ¹³⁷Cs deposition has been reconstructed from global measurements of ¹³⁷Cs in rain, seawater, and soil. We estimated the global distribution of ¹³⁷Cs deposition from 1945 to 2003 using these distribution data, ¹³⁷Cs deposition data observed at the Meteorological Research Institute, Tsukuba, Japan, from 1958 to 2003, and ¹³⁷Cs deposition data for 1945 - 1957 estimated from ice-core data. We compared the simulated results with ¹³⁷Cs sections from the South Pacific, Indian, and South Atlantic Oceans obtained during the BEAGLE2003 cruise in 2003. The simulated ¹³⁷Cs sections were in good agreement with the observations, except for the effects of mesoscale eddies, which not be simulated by the model because of its coarse resolution. OGCMs can simulate the general pattern of ¹³⁷Cs distribution in the world’s oceans and improve our understanding of the transport mechanism leading to those ¹³⁷Cs distributions on a time scale of several decades. The model simulation results suggest that the ¹³⁷Cs deposited in the North Pacific advected to the South Pacific and Indian Ocean, and then to the South Atlantic over about four decades. The North Pacific is thus an important source area of ¹³⁷Cs to the Southern Hemisphere. Show less