Interhemispheric drift of radioactive debris and tropical circulation

Abstract

Following French nuclear tests on Mururoa Atoll in the south Pacific (test site, 21° S, 138° W) during the northern summers of 1966–68, a high level of surface-air radioactivity was measured at stations in both the hemispheres. In particular, the effect of the 1968 French test was detected in the northern hemisphere up to a latitude of about 50° N. Relevant data for a few stations are presented. Available meteorological data for 1968 are examined to determine the air currents by which radioactive debris from the tests carried out during this year drifted to the northern hemisphere. South Pacific data appear to rule out the possibility that the debris from the test site was transported to the equator and beyond exclusively by the lower-tropospheric trade wind circulation. On the basis of global tropospheric cirulation data presented it is suggested that the interhemispheric transport was effected by the trade wind circulation after the debris had been transported by the tropical westerlies to latitudes nearer the equator by lateral diffusion. Available data suggest that the lateral diffusion coefficient of the tropical westerlies of the southern hemisphere may be of the order of 106 m2 sec?1 or more. In the west Indian Ocean the trade winds near the equator have strong meridional components, hence when the debris arrives by way of the tropical westerlies it is rapidly transported across the equator by the trade wind circulation. The study appears to support qualitatively earlier work in respect of transequatorial fluxes of air in the lower troposphere in the west Indian Ocean during the summer monsoon season and suggest generally that large transequatorial fluxes should occur in such regions where the intertropical convergence zone (ITCZ) lies well away from the equator in the opposite hemisphere. Meridional motion associated with ITCZ may transport the debris equatorward or poleward depending upon its location within a hemisphere.

In case of poleward transport, the debris may arrive at a latitude from where the westerlies can spread it to higher latitudes.