Transport of continental air to the subantarctic Indian Ocean

The occurrence of high Rn-222 episodes (radonic storms) observed at three islands (Crozet, Kerguelen, and Amsterdam) in the subantarctic Indian Ocean is simulated using a three-dimensional chemical tracer model. The chemical tracer model is described and the simulated time series of Rn-222 concentrations at the three islands are compared to observations. The origin, seasonal frequencies, and periodicities of the storms are examined. It is found that the storms are due to fast boundary layer advection of air from South Africa, made possible by the conjunction of a subtropical high SE of Madagascar and a midlatitudes low off the southern tip of Africa. The implications of the results for the transport of continental air to the subantarctic Indian Ocean are discussed.