Abstract
The occurrence of high levels of atmospheric 222Rn (radonic storms) at 3 subantarctic islands in the Indian Ocean is simulated with a 3-d chemical tracer model (CTM) based on the meteorology of the GISS general circulation model (GCM). Radon-222 (half-life 3.8 days) is a sensitive tracer of continental air over the oceans. The CTM simulates well the observed intensities of the radonic storms at the islands, their seasonal frequency (highest in winter), and their periodicity (25—28 days). 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 (Mascarene High) and a mid-latitudes low off the southern tip of the African continent. Transit times of air from South Africa to the islands range from 1 to 5 days. The Mascarene high is a semi-permanent feature of the circulation from May to October, and is responsible for the seasonal frequency of the radonic storms. The low is transient but exhibits an oscillation of period 23—28 days which appears to be responsible for the observed periodicity of the storms. Transport to the subantarctic Indian Ocean is the principal mechanism for ventilation of South Africa in the CTM, but most of this transport takes place in the free troposphere following deep convection over the continent. The boundary layer advection mechanism associated with radonic storms accounts for only a small fraction of the total continental air exported to the subantarctic Indian Ocean.
