A 10-YEAR TRAJECTORY FLOW CLIMATOLOGY FOR AMSTERDAM ISLAND, 1980-1989

This paper documents the meteorology and flow climatology of Amsterdam Island, a World Meteorological Organization baseline observatory loca ted in the southern Indian Ocean. The island is strongly influenced by two meteorological systems: the subtropical anticyclone and the weste rlies, known as the roaring forties. Westerly surface winds dominate g round-level flow on the island, which receives an annual average preci pitation of 1100 mm. A 10-year back air trajectory climatology at 850 hPa to the island showed the same predominant westerly-southwesterly f low (55% of the period). Special emphasis was placed on trajectories t hat had their origin on the African continent because this was the pre dominant nonmarine source of trace materials that could be transported to Amsterdam Island. This transport, most frequent in the winter and early spring, coincided with the winter peak of continental radon. A c omparison of trajectories and 12-h precipitation amounts on the island showed that most of the rain coincided with southwesterly flow, but m any large individual events were frequently associated with northerly and westerly flow regimes. Meteorological information about local cond itions and flow climatology is necessary to aid in the interpretation of atmospheric chemistry measurements made at global stations such as Amsterdam Island.