Abstract. The AMMA (African Monsoon Multidisciplinary Analysis) program is dedicated to providing a better understanding of the West African monsoon and its influence on the physical, chemical and biological environment regionally and globally, as well as relating variability of this monsoon system to issues of health, water resources, food security and demography for West African nations. Within this framework, an intensive field campaign took place during the summer of 2006 to better document specific processes and weather systems at various key stages of this monsoon season. This campaign was embedded within a longer observation period that documented the annual cycle of surface and atmospheric conditions between 2005 and 2007. The present paper provides a large and regional scale overview of the 2006 summer monsoon season, that includes consideration of of the convective activity, mean atmospheric circuCorrespondence to: S. Janicot (serge.janicot@locean-ipsl.upmc.fr) lation and synoptic/intraseasonal weather systems, oceanic and land surface conditions, continental hydrology, dust concentration and ozone distribution. The 2006 African summer monsoon was a near-normal rainy season except for a large-scale rainfall excess north of 15 • N. This monsoon season was also characterized by a 10-day delayed onset compared to climatology, with convection becoming developed only after 10 July. This onset delay impacted the continental hydrology, soil moisture and vegetation dynamics as well as dust emission. More details of some less-well-known atmospheric features in the African monsoon at intraseasonal and synoptic scales are provided in order to promote future research in these areas.

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Aim  Amphibians are a model group for studies of the biogeographical origins of salt‐intolerant taxa on oceanic islands. We used the Gulf of Guinea islands to explore the biogeographical origins of island endemism of one species of frog, and used this to gain insights into potential colonization mechanisms. Location  São Tomé and Príncipe, two of the four major islands in the Gulf of Guinea, West Africa, are truly oceanic and have an exceptionally high biodiversity. Methods  Mitochondrial DNA is used to test the endemic status of a frog from São Tomé and compare it with congeneric taxa from tropical Africa. Existing data on surface currents, surface salinity, atmospheric circulation and bird migration in the Gulf of Guinea are summarized to address hypotheses concerning colonization mechanisms. Results  The endemic status of Ptychadena newtoni (Bocage) is supported here by mitochondrial DNA sequences, and analysis of this and other molecular data indicates that an East African species close to Ptychadena mascareniensis (Duméril and Bibron) is its nearest relative. We refute the possibility that this population was anthropogenically introduced, in favour of a natural dispersal mechanism. Main conclusions  With six endemic frogs and one caecilian, the Gulf of Guinea islands harbour a diverse amphibian fauna. Five of these species appear to have their closest relatives in East Africa. Insufficient evidence exists for transportation by storms, birds or rafts alone. However, we propose a synergy of rafting, favourable surface currents and a reduction in salinity of surface waters. Catastrophic events, or wet periods in climatic history, could allow freshwater paths to open far enough to enable continental flora and fauna to reach these and other isolated oceanic islands.

Abstract. In the tropical oceans, where the temperature is roughly homogenous in the upper layer, vertical variations of salinity may be responsible for significant density stratifications. In this study on the western Tropical Atlantic Ocean, it is shown that fresh surface waters of the Amazon River discharge may induce a strong halocline in the 3-30 m depth range. This halocline induces a pycnocline that acts as a barrier for mixing between the surface and the subsurface waters. The fresh surface waters are generally associated with positive anomalies of the sea surface temperature. These positive anomalies cover a large part of the basin in boreal summer-fall, following the maximum Amazon River discharge, which may have a significant importance for the ocean-atmosphere exchanges.

[1] The total of 13 existing cross-equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top-to-bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the results is that, on the equator, a mean westward Equatorial Intermediate Current below the Equatorial Undercurrent exists.