Oceanography of the Gulf of Aden : John Murray–Mabahiss Expedition 1933–1934 Revisited

Oceanography of the Gulf of Aden was the focus of several recent publications using STD casts, XBT and AXBT. The present paper is based on the unpublished data set of John Murray–Mabahiss Expedition to the Indian Ocean 1933–1934. In addition to its historical value, these bottle data have an exceptional significance. Unlike most cruises, the data were collected in two successive monsoon seasons and in cross sections between the African and Arabian coasts of the Gulf, in addition to the longitudinal section along the ship’s transit. This gave a rare glimpse of what the density and oxygen were in 1933/34. The paper describes the water masses and circulation in the southern Red Sea and Gulf of Aden before the Second World War, or in other words, the state of the art at that era, when oceanographic data were very scarce. The study reveals pronounced spatial and seasonal variability in the hydrographic structure and circulation pattern in late summer and winter monsoons. Upwelling and frontal zones, which are known to be characterized by a high biological productivity, are developed during the summer season. The deep high saline outflow from the Red Sea into the Gulf of Aden is stronger in winter.Several water masses are identified as: the Surface Water Mass (SW) of high salinity, the Gulf of Aden Subsurface Water Mass (GASSW) of intermediate minimum salinity, the Red Sea Water Mass (RSW) of intermediate maximum salinity, the Persian Gulf Water Mass (PGW) and the Deep Water Mass (DW). The Gulf of Aden subsurface water mass (GASSW) is poor in oxygen (<0.5 ml l−1). This subsurface water of anoxic oxygen concentration occurs at shallower depths (<20 m) in summer, particularly in the upwelling regions. The deoxygenating process and its impact on the ecosystem and communities, as well as the oceanic variability, are key elements in managing the living resources of the region.The conclusions reached from this work are in general agreement with those obtained from the more recent studies. The broad features of the oceanography of the region could have been made available to oceanographers 80 years ago, if the data and results of John Murray–Mabahiss Expedition (JMME) had been timely published following the return of the expedition.