Alimentation des eaux équatoriales par la mer des Salomon : impacts climatiques et bio-géochimiques – SOLWARA

The oceanic circulation in the South Pacific redistributes waters from the large subtropical gyre toward the equator through a subtle journey, mostly suggested by model results and meridional gradients of trace elements. These waters are first transported from the central South Pacific, westward, in the South Equatorial Current. A part reaches the equator through a relatively direct northwestward route. Another part enters the Coral Sea, flows northward along the Australian coast, and crosses the Solomon Sea before joining the equator. At the equator, these waters form the most important supply to the Equatorial Undercurrent, then travel eastward before emerging in the cold water tongue of the central equatorial Pacific. It has been suggested that a change in thermohaline properties of these water masses, or of their transport to the equator, could influence the ENSO (El Niño Southern Oscillation) cycle at decadal scale and thus lead to basin wide climate feedbacks. This circulation is also associated with the equatorward supply of trace elements, including iron: these elements control biological activity in the equatorial cold water tongue, and are therefore a key factor in CO2 exchanges mechanisms between ocean and atmosphere. Such supply is believed to result from continental margins/ocean exchange, and quantification of that exchange is necessary to understand trace elements source mechanisms, which are linked to erosion and climate. The "direct" path toward the equator is taken into account by the large scale observations/modeling programs (satellites, Argo network, operational models,'), but circulation through the southwest Pacific requires specific studies, in particular to understand margin/ocean exchanges. In the Coral Sea, west of the dateline, the South Equatorial Current encounters major topographic obstacles, leading to the appearance of currents along islands or submerged chains, and subsequently the formation of zonal jets that join the western boundary current system along the Australian east coast, and then the Solomon Sea. The Solomon Sea, which is the unique pathway to the north, is a choke point where boundary currents become intense. Waters entering from its south exit to the north by 3 straits, and the transport partition among these straits may affect the response of the equatorial system to changes in the South Pacific climate. Because of their volcanic origins and the strong erosion pressure they suffer, the Solomon Islands and Papua New Guinea (PNG) are potentially important sources of trace elements such as iron. Preliminary experiments conducted by partners of this project over the past three years have provided an opportunity to study some aspects of the thermocline waters circulation in the Coral Sea and of their geochemical properties at the equator. That research has pinpointed the Solomon Sea as an important and virtually unexplored component in the climate and geochemical system. The oceanic characteristics of this region are not adequately described by observations nor completely rendered by models, and several key aspects remain unknown. The strong boundary currents, combined with the complicated topography and very narrow straits of the Salomon Sea make remote measurements (satellite) and numerical modeling difficult. We hereby propose a major research program dedicated to the Solomon Sea, involving in-situ observations, repeated monitoring and high-resolution modeling, in order to improve our knowledge of in ' and outgoing transports, sources of geochemical elements, and water masses transformations along their path toward the equator. This is a multidisciplinary approach, where emphasis will be placed on observations of margin/ocean exchanges, pathways, transport variability and mixing of water masses, as well as advanced high resolution numerical models. Such a combined analysis will provide a sound synthesis on the role of this region in the climate system and its variability. A depiction of waters transport and transformations from their subduction zones in the South to the equatorial area will provide information to improve our understanding of low-frequency modulations of ENSO and the supply of trace elements to the equatorial upwelling. Eventually, this research could lead to improve the oceanic component of climate models, lay the basis of a long-term monitoring network, and increase the skills of climate forecast.