Project Summary

Model results and few observational studies suggest that South Pacific thermocline waters are transported from the subtropical gyre center in the westward flowing South Equatorial Current, towards the southwestern Pacific Ocean. This features a major circulation pathway that redistributes water from the subtropics to the equator. The transit in the Coral and Solomon Seas is potentially of great importance to tropical climate prediction because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Niño-Southern Oscillation cycle and thereby produce basin-scale climate feedbacks. Nevertheless, neither the oceanic nor atmospheric features are fully appreciated from observations or properly depicted by models in this region, and several key aspects of these climate-important components remain unknown. Preliminary experiments have been carried out by the LEGOS over the past three years to investigate specific aspects of the thermocline water circulation in the Coral Sea, and it is proposed here to open a new component in the Solomon Sea.

The Solomon Sea is both a choke point and a place of intense boundary current and possible water mass mixing for the thermocline waters heading towards the equatorial undercurrent and eventually emerging in the cold tongue. The partition between its 3 northern straits could be determinant to the water route to the equator with potential consequence on the equatorial response to changes in the southwest Pacific circulation. The “Solwara” project, which means “Sea” in Solomon Sea Creole, aims at better understanding the Solomon Sea inflow, outflow and circulation. Its specific objectives are 1) to measure, model and understand the transport to the equator over seasonal to interannual time scales; 2) to evaluate the different inflow and outflow components and associated variations; 3) to describe the complicated regional circulation as suggested by numerical models and 4) to evaluate the water mass transformation and mixing within the Solomon Sea.

The proposed study is based on combination of modeling and observational approach. Models need to be examined and improved over this region because of complicated flows with boundary currents that, combined with unresolved topographic features, narrow straits and sharp shelves, generates large uncertainties. An important collaborative in situ observation program is proposed, with monitoring using gliders, and a large hydrographic cruise with mooring deployment. Gliders have been deployed twice in a similar configuration in the Coral Sea, and we propose to proceed with deployments in the Solomon Sea in a continuing collaboration with Scripps, so that the flow entering the Solomon Sea through its southern boundary is measured seasonally. The second key observation component is a large cruise including mooring deployments in the 3 northern straits in collaboration with CSIRO and Scripps. This cruise will provide an unprecedented “snapshot” description of the circulation, the few previous surveys being either low resolution or low coverage.

This study will help to identify oceanic model flaws in this area and correct them, and possibly to devise a sustained monitoring program to adequately sample the time-variability of climate-important currents. Improving the realism of the oceanic circulation in this region in climate models may improve the realism of decadal variability, and thus permit a better understanding of its mechanisms and possibly its predictability. This research will lead to an advanced knowledge of the oceanic features of the Solomon Sea, as well as the development of numerical modeling tools that will be of great relevance to local environmental applications. The “Solwara" project will be an important contribution to the emerging program SPICE whose goal is to analyze the Southwest Pacific climate issues in a coordinated approach involving Australia, New Zealand and the USA in collaboration with the Pacific Island Nations (clivar.org; spice.legos.obs-mip.fr)

2013 - Solwara: a SPICE/CLIVAR programme