Variability of the surface and thermocline circulations in the Solomon Sea

International audienceIn the southwest tropical Pacific, subtropical waters from the SEC flow in the Solomon Sea, mainly through the western boundary New Guinea Coastal Undercurrent (NGCU), before joining the equatorial western Pacific by three narrow straits. Because the NGCU is a primary source of the EUC, variations of its characteristics are expected to play a role in the equatorial thermocline features and more generally on decadal climate variability. Moreover, the highest levels in sea level variability in the entire South Tropical Pacific Ocean are found in the Solomon Sea region. Therefore, the study of the poorly known Solomon Sea is a key issue from a climate perspective (e.g. SPICE/CLIVAR program). In this study, we consider the circulation in the Solomon Sea and its variability through both modeling and altimetric data analysis. Since the geography of the region is extremely intricate, a high-resolution (1/12°) model with a realistic bathymetry has been implemented based on the NEMO code, and specifically retreated Topex-Poseidon along-track sea level data were used in addition to standard gridded altimetric data. We will first describe the fine-scale mean thermocline circulation based on our numerical simulation. It involves a complex system of western boundary currents (WBC). Transport limitation through Vitiaz Strait is responsible for a splitting of the NGCU in two branches. The main branch flows through Vitiaz Strait and along the New Guinea coast. The other branch , denoted as the New Britain Coastal Undercurrent, flows along the south coast of New Britain. The NBCU mainly flow through Solomon Strait and join the New Ireland Coastal Undercurrent, providing the most direct western boundary-equator connection. At annual time scales, the Solomon Sea thermocline WBC variability results from the combination of equatorial dynamics, of remotely forced Rossby waves north of 10°S and of the spinup and -down of the subtropical gyre as a response to Rossby waves forced south of 10°S. Altimetric along-track data appear especially helpful for documenting the observed fine structure of the surface coastal currents. The annual variability of the WBC that emerged from altimetry compared quite well with the variability described earlier at thermocline level in our model. At interannual time scales, the modeled NGCU thermocline transport variability is related to ENSO : it strenghens during El Nino and weakens during La Nina events. The transport anomalies are mainly transmitted to the NBCU and through Solomon Strait than through Vitiaz Strait. An analysis of altimetric data show that WBC surface transport anomalies counterbalance changes in western Equatorial Pacific warm water volume, confirming the phasing of South Pacific WBC to ENSO