On the role of the Solomon Sea on ENSO modulation: a preliminary modeling study

The Pacific decadal variability, such as the low-frequency modulation of El Niño Southern Oscillation, has been hypothesized to be influenced by changes in the strength or water mass properties of the meridional circulation associated with the subtropical cells (STC). The STCs provide a connection between subtropical waters and the equatorial Pacific at thermocline level, by both an interior pathway and by the low latitude boundary currents. The LLWBC branch of the South Pacific STC has been pointed as the main source of the Equatorial Undercurrent and of the Equatorial Cold Tongue by both observational and modeling studies. Those currents transit in the poorly documented Solomon Sea, located in the western boundary of the tropical South Pacific. The very intricate bathymetry of the semi-enclosed Solomon Sea complexifies the STC pathways to the equator. In this study, we characterize the fine-scale thermocline mean circulation in the Solomon Sea. As available observations of the region are sparse, our approach is based on modeling. High-resolution is required to realistically represent the complex topography. It is achieved through the implementation of a hierarchy of Ocean General Circulation Models (OGCMs) : a 1/12° resolution model of the Solomon Sea is interactively into a ¼° regional model of the southwest Pacific, itself embedded through open boundary conditions in a global 1/4° OGCM. We show that the circulation involves an inflow from the southern open Solomon Sea which is distributed via WBCs between the three narrow straits that connect this region to the equatorial Pacific. The system of WBCs appears to be complex, and provides connection of subtropical water to the EUC at different longitudes, which may be associated to different climate impacts. Early experiments of data assimilation are performed to assess the ability of localised 3-D glider (underwater vehicle) and synoptic altimetric sea level anomalies observations to control the main Solomon Sea WBC. An observing system simulation experiments (OSSE) strategy is implemented and data assimilation is performed in our multigrid model through a local reduced order analysis, based on a sequential scheme derived from the SEEK filter. Water mass transformations are characterized in our high-resolution model in a lagrangian quantitative framework. We show that strong diapycnal mixing, partly due to internal tide waves dissipation, is responsible for a reduction of the temperature and salinity vertical gradients. More specifically, diapycnal mixing erodes the high salinities associated with the waters of subtropical origin carried by the STC, and this erosion is associated to a downward heat transfer. Therefore, the Solomon Sea might damp the spiciness anomalies formed in the southeast Pacific and advected equatorward to the EUC by the LLWBC branch of the South Pacific STC, with reduced impact of the STC on the Tropical Pacific climate variability