Internal tides in the Solomon Sea in contrasted ENSO conditions

Abstract: The Solomon Sea has intense Low Latitude Western Boundary currents transiting to the equator, where active mesoscale structures exist with energetic internal tides. In this marginal sea, the cumulated effects of these dynamical constraints can play a role in the observed water mass transformation. The objective of this paper is to document the M 2 internal tides in the Solomon Sea and their impacts on the circulation and water masses, based on two regional simulations with and without tides. Since the Solomon Sea is under the influence of ENSO, the characteristics of the internal tides are also analyzed for two contrasted conditions: the summer 1998 El Niño and the fall 1999 La Niña. The generation, propagation and dissipation of the internal tides are sensitive to changes in stratification and mesoscale activity, and these differ between the El Niño and La Niña periods. Mode 1 is the dominant mode to propagate baroclinic tidal energy within the Solomon Sea, but Mode 2 is more energetic during the summer El Niño period when the stratification is closer to the surface. The fall La Niña period with a higher level of mesoscale activity exhibits more incoherent internal tides. These results illustrate the complexity in predicting internal tides in marginal seas in order to clearly observe meso and submesoscale signatures from altimetric missions, including the future SWOT mission. Diapycnal mixing induced by the internal tides contributes to a stronger erosion of the salinity maximum of the upper thermocline water, and to cooling of the surface temperature interacting with the atmosphere. Such effects are particularly visible far from the strong currents, where particles may experience the tidal effects over a longer time. However, when averaged over the Solomon Sea, the tidal effect on water mass transformation is an order of magnitude less than that observed at the entrance and exits of the Solomon Sea. These localized sites appear crucial for diapycnal mixing, since most of the baroclinic tidal energy is generated and dissipated locally here, and the different currents entering/exiting the Solomon Sea merge and mix. Finally, the extreme ENSO condition case studies highlight the dominant role of local circulation changes that modify the internal tides more than the vertical stratification changes.