Abstract
Abstract. Intense equatorward western boundary currents transit the Solomon Sea, where active mesoscale structures exist with energetic internal tides. In this marginal sea, the mixing induced by these features can play a role in the observed water mass transformation. The objective of this paper is to document the M2 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 January–March 1998 El Niño and the April–June 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 these contrasted El Niño and La Niña case studies. Mode 1 is the dominant vertical mode to propagate baroclinic tidal energy within the Solomon Sea, but mode 2 becomes more energetic during the El Niño period when the stratification is closer to the surface. The La Niña period with a higher level of mesoscale activity exhibits more incoherent internal tides. These results illustrate the complexity of predicting internal tides in marginal seas in order to clearly observe meso- and submesoscale signatures from altimetric missions, including the future Surface Water Ocean Topography (SWOT) mission. Diapycnal mixing induced by 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 in quieter regions, 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 suggest the strong role of local circulation changes, as well as stratification changes, in modifying the internal tides.
Highlights
The Solomon Sea is a marginal sea that is the last passageway for the low-latitude western boundary currents (LLWBCs) of the southwest Pacific that connect the subtropics to the Equator and supply water of subtropical origin to the Equatorial Undercurrent (EUC), warm pool, and Indonesian Throughflow (Tsuchiya et al, 1989; Grenier et al, 2011)
Abundant literature exists on the South China Sea and the Indonesian seas, illustrating this distinctive feature of marginal seas but not in the Solomon Sea, even though the southwest Pacific and in particular the Solomon archipelago are recognized as generation areas of energetic internal tides (Niwa and Hibiwa, 2001)
We focus on the 3-month periods of El Niño and La Niña, and we concentrate on SW waters since the transit time of SW waters is short enough to be influenced by these extreme conditions (Melet et al, 2011)
Summary
The Solomon Sea is a marginal sea that is the last passageway for the low-latitude western boundary currents (LLWBCs) of the southwest Pacific that connect the subtropics to the Equator and supply water of subtropical origin to the Equatorial Undercurrent (EUC), warm pool, and Indonesian Throughflow (Tsuchiya et al, 1989; Grenier et al, 2011). Depth-mean energy dissipation in the Solomon Sea is elevated by a factor of 8 relative to the rest of the equatorial Pacific, and energy dissipation is maximum in the surface and thermocline layers (4.1–23 × 10−8 W kg−1) To model this increased water mass mixing correctly in the Solomon Sea, tidal effects need to be included. This parameterization is based on the assumptions that all of the energy of the internal tides within a marginal sea is dissipated within that sea, and that energy dissipation is assumed to be maximum in the thermocline Their modeled salinity without this tidal parameterization is biased high over the 24.5–27.5σ θ range compared to observed properties, it remains biased low with the parameterization, such that the erosion of SPTW is too strong.
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