Turbulent Mixing in the Indonesian Seas

Overview & Synopsis

The Indonesian seas are the area where the largest energy transfers from barotropic to baroclinic tides occur as shown by tidal model based studies. These processes make it a hotspot for turbulence as indirectly evidenced through the strong watermass mixing of Pacific waters flowing through the Indonesian Seas toward the Indian Ocean. The first part of this work aimed at better characterizing the strong spatial variability of turbulent mixing based on the analysis of a unique 26 years historical dataset of temperature and conductivity profiles. The classical Thorpe scale method, used to infer the dissipation rates from density measurements, was improved and validated against turbulence measurements. We showed that the turbulent kinetic energy dissipation is enhanced within straits and narrowing passages, in shallowing topography and mostly driven by semidiurnal, M2 internal tides. We then focused on internal solitary waves, a process easily identified from satellite imagery but rarely studied precisely in the area. Based on two-dimensional non-hydrostatic simulations we gave evidence of the full lifetime cycle of internal solitary waves observed in a coastal area in the Sulawesi Sea. We thus showed that the internal tide generated at the Sibutu passage evolves during its propagation leading to the formation of solitary waves that extract a significant part of the internal tide energy and eventually break in the northern coast of Sulawesi Island. We investigated the mechanisms driving the enhanced dissipation rates accompanying the shoaling internal waves in this region. To further explore these high-frequency processes tidally induced, we conducted a short-time mooring experiment in the Lombok Strait, one hot spot for internal solitary waves and proved that the maximum dissipation rate occurs at semi-diurnal frequency thus giving evidence of the semi-diurnal internal tide as a main driver for turbulence there.