Internal tidal dynamics and associated processes at highly supercritical slopes in Banda Sea: Lessons from the oceanic island of Ambon, eastern Indonesia

Interactions between the deep sea and steep-sloping oceanic islands generate physical processes with potential impacts on sediment resuspension and ocean productivity. While studies have been conducted in the open oceans, those focusing on oceanic islands of the deep Banda Sea in eastern Indonesia are lacking. Here, we present the first observational evidence of vertical mechanisms (i.e. internal tidal reflection, internal hydraulic jumps) at the highly supercritical slope in outer Ambon Bay (OAB) in Ambon Island – an oceanic island in the Banda Sea. A 23-h CTD yoyo experiment combined with ADCP measurements conducted during spring flood and ebb tides demonstrated tidally-varying vertical temperature, salinity and density profiles. During spring flood tide, incoming internal tides were reflected by OAB's highly supercritical slope back to the deep sea with isopycnals and isotherms showing sharp downward plunges (downward vertical velocity = 6.5–8.3 × 10⁻³ m/s), and the internal tidal amplitude reaching 90–110 m. The reflection during flood tide caused seaward overturning flow at deeper depths despite the prevailing landward flow at the upper layers. During spring ebb tide, internal hydraulic jumps (upward vertical velocity = 6.1–6.48 × 10⁻³ m/s) occurred to rebound the downward plunges of isopycnals and isotherms as flood tide relaxed with the observed amplitude of internal tides of 90 m. We also observed weakened seaward ebb flow during the isothermal uplifting (when hydraulic jumps occurred), and subsequent intensified landward flow at the end of spring ebb tide indicating strong upslope flow when isotherms reached the maximum shoaling depths. Taken together, the observed vertical mechanisms indicate the conservation of energy at the highly supercritical slope of OAB evident by the comparable vertical velocities. An embedded turbidity-chlorophyll profiler in the CTD reveals that internal tidal activities at the highly supercritical slopes OAB may induce bottom nepheloid layers, and influence ocean productivity by regulating the vertical distribution of phytoplankton biomass.