Sill Overflow Processes in the Philippine Archipelago

We present an analysis of small scale processes associated with bathymetric sills in the Philippine Archipelago, with an emphasis on in situ observations made during the 2007-2009 Philippines Straits Dynamics Experiment (PhilEx). Due to their location at the margins of larger basins, the dynamics at these sills set the conditions under which water, heat, salt, and energy are transported internally and at the boundaries of the archipelago. The main connection between the internal Sulu Sea and the South China Sea to the north is through Mindoro Strait. Moored Acoustic Doppler Current Profiler (ADCP), Conductivity, Temperature, Depth (CTD), and lowered ADCP (LADCP) data are used to study the currents and stratification at Panay Sill, at the southern end of Mindoro Strait. We observe a strong bottom-intensified overflow; temporal mean velocity is greater than 0.75 m/s at 50 m above the sill. Both ADCP observations and a hydraulic control model based on hydrographic measurements estimate a transport of approximately 0.32 x 10^6 m^3/s. Analysis of Froude number variation across the sill shows the flow is hydraulically controlled. Turbulent dissipation and bulk diapycnal diffusivity measurements at the sill and downstream suggest mixing well above oceanic background levels. Downstream, Panay Sill overflow water is shown to ventilate a slab in the Sulu Sea between approximately 575 and 1250 dbar with a residence time of about 11 years. Below this, the salinity increases with depth. Deep Sulu Sea water is shown to most likely derive from the Sulawesi Sea to the south. We propose that Sulawesi Sea water between 245 to 527 m is mixed and transported by the energetic tidal environment at Sibutu Passage. Oxygen concentrations within the deep Sulu Sea suggest that the Sulawesi overflow is 0.15 x 10^6 m^3/s, with a residence time of Sulu Sea deep water of 60 years. The balance of ventilation from the northern and southern sources is likely to change on a range of time scales, associated with thermocline depth variability. Finally, we take advantage of our timing and location to investigate the energetics of large-amplitude non-linear internal waves generated at Sibutu Passage in the southern Sulu Sea. Water column displacement and velocity profile time series are used to track the passage of two solitary-like waves close to their generation site. These waves are repeatedly observed by hull-mounted ADCP as they travel north through the Sulu Sea. Wave amplitude is observed to be 43.5 m, and are reasonably fit by both a shallow water Korteweg-de Vries model and a finite depth Joseph model. Total potential energy per meter of wave crest in the modeled solitary waves are 1.6 x 10^8 J/m and 9.8 x 10^7 J/m for the K-dV and Joseph waves, respectively, while kinetic energy in the main wave crest contains 4.7 x 10^7 J/m, estimated from ADCP measurements. Attempting to compensate for the early development of the waves, a more broadly defined packet contains an estimated 1.5 x 10^8 J/m kinetic energy, comparable to the modeled potential energy.