Why is no Near-Inertial Peak Observed in the Luzon Strait, a Major Internal Tide Generation Region?

The Luzon Strait, a double submarine ridge between Taiwan and the Philippines, is home to the largest internal tides observed in the world oceans. Yet, the usually-ubiquitous near-inertial spectral peak is absent in spite of strong wind forcing during much of the observational period. Observations were collected during the southwestern monsoon season (Summer 2011) along northern and southern transects. Each of these transects was instrumented with 4 moorings with ADCP (Acoustic Doppler Current Profilers) and McLane moored profilers. Over the first 500 meters, in the north, there is no measurable energy in the near inertial band and, in the south, we observe near-inertial kinetic energy peaks 10 times weaker than the tidal peaks. These observations differ from other parts of the global ocean where inertial current forced by fluctuations in the local wind stress dominates the upper ocean circulation. Here, the strong monsoon winds (≈ 15 m s^-1) should generate mixed layer inertial currents which, after propagating downward, should also excite near-inertial waves beneath the mixed layer. The presence of these currents depends of the scales of the wind stress, the background stratification and the surrounding oceanic motions, among other factors. We hypothesize that the presence of the northern branch of the Kuroshio and/or Doppler shifting by the extreme internal tides in the area may be responsible, and present evidence for each scenario.