Observations of internal waves and parametric subharmonic instability in the Philippines archipelago

Abstract

Internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing (by wind and tide) and interior diapycnal mixing. We report here on measurements from Mindoro Strait in the Philippines (connecting the South China Sea to the Sulu Sea) of an internal wave field with a number of surprising properties that point to previously‐unrecognized processes at work in the region. Continuum spectral levels are very close to typical “background” values found in the open ocean, but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated—and higher at the northern mooring (MP1) than the southern (MP2). Two particularly energetic depth ranges stand out at MP1: an upper layer centered near 300 m, and one at the bottom of the water column, near 1800 m. The upper layer contains both internal tides and a near‐inertial band with upward and downward propagating waves and an apparent spring‐neap cycle. The combination is suggestive of Parametric Subharmonic Instability as the forcing for the near‐inertial band—a conclusion supported by bicoherence estimates. Mixing, estimated from density overturns, is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide—both diurnal and semidiurnal. Near‐inertial currents in this deep layer are dominantly rectilinear and not well‐correlated with the mixing. Bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography, suggesting additional enhancement elsewhere in the archipelago.