2:00 PM - 2:15 PM
[ACG38-08] Near-inertial oceanic internal waves observed by LADCP (lowered acoustic Doppler current profiler)
Keywords:oceanic internal waves, inertial waves, ocean observation
Frequency spectra of oceanic internal waves often show peaks around the inertial frequency of the observed location. These near-inertial waves are ubiquitous as many generation machnisms are possible such as surface wind, ocean currents interacting with bottom topography, and spontaneous break down of eddies. On the other hand, observation data of near-inertial waves are limited because of their intermittency, due partly to the transient and unsteady nature of their generation. The polarisation relationship of the internal wave constrains the particle movements of near-inertial waves within a horizontal plane which makes near-inertial wavs "invisible" to conventional conductivity temperature depth profilers (CTD) observations. Here, we attemp to observe these waves by using lowered acoustic Doppler current profiler (LADCP) used with CTDs in ship-based hydrographic observations. Since the near-inertial waves are peaks, we boldly assume that observed horizontal velocity is inertial waves, superposed upon which are other components (e.g. internal waves at other frequencies and geostrophic flows). At one station, the sensor package passes a depth twice, decent and ascent, giving meridional and zonal components of horizontal velocities each, which enables estimate of amplitude and phase of the inertial wave.
We applied this method to several sections in the Indian Ocean. On a section nominally along 55°E, one CTD station lasting about 4 hours could capture inertial waves at poleward of about 30°S where the inertial period is 24 hours. Their group velocity was more often downward than upward, not inconsistent with surface generation. Many had wavelengths longer than 2000 m, suggesting vertical structures of gravest modes. The magnitude was of the order of cm/s. On the other hand, there were unnegligible waves with upward group velocity with depths scatterd around the water column; suggesting contribution from other generation mechanims than wind.
We applied this method to several sections in the Indian Ocean. On a section nominally along 55°E, one CTD station lasting about 4 hours could capture inertial waves at poleward of about 30°S where the inertial period is 24 hours. Their group velocity was more often downward than upward, not inconsistent with surface generation. Many had wavelengths longer than 2000 m, suggesting vertical structures of gravest modes. The magnitude was of the order of cm/s. On the other hand, there were unnegligible waves with upward group velocity with depths scatterd around the water column; suggesting contribution from other generation mechanims than wind.