Deep fades in intensity: Exploration of measurement-Monte Carlo parabolic equation mismatch in the Philippine Sea

The oceanography of the Philippine Sea is partially characterized by energetic mesoscale and strong locally generated internal tides. Despite the simplification of range-independence and the exclusion of internal tides from Monte Carlo parabolic equation (MCPE) simulations, predictions of scintillation index, variance of log-intensity, and the distribution of intensity for acoustic paths with upper-turning-points (UTP) below the extreme upper ocean generally agree with measurements made during an experiment in 2009. These measures of the fluctuations did not appear to be strongly influenced by the number of UTPs in the path, though a compensating effect due to differences in UTP position cannot be ruled out. Enhanced variability in the form of deep fades is observed for paths turning in the extreme upper ocean; this enhanced variability is not predicted by the MCPE model employed. Seaglider-based observations of mixed-layer depth (from 2010 to 2011) and moored measurements of internal-tide-related sound-speed perturbations are presented. A plane-wave internal-tide model and results from acoustic mode propagation through range-independent profiles measured in situ are compared with the observed character of the intensity fades.