An Observational and Modeling Study to Quantify the Space/Time Scales of Inner Shelf Ocean Variability and the Potential Impacts on Acoustics

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1 DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. An Observational and Modeling Study to Quantify the Space/Time Scales of Inner Shelf Ocean Variability and the Potential Impacts on Acoustics John A. Colosi: PI Department of Oceanography Naval Postgraduate School Monterey, CA Ph: , FAX: , Award Numbers: N WX-00432, N WX LONG-TERM GOALS 1. Deterministic and statistical modeling of dynamical ocean processes. 2. Integration of ocean and acoustical models. 3. Predictability of ocean acoustic mean fields and low order moments. 4. Characterization and predictability of ocean ambient noise fields. 5. Effects of acoustic fluctuations on ocean acoustic remote sensing, communication, and navigation. OBJECTIVES The inner shelf, connecting the surf zone to the broader continental shelf region is a dynamic environment of great ecological, economic, and military importance. With many interrelated dynamical processes in the water column and on the boundaries, this region poses several challenges for ocean forecasting. A critical consideration in this region is the acoustical environment since acoustics provides an important tool for ocean remote sensing, communication, and navigation. Using observations, numerical modeling, and reduced physics theory our group seeks to provide a better understanding of the critical inner shelf dynamical process at work in shaping the acoustical environment and ambient noise. These processes may be treated deterministically or statistically. Fundamental ocean processes of interest include shelf eddies, filaments and fronts, mixing and boundary processes, wind driven circulation, linear and nonlinear internal waves and tides, surface and infra-gravity waves, edge waves, and rip currents. APPROACH The approach here is to develop an effective observational program to monitor inner shelf variability, and isolate important processes. Given the complex feedback mechanisms at work on the inner shelf and the wide range of space/time scales, the observational task must be integrated with a significant large-scale modeling effort, and reduced physics models. 1

2 Our group has observational and reduced physics internal wave modeling capability and we intend to work closely with the large-scale modelers to help improve model physics and forecasting ability. Work in the last fiscal year, has focused on collaboration with numerical modelers from Scripps and Georgia Tech to design the summer pilot study which was conducted successfully off Point Sal in central California. WORK COMPLETED For the pilot study, our group was primarily focused on making observations in the deeper waters of the inner shelf between 30 and 50 meters depth (See Fig. 1) Figure 1: The 2015 summer pilot study at Point Sal. Blue dots show 9 mooring sites each instrumented with a mooring of 5 temperature and pressure sensors and an ADCP mooring. Other symbols indicate thermistor and ADCP sites instrumented by Prof. Jamie MacMahan s group. The instruments were deployed June 13-14, and recovered August 7-8. Instrument performance was excellent with 44 out of 45 of the SBE39, temperature and pressure sensors provided quality data and 8 out of 9 of the ADCPs provided good data. Processing of this data is underway, and a few preliminary results are discussed below. 2

3 RESULTS A. Time Scales Figure 2 shows a raw depth/time series of potential temperature from the southern most mooring in 50 m water depth and the frequency spectra. Several time scales of variability are evident: 1) an energetic eddy field, 2) a sub-inertial, diurnal fluctuation, 3) an energetic and nonlinear internal tide (See tidal harmonics), 4) a power law internal wave continuum akin to the GM internal wave spectrum, and 5) energetic, high- frequency internal solitary waves. Figure 2: Raw time/depth series of temperature from the S50 mooring, and the frequency spectra. In the spectral plot, the black line shows a frequency to the minus-two power law. B. Large Scale Variability Figure 3 shows an example of low pass filtered temperature fluctuations observed on the southern most mooring. The energetic diurnal fluctuations and eddy field are evident. 3

4 Figure 3: Mean S30 potential temperature profile (upper), and a low pass filtered depth/time series of potential temperature fluctuations (lower). In the lower panel a linear trend of roughly 1 deg. C per 20 days has been removed. The cut off frequency for the low pass filter is the inertial frequency. C. Internal Tide Bores Figure 4 shows an example of an undular bore observed on the Northern most mooring in 30 m water depth. These undular bores are common over the whole observational area and throughout the experiment. The nonlinear internal tide is propagating on-shore with a speed of cm/s and the tidal bore front is mostly aligned with the isobaths. The horizontal wavelength of the internal tide is roughly 10 km. 4

5 Figure 4: An undular bore observed on the N30 mooring. Curves track the depth of isotherms (potential temperature), demonstrating vertical displacements. D. Internal Solitary Waves Figure 5 shows examples of internal solitary waves. These internal solitary waves are common over the whole observational area and throughout the experiment. The internal solitary waves have durations between 7 and 15 minutes, and thus horizontal wavelengths of order a few hundred meters. Figure 5: Examples of internal solitary waves from the N30 mooring. Upper panel: isotherm displacements showing waves of elevation. Lower Panel: Raw temperature records showing energetic internal solitary waves of depression which are so large isotherm tracking cannot be carried out. 5

6 IMPACT/APPLICATIONS 1. The observations here will be used to help better design the main inner shelf DRI field effort in The observations will be used to test and refine ``large-scale ocean models of the inner shelf and surf zone. 3. The observations will be used to test reduced physics, non-hydrostatic models of nonlinear internal wave propagation and dissipation. TRANSITIONS None RELATED PROJECTS 1. MURI Integrated Ocean Dynamics and Acoustics (Tim Duda, WHOI MURI Leader) REFERENCES/ RECENT PUBLICATIONS 1. Lynch, J.F., T.F. Duda, and J.A. Colosi, ``Acoustical horizontal array coherence lengths and the ``Carey Number, Acoustics Today, 10, pp10-19, Jones, B.A., J.A. Colosi, and T.K. Stanton, ``Echo statistics of individual and aggregations of scatterers in the water column of a random oceanic waveguide, J. Acoust. Soc. Am., 136, pp90-108, Jones, B.A., J.A. Colosi, T.K. Stanton, R. C. Gauss, J.M. Fialkowski, and J. Michael Jech ``Classification and statistics of long range mid-frequency sonar measurements of aggregations of fish, J. Acoust. Soc. Am., submitted, Raghukumar, K. and J.A. Colosi, ``High frequency normal mode statistics in a shallow water waveguide: I. The effect of random linear internal waves, J. Acoust. Soc. Am., 136, pp66-79, Raghukumar, K. and J.A. Colosi, ``High frequency normal mode statistics in a shallow water waveguide: I. The combined effect of random linear surface and internal waves, J. Acoust. Soc. Am., 137(5), pp , Colosi, J. A., ``A re-formulation of the Λ Φ diagram for the prediction of ocean acoustic fluctuation regimes, J. Acoust. Soc. Am., 137(5), pp , Morozov, A.K., and J.A. Colosi, ``Entropy rate defined by internal wave scattering in long range propagation, J. Acous. Soc. Am., 138, , Andrew, R. A. White, J. Mercer, P. Worceter, M. Dzieciuch, and J.A. Colosi, ``A test of deep water Rytov theory at 284 Hz and 107 km in the Philippine Sea, J. Acous. Soc. Am., accepted, Radko, T. J. Ball, J.A. Colosi, J. Flanagan, ``Double-diffusive convection in a stochastic shear, J. Phys. Oc., in revision,

7 10. Ramp, S.R., J.A. Colosi, P.F. Worcester, F.L., Bahr, K. Heaney, J.A. Mercer, and L.J. Van Uffelen, ``Direct observations of the mesoscale variability in the Western Philippine Sea, J. Phys. Oceanogr. 11. Colosi, J.A. Sound Propagation through the Stochastic Ocean, Cambridge University Press, in press. PATENTS None HONORS/AWARDS/PRIZES 1. Cecil H. and Ida M. Green Scholar, Scripps Institution of Oceanography, 2000, Fellow, Acoustical Society of America, Medwin Prize in Acoustical Oceanography, 2012, for furthering the understanding of ocean sound-speed structure and its effects on acoustic propagation in both deep and shallow water. Acoustical Society of America. 4. A. B. Wood Medal, 2001, for significant contributions to the understanding of acoustic scattering by internal waves in long-range propagation. Institute of Acoustics and Acoustical Society of America. 5. ONR Young Investigator Award,