Hawaiian Ocean Mixing Experiment
Absolute and eXpendable Velocity Profiling
Tom Sanford, Eric Kunze, Craig Lee
Applied Physics Lab - University of Washington
Jonathan Nash
COAS - Oregon State University
comments? ... Email Jonathan Nash


To characterize the internal wave energy fluxes radiating from the Hawaiian Ridge and the associated mixing, the absolute velocity profiler (AVP) occupied 14 stations over a three week period during October, 2000.  A summary of our preliminary results are shown in the six figures below.

Our measurements of energy flux <u'p'> indicate that the conversion of the barotropic to baroclinic semidiurnal tide occurs at specific locations around the Hawaiian Islands, most notably at French Frigate Shoals, near Nihoa Island, and in the Kauai Channel (Figure 1).  Turbulent kinetic energy dIssipation strengthens near the surface and the bottom.  The mixing, as described by an eddy diffusivity, increases to 10-3 m2/s near the bottom in response to an increase in S2/N2, presumably from short wavelength internal waves (Figure 2).  Near the 3000-m isobath, the dissipation is found to scale approximately with the energy flux (Figure 3).  A more detailed analysis of our turbulence observations can be found here.

An expendable current profiler (XCP) survey was carried out at Kaena Ridge to investigate the specifics of the generation process (Figures 4-6).  Seven repeats over a 20 hour period of an 18-km section were made to resolve the semidiurnal motions.  This survey shows that the energy flux increases rapidly near the 1000 m isobath, and produces two paths of energy propagation originating at a depth near 700 m and propagating both upward and downward and away from the ridge.

For more general information about the Hawaiian Ocean Mixing Experiment, visit the HOME Homepage

An description of the Kauai Channel XCP observations and an investigation into internal tide generation mechanisms has been submitted to the Journal of Physical Oceanography : "Structure of the Baroclinic Tide Generated at Kaena Ridge, Hawaii", J.D. Nash, E. Kunze, C.M. Lee, and T.B. Sanford (2 MB pdf file)

An introductory talk about the HOME experiment A talk about the basic physics governing internal tide generation and dissipation was given by Jonathan Nash at the University of Victoria in March 2002, and is available for download here (3.6 MB pdf file)

Figure 1: Internal wave energy flux from 14 Hawaiian Ridge stations, as estimated with <u'p'> from the Absolute Velocity Profiler (AVP).  Calculations using the downward profiles (blue) are distinguished from those made from upward profiles (red) as an indication of the method error. (print version)
Figure 2:  Vertical profiles of energy flux <u'p'>, turbulent dissipation (epsilon), Eddy diffusivity (Krho), 10-m vertical shear squared (S2), and inverse Richardson number (S2/N2).  Profiles represent 100-m vertical averages over all AVP stations at a nominal depth of 3000 m.   95% bootstrap confidence limits are indicated by shading and represent the variability between the 16 station occupations of the 100-m vertically averaged data. (print version)

Figure 3: At the 3000-m isobath, the turbulent dissipation scales approximately with the energy flux, as indicated below.  Unique symbols are used for each station occupation; the dissipation represents the mean and bootstrap confidence intervals in a given half-decade interval of energy flux. (print version)

Figure 4:  Plan view of the expendable current profiler (XCP) survey location.  Five stations, each separated by 4.5 km, were occupied seven times over a 20 hour period (at 3 hour repeats).  Red arrows indicate the depth-integrated energy flux at each station location. (print version)

Figure 5: Lower panel: vertical section of the downslope energy flux, as calculated from the XCP survey shown in Figure 4.  Upper panel: Depth-integrated energy flux for this section.  All of the outward-propagating energy flux is formed between 5 km and 10 km from the ridge-crest (between stations 4 and 3). (print version)

Figure 6:  Time-depth plot of downslope velocity fluctuation at XCP stations 3 and 5 (as in figures 5 and 6).  The upper panels (a ridge top station) indicate that the phase propagation is entirely downward, consistent with upward energy propagation.  In the lower panels, energy propagation is upward and downward away from a depth near 700 m, as indicated by the strong phase propagation downward from the surface and upward from the bottom.  Combining figures 5 and 6 gives a consistent image of the internal wave energy flux being formed inshore of the 1300-m isobath (station 3), and propagating outward from the ridge crest and both upward and downward from a depth near 700 m. (print version)