There are a variety of energy sources for mixing: mesoscale eddies, the background internal wave field, and tides. Because the tide is so consistently energetic, and its frequency so well defined, HOME is focusing on tidally forced mixing. The Hawaiian Ridge is particularly well suited for a study of internal tides because the incident tide is normal to the ridge.
Our plan in HOME is to have a Nearfield experiment specifically to resolve tidal processes at an active mixing site or "hot spot." In order for the Nearfield experiment to succeed, it is imperative that a representative site be found. The Survey addressed the problem of finding a Nearfield site through both broad-ranging and focused surveys. A suitable site was found in the Eastern Kauai Channel where the Nearfield observations will be undertaken in October of 2002.
The three major goals of the Survey program were to:
The HOME Survey program consisted of a total of 5 cruises to the Hawaiian ridge. These cruises were conducted by the world's leading oceanographers on state-of-the-art scientific research vessels, the R/V Revelle and the R/V Wecoma . Each vessel was equipped with the very latest technology for measuring temperature, salinity, horizontal and vertical velocity, turbulent energy dissipation, optical attenuation and acoustic backscatter. Horizontal resolution was as fine as O(1 m), with ranges of roughly 1000 km along-ridge and 100 km across-ridge. The finest vertical resolution was O(1 cm), ranging over the entire ocean depth.
Following is a summary of each cruise, all cruises took place in 2000:
| Vessel | Duration | Summary |
| R/V Revelle | 8/29-9/29 | Broad along-ridge surveys of stratification and shear were conducted with Sea Soar. |
| " | 10/04-11/06 | Across-ridge surveys of shear, stratification, and dissipation at sites identified in the first cruise as hot spots conducted with AMP. |
| R/V Wecoma | 10/04-11/06 | Full depth surveys of representative topographic sites, and of hot spots identified in the first two cruises were conducted with AVP | .
| " | 11/02-11/05 | Instrument moorings deployed at hotspots for measurement of tidal variablity. |
| " | 11/10-12/10 | Deep surveys of mixing hot spots using Marlin. |
All HOME Survey goals have been met. Physical quantites, such as shear, stratification, and dissipation, which are the ocean's signatures for turbulent mixing, have been adequately measured on and around the Hawaiian Ridge. Areas of intense mixing have been located which will be very suitable for the Nearfield experiment.
In addition, the processes causing turbulence along the ridge are
also beginning to come to light. HOME measurements provide striking
evidence that energy is being transferred from the incident tide to
internal waves which then break, giving rise to turbulence. This
conversion from the tides to turbulence represents a cascade of energy
from the very large tidal scales of 1000's of kilometers, down to the
relatively tiny scale of turbulent dissipation, approximately 1 cm.
Evidence for this cascade is pieced together from several different
aspects of HOME. The figure above shows the energy flux associated
with internal waves on and around the Hawaiian Ridge. The green arrows
represent fluxes calculated by a computational simulation, and the red
arrows represent energy fluxes as measured by the
AVP
as part of HOME. The length of the arrows indicates the strength of
the energy transport. The fact that the arrows are always pointed away
from the ridge indicates that the ridge is a source of internal wave
energy. This is in close agreement with computational models which
indicate that the Hawaiian Ridge is a sink of tidal energy. The tidal
energy seems to be converted to internal wave energy at the
Ridge. Notice that some arrows are longer than others. This indicates
that the energy conversion from tide to internal is stronger at
certain locations. The Nearfield experiment, which will look much more
closely at the generation of turbulence, will take place in the
channel east of Kauai, a region of intense energy conversion.
The next figure is a contour plot of ocean temperature as a
function of time and depth as recorded by a HOME mooring in the
Eastern Kauai Channel. Notice how the temperature is cooler with
depth, as you'd expect. The other important feature is the prominent
oscillation of the temperature contours, which are direct evidence of
internal waves. The period of the oscillation is very close to the
tidal period of approximately 12 hours. Also note that the peak to
peak amplitude is nearly 300 m! These internal wave are quite steep,
and there is evidence, not presented here, of internal wave breaking
on the Hawaiian Ridge.
The top panel of the figure at left shows the variance of vertical isopycnal displacement as measured across the ridge. An "isopycnal" is a line of constant water density, which oceanographers expect to be completely flat if there were no internal waves or currents. Hence, the diplacement of the isopycnals is another measure of internal wave activity. From the figure we see that isopycnal displacement is peaked south of the Ridge. The bottom panel shows the variance of vertical velocity gradient along the same isopycnals. Vertical velocity gradient is a signature of turbulence. Note that it is peaked over the ridge.
The last step in the tides to turbulence cascade is the turbulence
itself. The bottom figure is a plot of turbulent dissipation at the
French Frigate Shoals by AVP,
AMP, and
Marlin
. The bottom panel is a profile of the Ridge. The top panel displays
individual measurements by the different instruments (colored points),
and an average of all the instruments (solid lines). Note that
turbulent dissipation reaches a maximum at the peak of the ridge, and
decays away from the ridge. The dotted line shows the open ocean value
of turbulent dissipation. Note that the measured values are always
larger, in accordance with the theory that mixing is enhanced at
abrupt topography.
Though these results illuminate the processes taking place in the world's oceans to keep the ocean well mixed, many questions remain unanswered. There are a number of physical processes which could explain these results. More precise investigations will be made during the HOME Nearfield experiment which will take place in October 2002.
Analysis of HOME Survey data is continuing. Future goals include a careful accounting of the energy budget for Tides-to-Turbulence processing on the Hawaiian Ridge.