The HOME Farfield Experiment

A significant fraction of the energy lost from the barotropic tide is carried away from the Hawaiian Ridge by the radiation of low-mode internal tides. The exact amount of energy that might be carried by these waves is unknown at the moment; the estimates have varied considerably the past few years. Modeling provides one answer that appears to be quite accurate for the radiation in the nearfield. But it is an open question as to the amount of energy that entirely escapes the Hawaiian Ridge to be dissipated in the open ocean or on the opposite shores of the ocean basin. It may be that as the waves propagate from the Hawaiian Ridge they initially lose considerable energy in the first few wavelengths (few 100 km), before settling down to coherent waves.

You can see transparencies from a talk presented at the AGU 2004 Ocean Sciences Meeting entitled "Radiation of Mode-1 Internal Tides from Topographic Features" HERE. You can also see the poster presented at the 2006 AGU Ocean Sciences Meeting, Honolulu, HI: "Radiation of Internal Tides from the Hawaiian Ridge: Implications for the Large-Scale Tidal Energy Budget", (2.5 MB PDF File).

Mode-1 internal tides radiating from Hawaii derived from an objective map of TOPEX/POSEIDON (T/P) altimetry data (note the altimetry tracks). The bottom two panels show the initially assumed wavenumber spectrum, and final spectrum of the map. The black semicircle in those panels shows the theoretical wavenumber of mode-1 internal tides. Click on the image to see a movie of the mode-1 radiation (554KB animated gif file - compatible with all browsers).

The HOME Farfield experiment is designed to measure the internal-tide, low-mode energy that escapes into the farfield. These measurements were made a two key locations of the Ridge identified by modeling and satellite altimetry as has being locations of considerable internal-wave radiation. The locations of the measurements is shown on the image to right. Since there are not sufficient resources to measure at all points along the Ridge, the goal is to use the measurements at these two locations to calibrate or test numerical models of the radiation. Once the models are "tuned up" so that we are confident in their accuracy, the model results may be integrated to obtain an energy estimate for the entire Ridge.

The measurements consisted of line-averaging tomography indicated on the figure, together with thermistor's located on the tomography moorings. The combination of line-integral and point measurements provides a balance of spatial coverage and point measurements that best resolves the internal waves, given the limited resources [not meant to be a complaint about the resources, merely to state the obvious that, it is impossible to obtain all the data one would want]. The scale of the array is ideal for detecting the mode-1 waves that have 150-km wavelength. In addition, hydrographic casts were obtained from R/V FLIP for one month near the southern array, and the combination of these data and the thermistor data give an indication of the incoherent and high-mode content of the internal-tide radiation.