HOME Nearfield

Goal

The goal of the Nearfield Program is to observe mixing processes on the Hawaiian Ridge in sufficient detail that the dynamics can be determined. The site of the experiment will be selected based on the findings of the Survey. In conjuction with the Modeling program, Nearfield data will be assimilated into models to assist in generalizing local results to the entire Ridge and the global ocean.

Schematic of the Deep, Slope, and Ridge moorings and the R.P. FLIP

Objectives

Given the Nearfield goal, specific objectives include:

Documenting spatial and temporal patterns in kinetic energy, shear variance, buoyancy frequency (strain), and turbulent dissipation, as a function of distance from potential topographic sources and of semi-diurnal and fortnightly phase. Survey findings can be extended with higher spatial and temporal resolution.
Identifying the larger scale energy sources that drive the fine scale fields. Are there threshold values for the driving forces, above which mixing levels rapidly increase?
Quantifying mixing rates and developing parameterizations which accurately describe mixing variability. With the support of models, we wish to extrapolate results along the Ridge.
Identifying those aspects of the internal tide that are coherent with astronomical forcing and comparing energy and modal structure between coherent and incoherent components.
Documenting bottom boundary layer formation and the penetration of mixed water into the ocean interior.
Identifying the fine scale internal waves which actually "break". Are these waves driven by the scattering of the barotropic tide? Do they have a tidal intrinsic frequency, as linear scattering theory would predict. Are they lee waves, driven by tidal or messoscale flows?
Quantifying the internal wave modal distribution in the Nearfield for comparison with corresponding Farfield observations. Is there an apparent temporal modulation in high-mode energy levels? If all of the high-mode energy were dissipated on a fortnightly basis, is the energy loss comparable to the observed dissipation rate?
Comparing observed ray-like behavior of the internal tide with model predictions. The predicted distance that rays penetrate into the ocean interior depends strongly on the parameterization of dissipation in the model.

Observational Plan

To achieve the Nearfield objectives, a comprehensive process oriented experiment is being planned. We anticipate observing processes such as tidal scattering (2-D and 3-D), lee wave formation, and bottom boundary mixing. The Nearfield Experiment is designed with the flexibility to be sited anywhere east of 165°W along the Ridge. The experiment domain will extend from the Ridge crest (0-500 m water depth) to the deep sea (about 40 km off-crest), with topography favorable for the study of the selected mixing processes both in the upper ocean and the abyss. The upper regions of the domain are accessible to the full range of HOME sensors. Specialized deep-sampling instruments, working off-crest, can determine the depth variability of mixing processes and rates.