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IWAP

Internal Waves Across the Pacific (IWAP), a collaborative NSF project with Drs. MacKinnon, Winters, Pinkel, and Munk (Scripps), aims to understand the distribution of mixing by long-range-propagating internal waves. These waves, which I call "internal swell" in analogy with surface waves that break on beaches, arise from the wind and the tides.

The latter kind, the "internal tide", deflects the ocean's layers up to 100 meters vertically, and consumes about the same amount of power as all of humankind's use of electricity. The geography of where this energy is dissipated in the ocean has profound implications for climate change and the Earth's large-scale current system.

A large internal tide forms as the lunar tide flows back and forth past the Hawaiian Ridge. It then travels northward at least 1500 km from there (figure 1), and can be seen from space as it does so. The Hawaii Ocean Mixing Experiment (HOME) has sought to understand the generation, propagation and dissipation of the internal tide near the Ridge.

With this project we have tracked the long-range propagation of the internal tide northbound from Hawaii, and hope to understand where it breaks. To understand the structure and energy loss of the waves as they propagate, we deployed over 15 km of wire and synthetic line upon which 6 robotic McLane Moored Profilers (figure 2) crawled up and down, in a long line north from Hawaii.

We also used a Seasoar, a lowered ADCP/CTD and a new fast-profiling CTD to understand the waves' structure. This field work will take place in two cruises during summer 2006. In the meantime, we will be modeling and analyzing data from historical moorings in order to better focus the experiment.

Models predicted a catastrophic loss of energy at latitude 28.8N due to a nonlinear interaction called parametric subharmonic instability (PSI). One important finding is that while the internal tide clearly survives its crossing of 28.8N, the signature of PSI is clearly visible at 28.8N (Figure 3).



 Figure 1: Bottom depth (color) and energy-flux vectors from three sources: a POM model, altimetry, and historical moorings. The line and ship track overlie a ray path and altimetric satellite track (black) emanating from the Hawaiian ridge. Measured fluxes (red arrows) and upper-ocean velocities measured from the ship (right, blue and red) show the internal tide propagating northward for over 1600 km.
 

Figure 2: Matthew Alford at a recovery of a McLane Moored Profiler. This newly operational instrument is capable of making repeated vertical traverses of the water column along a conventional subsurface mooring wire while carrying a CTD and an acoustic current meter.

  
Figure 3: Time series of meridional shear (colors; scale at lower left) and isopycnal depth (black; plotted every 50 m) at 30.5N (a), 28.8N (b), and 26.5N (c; see Figure 1 for locations). Each is 5.25 days long and plotted versus elapsed time. Smoother isopycnal displacements for the first (a) or last 1.25 days (b,c) of each time series are from the more coarsely sampled LADCP/CTD. Dashed lines indicate successive inertial periods at each latitude.





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