General circulation models (e.g. Samelson, 1998) indicate that the thermohaline circulation is highly sensitive to the global magnitude and distribution of mixing. Recent observations (e.g. Polzin et al 1997) highlight the inhomogeneity of mixing. Therefore, a global map of mixing would benefit climate modeling, as well as our understanding of biological productivity and pollutant dispersal. Since global measurements of mixing are impractical, another way of estimating the global mixing distribution is to map the energy flux into internal waves, and then to measure their subsequent long-range propagation.
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
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).
Download PDF file: "Internal Waves Across the Pacific"
Download Tyler Hughen's movie (110 MB; mp4a).
Also check out the recent article on this project on Discovery Channel News.
The first step is to map the distribution of the two primary internal-wave sources, the wind, which generates near-inertial waves, and surface-tidal flow over topography (Egbert and Ray, 2000), leading to internal tides. The wind-flux portion is calculated (Alford, 2001) by using the NCEP/NCAR reanalysis wind fields, which incorporate observations over many years and locations into a dynamically consistent framework, to drive a simple model of the mixed-layer response. The energy flux is then given by the scalar product of the wind stress and the mixed-layer current. Ongoing work involves examining the high-latitude dependence of the fluxes (as the response gets faster at high latitude, the NCEP winds become more and more inadequate for the job), and the interannual variability of the fluxes.
The global distribution of the energy flux available for internal waves is plotted below for the wind (top) and the tides (bottom, courtesy of G. Egbert). The greatest wind inputs occur at midlatitudes during wintertime, associated with travelling storms. Large tidal inputs (red regions) occur where the surface tide flows perpendicular to rough bottom features.
Download PDF file: "Improved Global Maps and 54-year History of Wind-Work on Ocean Inertial Motions"