Small-Scale Process Studies
 | Oregon Shelf Tides |
One possible scenario for the demise of the low-mode internal tides is that they impinge upon continental shelves and
dissipate their energy via a variety of mechanisms, including
reflection at a near-critical angle. To understand this process, Drs. Nash, Kunze (UVic), and I
deployed a line of 6 moored profilers for 44 days, and performed many lowered ADCP/CTD stations and XBT drops spanning the Oregon
continental shelf (figure 1). Strong low-mode fluxes appear to impinge upon the area from deep-water
sources, and figure 2). We aim to understand the fate of
this energy, as well as a very strong and complex field of locally generated internal tide energy. With our estimates of energy flux at each site, and
estimating the mixing via several methods, we hope to obtain an energy
budget for the region, and thus determine the role of the continental
shelves in dissipating internal-tidal energy generated in the deep
ocean.
| Mixed-Layer Restratification | ^ TOP |
For several decades mixed layers studies have concentrated largely on understanding, quantifying, and modeling vertical processes. As a result much is known about the vertical processes within mixed layers, particularly the structure of turbulence, and models have become increasingly sophisticated to simulate these observations. Attempts to measure horizontal processes were sometimes made, but they were not as successful. Initially, this resulted from inadequate navigation, but it was also a consequence of horizontal processes having weaker signals due to their longer space and time scales. In spite of these obstacles, a growing body of evidence demonstrates that lateral processes significantly affect mixed layer properties, particularly in some places.
Understanding the role of horizontal processes requires dense, synoptic
3-D-plus-time measurements on the appropriate scales. To attempt to
resolve the evolution and 3-D structure of the mixed layer, and its
underlying dynamics, we went aboard R/V Wecoma to the North Pacific in
Feb/Mar 2004. By towing SWIMS II in patterns centered on a drogued
drifter, we were able to construct maps of mixed-layer properties on
10-km scales (figure 2). Structures were complex and evolution was
rapid, but Phil Hosgood, a postdoc who has since moved back to England, successfully disentangled the space-time evolution and managed to publish two papers (see publications).
| Fine-Scale Conditions Accompanying Overturns | ^ TOP |
Using a rapid-profiling CTD and downward-looking sonars, my Scripps thesis advisor Rob Pinkel and I measured the shear and strain of the upper 400 m every four minutes for a month. With this data we were able to examine the fine-scale conditions that led up to overturns, gravitationally unstable regions where dense water overlies less dense: an indicator that turbulence is working against the stratification. High shear, high strain (when density gradient is less than average because the water has been "stretched" by internal waves or other processes) and the presence of short-wavelength, high-frequency waves (indexed by the "strain rate") can all precede overturning.
| Nonlinear Internal Waves | ^ TOP |
A strong internal tide propagating westward in the South China Sea impinges upon the continental slope and forms NONLINEAR INTERNAL WAVES, very steep disturbances on the thermocline. These can have vertical velocities in excess of one knot, and displace isotherms downward 200 m or more over the course of a few minutes. They can thus be of import to biological process, nutrient transport, as well as to surface and submarine navigation. To understand these, we instrumented two profiling moorings at and inshore of the continental shelf break, combining forces with Rob Pinkel, Jody Klymak, Ren-Chieh Lien and Lou St. Laurent to determine the waves' energy budget and breaking mechanisms. The project is funded as part of the Office of Naval Research's Nonlinear Internal Waves initiative.
| Near-inertial Internal Waves and Mixing | ^ TOP |
In an expedition to the Banda Sea, in the Indonesian throughflow, data collected in 1998 by Mike Gregg provided an opportunity to better understand the mixing associated with one kind of wave, the so-called near-inertial wave. These waves are near the local inertial or Coriolis frequency f, which is twice the sine of the latitude: 2 cycles per day at the poles decreasing to zero at the equator. In midlatitudes, f is about one per day, close to tidal frequencies, but in the Banda Sea (6S) f is only one cycle per 4.4 days. Consequently, both the mixing (figure 5) and the kinematic interactions (figure 6) of the near-inertial waves could be studied. This particular group of near-inertial waves appeared to have been generated by strong monsoon winds about 3 weeks before, and led to about 75 percent of the mixing that was observed during the cruise.
Since, I have been working on understanding the potential importance of near-inertial waves to mixing the deep ocean. Observations of long coherent shear features that span hundreds of kilometers, together with the presence of a deep seasonal cycle to near-inertial energy, both suggest that these extremely energetic features of the ocean may have a climate story to tell. Stay tuned.
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Movie of a Near-Inertial Wave »