After several days of exploring nearby sites, we’ve turned attention to a small hill on the side of the continental slope where computer simulations have suggested that large internal waves and energetic turbulence would be found. Using the computer controlled guidance system in the ship, we’ve taken a number of repeated transects across the top of this hill, watching the waves and turbulence evolve 1900 meters below the somewhat angry sea surface. Guided by an acoustic altimeter, our Fast-CTD is profiling to within 20 m of the sea floor. We can repeat profiles between 1500 and 2000 m every 8 minutes.
The view has been worth the price of admission: We are seeing large rhythmic internal wave crests radiating downward every tidal period, with near-vertical breaking fronts (color images, left below) almost 200 m tall. The actual turbulent regions associated with the breaking propagate downward also, at a fixed phase of the wave. Determining why the waves break where they do is one of the goals of the experiment. These data provide a great clue.
Figure 1. The Fast CTD real-time data display showing three crossings of the hill. The downward propagation of the wave crests is best seen in the center section of the left color panel. In the beginning and end segments, the crests are still propagating downward, but the Revelle is simultaneously driving “up-slope”, so the combined effect doesn’t show the motion as well. Breaking sections of the wave are indicated by the red dots in the upper-right panel. Check-out http://santvn.ucsd.edu/FCTD/ to see the Fast CTD data come in in real time.
Our Bi Hemispherical (?) team of veterans and volunteers has done a great job of keeping the Fast CTD running 24-7 since the initial deployment. We’ve taken over 1400 CTD profiles so far. We’re in the sweet spot in the cruise right now, with the gear midway between “dialed in” and “worn out”.
Next, we’ll move on to the shelf for a day and take a 24-hour record of ocean density, salinity, and suspended sediment data for Nicole Jones, Drew Lucas, and the T-SHELF team. Then it’s back out to deeper water to see what happens when the waves generated at this hill crash into the upper continental slope.
Figure 3. Proof: It’s still up there! A nice sunset on February 12.