Jim Thomson is principal oceanographer at the Applied Physics Lab at the University of Washington. He studies ocean surface waves and coastal processes and wrote about his expedition to the North Pacific in the fall.
Monday, Feb. 11
This project wasn?t supposed to be about waves; it was supposed to be about tides. With the wind blowing 30 knots and whitecaps spanning the horizon, both were in sharp focus today as we deployed our mooring in Chile?s Canal de Chacao (i.e., Plan A).
The rough water at the surface was obvious enough, but the fast water below was more subtle. We planned our deployment for the time of slack water ? the brief moment of stillness between the flood and ebb currents ? so that the mooring (and the boat) would not be carried away from the mark as we struggled to get 3,500 pounds of gear off the deck.
The captain taught me the Spanish word for slack tide (?estoba?), and a local fisherman explained that it happens a bit early at the site we wanted. He was right, and we were ready.
We started downstream of the mark and shuffled equipment off the deck until the mooring was trailing behind us. As the boat neared the site, we moved the final piece, the anchor, to the edge of the deck.
Passing over the site, we used the hydraulic winches to move the anchor up and out ? then released it. When we release anchors like this, I often perceive that it pauses for a moment, then dives to the depths and takes the instruments with it. Today, there was no pause. The drop was swift and true, and the deployment was sharp. I like to think of it as an air drop by a bush pilot: you might get only one pass, so make it good.
The mooring will stay there, beneath the surface, for just a few days. Our Chilean colleagues already have a long-term record of the tidal currents; we are here for the turbulence. That data accumulates at several gigabytes per day, so a few days is all we can manage (and all we need, really).
The measurements are state of the art, yet still limited in many ways. The fundamental problem is that turbulence is a four-dimensional phenomenon, varying in space and time, and a measurement from a single location is incomplete. Think of standing in the center of a mosh pit at a concert and trying to catalog all of the people who pass by ? the answer might depend on where you stood, for how long and how fast you were counting. And whether you could stay standing.
G. I. Taylor, a central figure in the discipline of fluid mechanics, postulated that one could swap time for space at a fixed position, if the turbulence did not evolve much. But it does evolve. Worse, our fixed position is not that fixed; the mooring dances in the currents like a kite on a string. It has trouble standing.
Our biggest challenge is to remove the contamination of motion from the data. That is a matter of precision and calculation. The time-space convolution, and the coherence of the turbulence, are the much bigger question, at which we can only chip away.
Now we are safe at harbor in Carelmapu, a fishing village near the outer edge of Canal de Chacao. Tomorrow we will begin a survey of the spatial patterns in the currents, using equipment mounted to the side of the ship.
In a few days we will signal the mooring to come to the surface, and we?ll recover it. We?ll find out if it stayed upright, and we?ll find out what it saw.
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