Deployment Notes: Antofagasta Coast, Chile - October 2021
The wind was ripping across the deck as we prepped the tripod frames, that relentless southwesterly breeze that defines the Atacama coast. We hit the water just off the Antofagasta shoreline, where the seafloor drops off into a jagged mess of submarine canyons. It is a chaotic environment. The water looks deceptively calm on the surface, but underneath, the Humboldt Current System is doing something violent. We were positioning the gear right in the teeth of an upwelling cell, where the cold, nutrient-heavy South Pacific Central Water surges upward toward the euphotic zone.
Monitoring this specific stretch of the Chilean coast is a nightmare if you rely on surface data. You can't just throw a buoy in and call it a day. The vertical shear here is extreme. I've spent years looking at these gradients, and Antofagasta is more aggressive than the Canary system. We are dealing with a vacuum effect; the Ekman transport pushes surface water offshore, and the deep water rushes in to fill the void. It creates a vertical conveyor belt that renders traditional current meters useless. If you aren't capturing the full water column, you're essentially guessing.
What We Found
The data came back exactly as I feared: the vertical velocity gradients were off the charts. We caught a moment where the surface current was pushing north at 0.3 m/s, while the water just 50 meters below was screaming in the opposite direction or surging upward. This isn't a steady flow. It's a collision. We saw massive volumes of cold water hitting the surface within a few kilometers of the shore, triggering a biological explosion. The biomass was so thick it actually messed with our acoustics.
We hit a wall with 'false bottoms.' The plankton blooms were so dense they created high-intensity backscatter layers that looked like the seafloor on the initial raw data. I had to run a rigorous sanity check against the CTD (Conductivity, Temperature, Depth) profiles to make sure we weren't misinterpreting a massive swarm of organic detritus as a boundary layer. Honestly, if we hadn't cross-referenced the temperature drops, we would have reported a shallowing seabed that didn't exist. The signal was incredibly noisy in the upper 30 meters, but once we filtered the biomass interference, the velocity profiles revealed a terrifying amount of energy moving vertically.
Equipment Performance
I insisted on the 300 kHz ADCP for this run, and it was the only right call. A 600 kHz unit would have lost too much energy to attenuation in that biomass-heavy soup, and 1200 kHz is far too shallow for the depths of these canyons. The 300 kHz gave us a clean signal through the full column, from the surface down to the benthic boundary. The bottom-mount configuration held up, though the tripod frame took a beating from the currents. We had a slight tilt on one unit (about 2 degrees), which usually ruins vertical velocity calculations, but since we were ground-truthing with CTD casts, we could correct the bias in post-processing. The battery life held steady, though the cold water of the HCS always makes me nervous about voltage drops during long deployments.
The real struggle was the bin contamination. Because the upwelling is so violent, we saw significant leakage between the acoustic bins. We had to tighten the blanking distance to avoid surface noise, but that meant we lost the top few meters of the shear layer. It's a trade-off. You either accept some noise or you lose the most active part of the water column. In this case, I'd rather have a noisy signal than a blind spot.
Recommendations for Future Deployments
If you're heading back to the 23.6°S coordinates, don't cut corners on the mounting hardware. The bathymetry is too unpredictable for light frames.
- Stick to 300 kHz transducers. Anything higher will choke on the plankton blooms common in the Atacama zone.
- Double-up on the tripod ballast. The vertical surge in these canyons can shift a light frame, ruining your tilt calibration.
- Schedule CTD profiles every 48 hours. You need the temperature data to distinguish between the actual bottom and biological false bottoms.
- Increase the sampling rate during peak upwelling events to capture the high-frequency oscillations of the HCS.
The interaction between the southwesterly winds and the jagged shelf creates a hydrodynamic environment that is uniquely volatile. You can't treat it like a standard coastal shelf. You have to respect the verticality of the system, or your models will be worthless.
Field report by Elena Rodriguez. Elena is a specialist in underwater acoustics and oceanographic instrumentation with a focus on high-shear coastal environments and sediment transport.
Field Deployment Report: Bottom-Mounted ADCPs in the Antofagasta Upwelling Cell