Deployment Notes: Monterey Bay, California - October 2023
The morning air was sharp and smelled of salt and diesel as we cleared the harbor. We hit the deployment site near 36.6° N just as the sun broke over the Santa Cruz mountains. Looking down into the water, the depth sounds were already screaming; the Monterey Submarine Canyon is a beast. One minute you're in relatively shallow coastal waters, and the next, the floor drops away into a vertical abyss. It's a hydrodynamic anomaly that makes standard coastal modeling a joke.
The surface was choppy, driven by those classic northwest winds that define the region's autumn. I could see the signs of Ekman transport in action—the surface water was being shoved offshore, leaving a void that the canyon's deep, nutrient-dense water was rushing to fill. The water state was volatile. We saw temperature fluctuations of nearly three degrees in the upper ten meters alone, a clear sign that the upwelling cell was firing on all cylinders.
What We Found
The data was wild. The most shocking part? The vertical shear was absolutely brutal. While the California Current was pushing surface waters south at a steady clip, the deeper layers—driven by the canyon's unique topography—were moving in completely different directions. We caught these high-velocity jets hugging the canyon walls that most generic surveys completely miss. These aren't just anomalies; they're the engines driving the bay's legendary biological productivity. If you aren't looking for them, you're missing the whole story.
I've spent time deploying sensors in the canyons off Peru, and the physics here are a mirror image. The water doesn't move as one block. It's layered, fractured, and chaotic. We hit intense salinity gradients that acted like acoustic mirrors. In some bins, the refractive environment was so severe that I had to double-check our sound speed profiles just to make sure the velocity calculations weren't drifting. It's a reminder that in a canyon environment, a standard speed of sound assumption is a recipe for bad data.
Equipment Performance
We ran a 300 kHz ADCP for this bottom-mounted stint. Honestly, it was the only right call. A 600 kHz unit would have been blind to the full vertical extent of the upwelling cell, but the 300 kHz gave us the reach we needed to see the transition from the canyon floor to the surface. However, the organic load was a nightmare. Monterey is currently in the middle of a massive phytoplankton bloom. The water was thick with biological particles, and for three days, we dealt with incredibly noisy data. I saw signal-to-noise ratios (SNR) plummet every time a swarm of krill drifted through the acoustic bins. It looked like sensor failure at first glance, but it was just the biology getting in the way. I had to get aggressive with the signal fence settings to scrub the noise without killing the actual velocity data. Also, we had a close call with side-lobe interference. We placed one unit slightly too close to a steep canyon ledge, and the sensor started picking up echoes from the rock wall instead of the water column. It's a classic mistake, but it nearly ruined a week of data.
Recommendations for Future Deployments
If you're heading into the Monterey Canyon, don't wing it. The bathymetry is too aggressive for a 'set it and forget it' approach. You need to ground-truth your depth readings before the ADCP hits the bottom to avoid wall interference.
- Use 300 kHz units for any deployment deeper than 200m to ensure you capture the full upwelling profile.
- Tighten your signal fence settings during bloom seasons to filter out biological noise (krill/plankton).
- Perform a manual sound speed cast every 48 hours to account for the extreme temperature and salinity gradients.
- Avoid placing sensors within 50 meters of a steep slope to prevent side-lobe contamination from the canyon walls.
- Double-check your mooring tension; the high-velocity jets along the canyon walls can cause significant sensor tilt.
The interplay between the California Current and the canyon's internal circulation is a complex dance. You can't just drop a sensor and hope for the best. You have to fight for a clean signal in this environment. Between the biological noise and the refractive layers, it's one of the toughest places to get a reliable profile, but it's the only way to understand why this patch of ocean is so alive.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with two decades of experience profiling deep-sea canyons and continental shelves.
Field Deployment Report: Velocity Profiling in the Monterey Submarine Canyon