Evaluating Doppler Shift Accuracy Amidst Atlantic Swell Interference in the Azores Archipelago Ports

Evaluating Mesoscale Eddy Influence on Port Basin Circulation in the Azores

Water velocity fluctuations in the ports of Ponta Delgada and Horta often exhibit erratic spikes of 0.3 to 0.7 m/s that defy standard tidal predictions. These aren't typical tidal surges. They are the result of the Azores Current's interaction with the steep volcanic bathymetry of the archipelago. When high-energy Atlantic swells hit the submerged flanks of these islands, they trigger localized vertical mixing and complex eddies that penetrate the harbor basins. This creates a chaotic velocity profile that makes standard surface-based current monitoring useless. Measuring these currents requires an understanding of the internal wave energy crossing the Mid-Atlantic Ridge. The salinity gradients here are relatively stable, but the temperature fluctuations during the transition from the boreal summer to autumn create thermoclines that can bend acoustic beams. If you don't account for the sound velocity profile (SVP) in these specific port geometries, your depth bins will be shifted. I have seen data where the bottom-track was off by several meters simply because the technician relied on a default sound speed of 1500 m/s instead of performing a real-time CTD cast.

The Bathymetric Constraints of the Horta Harbor Basin

Located at approximately 38.53°N, 28.63°W, the Horta harbor on Faial Island sits in a precarious position relative to the deep ocean. The seabed drops precipitously just outside the breakwaters. We see depth contours plummeting from 20 meters to over 1,000 meters within a very short horizontal distance. This creates a 'funnel effect' where deep-water currents are forced upward as they encounter the volcanic shelf, injecting high-momentum water into the port's perimeter. This steep slope introduces significant 'bin contamination' for bottom-mounted ADCPs. Because the seafloor is not flat, the acoustic return from the bottom track often hits an angled basalt surface. This reflects the signal away from the transducer, leading to 'noisy data' or total signal loss in the lowest bins. In my experience, you cannot trust the bottom-most 10% of the water column data in these volcanic ports without a rigorous sanity check against a fixed mooring reference.

Acoustic Propagation Challenges in the Azores Atlantic Interface

The primary headache in the Azores isn't turbidity—these waters are remarkably clear compared to estuarine environments. The problem is the aeration caused by heavy Atlantic swells. When large waves break against the harbor walls, they inject millions of micro-bubbles into the upper 5 meters of the water column. Air is a terrible conductor of sound. These bubbles scatter the acoustic signal, creating 'blind zones' where the ADCP cannot lock onto a target. I've noticed that during winter storms, the signal-to-noise ratio (SNR) in the upper bins drops precipitously. You get these jagged gaps in the time-series data. Some engineers try to fill these gaps with linear interpolation. That is a mistake. Interpolation hides the actual turbulence of the storm surge. It is better to flag the data as 'bad' than to pretend the current was steady when the sensor was actually blinded by bubbles.

Frequency Selection for Volcanic Port Deployment

Choosing between 300 kHz and 600 kHz for these ports is a trade-off between range and resolution. For the deeper channels near the port entrances, 300 kHz is the only viable option if you want to capture the full water column. However, the 600 kHz units provide much tighter bin resolution. In the smaller fishing berths of São Miguel, where depths rarely exceed 30 meters, the 600 kHz unit outperformed the 300 kHz in every metric. It gave us the granularity needed to see the shear layers near the seabed. I generally recommend a 600 kHz deployment for internal basin monitoring. The higher frequency allows for smaller bin sizes (e.g., 0.25m), which is critical when you are trying to identify the exact depth where the swell-driven current dies out. If you use a 300 kHz unit in a 20-meter basin, your vertical resolution is too coarse. You end up averaging the current over a meter of water, which smears the data and hides the very turbulence you are trying to measure.

Data Interpretation and Field Findings

When we analyze the return signals from the Azores, the 'correlation' value is the first thing I check. In these ports, a correlation below 60% usually indicates that the acoustic backscatter is too weak or too chaotic. We often see a pattern where current velocities increase sharply during the ebb tide, but the correlation drops. This suggests that the retreating tide is pulling sediment or organic debris from the harbor floor, increasing the 'noise' in the signal. One interesting finding was the presence of residual currents that persisted long after the tidal peak. In a 'clean' system, the current should drop to near zero at slack tide. In the Azores ports, we frequently see 0.1 to 0.2 m/s residuals. This is the 'signature' of the oceanic eddies pushing water into the harbor. It proves that the ports are not closed systems but are dynamically linked to the North Atlantic Current. If you ignore these residuals, your hydrodynamic models will never converge.

Operational Implications for Port Management

These current measurements have direct consequences for the docking of cruise ships in Ponta Delgada. Large vessels have massive windage and a deep draft. When a residual current of 0.5 m/s hits the bow at an angle, it creates a significant lateral drift. Tugboat captains need real-time data to counteract this. Relying on a static tide table is dangerous. They need to know the actual flow velocity in the channel right now. Furthermore, the sediment transport driven by these currents affects dredging schedules. The 'scouring' effect at the harbor mouth is more aggressive than expected. By mapping the high-velocity zones with an ADCP, port authorities can predict where siltation will occur. It is a simple application of physics: high-velocity zones keep the channel clear, while the eddies in the corners of the basin act as sediment traps. Monitoring these zones saves thousands of Euros in unnecessary dredging costs.

About the author: Dr. Kenji Sato. A specialist in underwater acoustics with 20 years of experience designing instrumentation for extreme marine environments. He has led numerous field campaigns across the Atlantic and Pacific focusing on high-resolution flow dynamics.