From a Stopwatch to Sound Waves: How Ocean Currents Are Measured

The first way to measure an ocean current needed just three things: a person, something that floats and a way to keep time. The principle has not changed much since. What has changed is the equipment — from a timed drifter alongside a ship to instruments that listen to the water and radar that watches it from shore.

Crew from the NOAA Ship Rainier use an Acoustic Doppler Current Profiler in a current survey of waters near Sitka, Alaska. Credit: NOAA.

The original method

An observer stands on a ship, throws a floating object — a drifter — into the water, and times how long it takes to travel along the ship's side. As technology improved, oceanographers turned to mechanical current meters: a ship would put a meter in the water, where a rotor turned with the current and measured it. That is still the basic process, done now with more accurate and sophisticated instruments.

Drifters today

In the open ocean, a modern drifter is something like a buoy, and it may carry GPS or satellite communications to relay its data. Some drifters can dive and stay down for long periods to measure currents at a particular depth, surfacing now and then to send their data and position to observers on land.

Listening to the current

The Acoustic Doppler Current Profiler is one of the most common tools. It sits on the seafloor or is attached to the bottom of a boat, and it sends a sound signal into the water column, where it bounces off particles. From the frequency of the returning signal, the distance it traveled and the time it took, the instrument works out the current's speed and direction.

Watching from shore

Many oceanographers also use radio antennas and high-frequency radar to measure currents at the surface. Like the acoustic profiler, these shore-based instruments rely on the Doppler effect to tell whether currents are moving toward or away from the shore, or how fast they are moving.

Measured in knots

At NOAA, current speed is measured in knots. One knot is one nautical mile per hour. One nautical mile equals 1.85 kilometers, and one knot is also 51.44 centimeters per second.

Survey results like these improve tidal current predictions and support new operational forecast models.

Sources

Based on "How do we monitor currents?" from NOAA's National Ocean Service, a work of the United States government in the public domain.