Researchers at the Nansen Center have sent sound signals across the Arctic Ocean, over distances comparable to traveling from Bergen to Lisbon by plane. The experiment shows that acoustic measurements can provide precise, year‑round observations of temperature variations under the sea ice.
To understand and document the rapid changes in the Arctic, good and continuous observations are essential. But large parts of the ocean are hidden under ice most of the year. That makes it very difficult to measure changes in temperature and ocean currents. Traditionally, scientists rely on point measurements using instruments that are either anchored to the seafloor or drifting freely in the water. Satellites give good observations of the surface, but under sea ice we need other methods.
Acoustic thermometry as aid
In the CAATEX experiment, the researchers used an “acoustic thermometer.” By sending sound signals through the water, they measured how fast the sound traveled – a speed that changes with the temperature of the ocean. Acoustic thermometers give precise measurements of ocean temperatures over very long distances. These large‑scale measurements complement traditional point measurements extremely well.
Sound waves can be detected 2,600 kilometers from the source
From 2019 to 2020, six bottom‑anchored moorings were placed in the Arctic Ocean; three north of Svalbard and three north of Alaska. Each mooring was roughly 4,000 meters long from seafloor to surface and carried 40 instruments measuring different ocean conditions at different depths. What made the CAATEX experiment unique was that two of the moorings had their own sound sources. Every third day, these sent out signals at around 35 Hz, which were picked up by the instruments on the other moorings. The signals could be detected more than 2,600 kilometers away from a sound source north of Svalbard.
Large variations through the year
The measurements taken across 2,600 kilometers take around half an hour of travel time. In that time, the ocean changes very little, which means the data gives an almost instantaneous snapshot of ocean temperature. At the same time, the measurements show that sound speed in the ocean under sea ice varies significantly throughout the year. These variations reveal changes in the ocean that no other observations capture.
A first step, temperature calculations come next
A recently published study focuses on how fast sound travels through the Arctic Ocean. The next step is to use these measurements to calculate the average sound speed at different depths across the Arctic. That will also allow researchers to estimate ocean temperature.
A method for the future
Using sound to measure temperature gives researchers something they have long lacked in the Arctic: stable and precise temperature measurements across large ocean areas, all year round. Once the temperature data is ready, it will give important insights into how the entire Arctic Ocean changes through the seasons and from year to year. Such large‑scale measurements are important for climate research, both to monitor climate change and to validate and improve model systems. To capture the slow changes in climate, the measurements must be repeated. It is now five years since CAATEX, and researchers at the Nansen Center are working with international partners to secure funding to repeat the measurements.
Key researchers: Hanne Sagen, Espen Storheim, Stein Sandven
