That’s a weird thermometer! Using changes in the travel time of sound to monitor changes in ocean temperatures

Each summer, the University of Rhode Island Graduate School of Oceanography (GSO) hosts undergraduate students from all over the country to participate in oceanographic research. These Summer Undergraduate Research Fellows (SURFOs) have not only been working with GSO scientists, but they also have spent part of their time learning how to communicate this science to the public. Read on to find out what they have been up to, and why everyone should be as excited as they are about their work.

Olivia is a junior at St. Olaf college in Northfield, MN where she studies physics and mathematics. She is from the central Minnesota area. When Olivia is not working on school work, she shares my time between many different clubs like Women in Physics, Water Polo, and Norseman Band to name a few. Another important part of Olivia’s life is taekwondo, a Korean martial art, where she is a 3rd degree black belt.

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Weather, coral reefs, and fisheries are just a few examples of systems that are sensitive to ocean temperature. However, ocean temperature is impacted by climate change because the ocean absorbs most of the extra heat trapped by greenhouse gases. So monitoring ocean temperature will give us great insight into how climate change is impacting the ocean temperature, and thus affecting systems that are sensitive to ocean temperature. 

Currently, we use thermometers and sea surface temperature satellites to measure ocean temperature. While thermometers provide accurate and discrete temperature data points, it is difficult to collect information from many locations or locations that are difficult to access. We would need a lot of time, money, and thermometers to cover every location of interest! Sea surface temperature satellites are also a great resource, but they can only measure temperatures from the surface ocean that are not covered in clouds, as they cannot see through clouds. Over the summer, I looked into an alternative monitoring system that bypasses these issues: changes in the travel time of sound through the ocean.

You may be wondering what sound, a type of wave, has to do with temperature. The speed of sound in the ocean is impacted by three aspects of the water: pressure, salinity, and temperature. We can find the depths at which the wave travels to calculate changes in pressure. And changes in the salinity (or saltiness) of the water are very small. So, this leaves temperature as the final culprit for changes in the travel time of sound in the ocean.

For my summer project, I used continuous ambient noise data from five hydrophones (underwater microphones, pictured below) off the coast of Washington and Oregon to analyze the changes in wave speeds of sound between the stations. I looked specifically at the waves with periods (time between wave crests) between one and ten seconds because they are known to be most sensitive to changes in temperature. Then I picked out the wave that takes the path between the stations and ignored waves that take different paths.  

Once I selected the waves that traveled between stations, I could find the travel time of those waves, as I knew the time when the wave arrived at each station. At that point, finding changes in wave speed is just a matter of stacking enough days worth of data.    

Developing this new system of monitoring changes in ocean temperatures will help us see changes in temperature on a multitude of time scales. While we benefit from many years of ocean temperature measurements to observe the impact of climate change on the ocean, changes in ocean temperature over short times is also important. For example, seasonal ocean temperature changes help us gain insight into temperature sensitive systems. In addition, using changes in the travel time of sound will make it easier to monitor changes in ocean temperatures in locations of interest that are difficult to reach, allowing us to better understand our oceans.

Jiwoon Park

I am a PhD student in chemical oceanography at University of Washington. I am studying how different forms of metals in the ocean are shaping microbial communities in the North Pacific Ocean. When not working, I like going for a walk, visiting farmers’ markets and playing keyboard.