Nuclear test ban treaty hydrophones help monitor ocean temperatures

Newswise —

During the Seismological Society of America (SSA)’s 2023 Annual Meeting, it was suggested that the ocean-based hydrophones integrated into the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO)’s seismic-acoustic monitoring network could offer improved insights into the long-term trends of ocean temperature changes.

The researchers emphasized the significance of discovering novel methods for monitoring ocean temperatures, as it is essential for calculating the pace of ocean warming, sea level increase, and climate-induced ocean circulation patterns amidst the ongoing escalation of average global temperatures.

Seismic ocean thermometry is a field that utilizes T waves, the sound waves created by earthquakes, which travel faster between hydrophones and their source when the water temperature is warmer. By examining the T wave data, researchers can deduce the broader average changes in ocean temperature over time. This technique has been utilized to estimate large-scale ocean temperature changes.

Due to the fact that T waves are initially generated as seismic waves in the earth's crust and are later transformed into acoustic waves that propagate through water, it can be challenging for land-based coastal stations to detect them. To address this issue, Wenbo Wu from the Woods Hole Oceanographic Institution and fellow researchers explored the feasibility of using CTBTO hydrophones to obtain a more distinct T wave signal.

By utilizing two CTBTO hydrophones situated in the Indian Ocean, the researchers were able to record T waves generated by earthquakes with magnitudes of 4 or lower. These recordings were subsequently examined to track the warming of the ocean over the course of decades, detect seasonal changes in ocean temperature, and identify mesoscale eddies, vigorous ocean circulation patterns. According to Wu, these findings were made possible by analyzing the T wave data obtained through the CTBTO hydrophones.

Previous studies in seismic ocean thermometry have typically relied on active source seismic signals, which are generated by human-induced blasts or hammering onto the ocean floor. Wenbo Wu highlighted that the primary difference in their research is the use of natural earthquakes to measure T waves, as opposed to the utilization of active source signals in previous studies.

As Wenbo Wu explained, hydrophones are already employed to monitor submarine earthquakes due to their ability to accurately record acoustic waves in the ocean. However, land-based stations are generally not as effective in recording T waves since the acoustic waves tend to dissipate as they propagate into the continents.

With more T waves to analyze, researchers can better pinpoint the timing of ocean temperature changes, said Wu.

Wenbo Wu also suggested that this technique could be particularly advantageous in areas where seismic activity is typically low, such as the Atlantic Ocean, by increasing the total number of usable earthquake sources.

Furthermore, Wu and his team examined how T waves of varying frequencies travel at distinct depths in the ocean, allowing them to monitor temperature shifts over time throughout the water column. This approach enables the production of a "vertical slice tomography," providing insight into the temperature characteristics of various layers within the ocean.

Wu emphasized that seismic ocean thermometry is a complementary method to other techniques employed by scientists to monitor changes in ocean temperatures, such as the use of autonomous Argo floats and hydrographic surveys conducted by ships. Each of these methods has its limitations; for instance, Argo floats primarily measure temperatures at shallow depths, and expensive surveys can only capture changes in one part of the ocean during a specific period.

The T wave data collected by hydrophones, as examined by Wu and their team, could help broaden the overall collection of temperature change data in the oceans and provide a more detailed examination of smaller regional characteristics as well.