Seismic Ocean Thermometry Using CTBTO Hydrophones
Abstract
Due to limited observational coverage, monitoring the warming of the global ocean, especially the deep ocean, remains a challenging sampling problem. Seismic ocean thermometry (SOT) complements existing point measurements by inferring large‐scale averaged ocean temperature changes using the sound waves generated by submarine earthquakes, called T waves. We demonstrate here that Comprehensive Nuclear‐Test‐Ban Treaty Organization (CTBTO) hydrophones can record T waves with a higher signal‐to‐noise ratio compared to a previously used land‐based T‐wave station. This allows us to use small earthquakes (magnitude <4.0), which occur much more frequently than large events, dramatically improving the resulting temporal resolution of SOT. We also find that the travel time changes of T waves at the land‐based T‐wave station and the CTBTO hydrophone show small but systematic differences, although the two stations are only about 20 km apart. We attribute this feature to their different acoustic mode components sampling different parts of the ocean. Applying SOT to two CTBTO hydrophones in the East Indian Ocean reveals signals from decadal warming, seasonal variations, and mesoscale eddies, some of which are missing or underestimated in previously available temperature reconstructions. This application demonstrates the great advantage of hydrophone stations for global SOT, especially in regions with a low seismicity level.
Copyright and License
© 2023. American Geophysical Union.
Acknowledgement
This material is based upon work supported by the Resnick Sustainability Institute and by the National Science Foundation under Grants OCE-2023161 and OCE-2241663. We thank two anonymous reviewers, whose comments have helped improve this manuscript.
Data Availability
The IMS hydrophone data are available directly from the CTBTO upon request and signing a confidentiality agreement to access the virtual Data Exploitation Centre (vDEC; https://www.ctbto.org/resources/for-researchers-experts/vdec). All seismic data were downloaded through IRIS Data Management Center https://ds.iris.edu/ds/nodes/dmc/, including the seismic networks II (GSN; https://doi.org/10.7914/SN/II), MY, PS, GE (https://doi.org/10.14470/TR560404). Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS) and the National Science Foundation (NSF), under Cooperative Agreement EAR-1261681. Bathymetry data were downloaded from and freely available at https://download.gebco.net/. The sediment thickness data were downloaded from https://ngdc.noaa.gov/mgg/sedthick/ (Straume et al., 2019). The Argo data were downloaded from https://sio-argo.ucsd.edu/RG_Climatology.html (Argo, 2000). Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System. The ECCO data were available at https://podaac.jpl.nasa.gov/dataset/ECCO_L4_TEMP_SALINITY_05DEG_DAILY_V4R4 (Fenty & Wang, 2020). The processing code is available at https://github.com/joernc/SOT (joernc et al., 2023).
Conflict of Interest
The views expressed in the paper are those of the authors and do not necessarily represent those of the CTBTO.
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Additional details
- ISSN
- 2169-9356
- Resnick Sustainability Institute
- National Science Foundation
- OCE‐2023161
- National Science Foundation
- OCE‐2241663
- Caltech groups
- Division of Geological and Planetary Sciences, Seismological Laboratory, Resnick Sustainability Institute