Zhan Zhongwen

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Last Name
Zhan
First Name
Zhongwen
ORCID
0000-0002-5586-2607

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  • Article
    Vertical‐slice ocean tomography with seismic waves
    (American Geophysical Union, 2023-04-15) Callies, Jörn ; Wu, Wenbo ; Peng, Shirui ; Zhan, Zhongwen
    Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.
  • Article
    Seismic ocean thermometry using CTBTO hydrophones
    (American Geophysical Union, 2023-09-08) Wu, Wenbo ; Shen, Zhichao ; Peng, Shirui ; Zhan, Zhongwen ; Callies, Jorn
    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.
  • Article
    Seismic ocean thermometry of the Kuroshio Extension region
    (American Geophysical Union, 2024-02-24) Peng, Shirui ; Callies, Jorn ; Wu, Wenbo ; Zhan, Zhongwen
    Seismic ocean thermometry uses sound waves generated by repeating earthquakes to measure temperature change in the deep ocean. In this study, waves generated by earthquakes along the Japan Trench and received at Wake Island are used to constrain temperature variations in the Kuroshio Extension region. This region is characterized by energetic mesoscale eddies and large decadal variability, posing a challenging sampling problem for conventional ocean observations. The seismic measurements are obtained from a hydrophone station off and a seismic station on Wake Island, with the seismic station's digital record reaching back to 1997. These measurements are combined in an inversion for the time and azimuth dependence of the range-averaged deep temperatures, revealing lateral and temporal variations due to Kuroshio Extension meanders, mesoscale eddies, and decadal water mass displacements. These results highlight the potential of seismic ocean thermometry for better constraining the variability and trends in deep-ocean temperatures. By overcoming the aliasing problem of point measurements, these measurements complement existing ship- and float-based hydrographic measurements.