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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, calledTwaves. We demonstrate here that Comprehensive Nuclear‐Test‐Ban Treaty Organization (CTBTO) hydrophones can recordTwaves with a higher signal‐to‐noise ratio compared to a previously used land‐basedT‐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 ofTwaves at the land‐basedT‐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. 

Abstract 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. TheseTwaves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, aTwave'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. 

Estimating the large-scale variability and trends in subsurface ocean temperatures is limited by sparse in situ observations inadequate for resolving mesoscale eddies. Travel times of seismically generated sound waves, sensitive to path-integrated temperature, provide complementary integral constraints. We here use earthquakes along the Japan Trench and receivers at Wake Island to sample the Kuroshio Extension region in the Northwest Pacific. We develop a Gaussian process framework, optimized via maximum likelihood, to estimate temperature anomalies and uncertainties from this seismic data and to combine it with in situ data from Argo profiles and shipboard data. This framework shows seismic measurements are quantitatively consistent with in situ data and substantially reduce uncertainties in large-scale variability and trends. Relative to their prior, error variances of area-mean temperature fluctuations due to mesoscale eddies from 2008 to 2021 are reduced by 30% by the in situ data, 39% by the seismic data and 50% by the combination. For path-mean estimates, the combined reduction is 83% in error variances, compared to 45% from in situ data alone. The data show a steady subsurface warming of 11.8±5.0 mK/yr (2σ uncertainty) from 2008 to 2021 and no substantial trend between 1997 and 2008. 

Near-surface measurements of meridional velocity suggest that wind forcing excites equatorial waves in the biweekly band in the Indian Ocean. The characteristics of these waves in the deep ocean are poorly constrained, and it is unclear how well models capture the deep variability. In this work, biweekly temperature variations in a few low vertical modes in the deep east Indian Ocean are observed using seismically generated sound waves. These so-called T waves are generated by earthquakes off Sumatra and received by a hydrophone station off Diego Garcia. Changes in their travel times reflect temperature-induced sound speed variations in the intervening ocean. Regression analysis indicates that these variations are caused by westward-propagating Yanai waves. A comparison between T-wave data and model output shows generally good consistency in biweekly variations dominated by the first three vertical modes, although the biweekly variance differs by up to a factor of 2 between the data and the models. A similar degree of discrepancy appears in the comparison between the models and deep mooring measurements. These results highlight the potential of using T-wave data to study biweekly Yanai waves in the deep equatorial ocean and to calibrate numerical simulations of the variability they cause. 

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.