Seismic Ocean Thermometry of the Kuroshio Extension Region

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. The transfer of excess heat from the surface to the deep ocean is crucial in determining how rapidly global warming progresses. Despite progress, tracking the warming of the deep ocean remains an observational challenge because natural temperature variations can obscure the warming signal. These variations are especially intense on the western margin of ocean basins, where strong currents bring warm subtropical water poleward. These currents meander, shed eddies, and can switch between states every few years, all processes that lead to large local warming or cooling. This study helps alleviate the challenge to distinguish between these natural fluctuations and the forced signal by measuring deep ocean temperature using sound waves generated by earthquakes. We measure the travel time of these waves between Japan and Wake Island in historical seismic and acoustic records going back to the late 1990s, sampling one such current system. Because the waves travel faster in warmer water, they arrive slightly earlier if warming has occurred along their path. We measure such changes in arrival time for different travel paths that are sensitive to different parts of the current system, so warming on one side of the current can be distinguished from warming in the other side. Seismic T waves generated off Japan and received at Wake Island sample the Kuroshio Extension region Travel time changes between repeating earthquakes along the Japan Trench constrain the azimuth-dependent temperature change between events The data constrain deep temperature changes due to the meandering current, mesoscale eddies, and water mass displacements back to the 1990s