Seismic ocean thermometry

Monitoring ocean heat uptake is crucial, because it is by far the largest sink of excess energy in the climate system. Yet, sampling the ocean with sufficient spatial and temporal resolution is challenging, because large-scale and long-term trends are often much weaker than small-scale and short-term fluctuations. Data collection was sparse before the implementation of the Argo program, and still few measurements reach below 2000 m depth. Here, we propose to augment the existing observing system with a novel and low-cost method that uses acoustic waves generated by natural earthquakes. As in previously employed acoustic thermometry, acoustic travel times are integrating and thus intrinsically averaging measurements of temperature anomalies along the wave's path. We apply this method to a 3000 km long section in the East Indian Ocean that was insonified by the 2005 Nias-Simeulue earthquake and its aftershock sequence. Over the period 2005-2016, we find a warming trend and seasonal fluctuations that match Argo data, demonstrating the feasibility and accuracy of the approach. This seismic thermometry has the potential to accurately measure large-scale ocean temperature change back to the 1990's or earlier, when modern global seismic data became available, and the very low frequency seismic waves (around 2Hz) have considerable sensitivity to the ocean below 2000 m depth. Our reliance on natural sources avoids any potential interference with marine life.