ABSTRACT

Of the energy trapped on Earth by increasingly abundant greenhouse gases, over ninety percent is absorbed by the ocean. Monitoring the resulting ocean warming remains a challenging sampling problem, despite drastic improvements of the observing system over the past two decades. This project will complement the existing observations by inferring largescale and deep ocean temperature changes from sound waves that are generated by repeating natural earthquakes. These waves propagate across entire ocean basins, and changes in their travel time reflect changes in the average ocean temperature they encounter along their paths. Preliminary work has shown that the seismic ocean thermometry to be used constrains temperature changes averaged over a 2900 km long section in the equatorial East Indian Ocean with an accuracy of 0.007 K. This initial example of seismic ocean thermometry would be expanded into a broadly applicable method that harvests some of the abundant information on ocean warming that is generated every year by tens of thousands of shallow submarine earthquakes. This project will contribute to understanding of the ocean?s heat uptake and rate of transport to the deep ocean, that drives climate change. Measuring and understanding the patterns of heat uptake and its partitioning between the surface and deep ocean is crucial for improving projections of the climate?s trajectory in the coming decades and centuries. Furthermore, ocean warming contributes substantially to sea level rise, and the patterns of uptake imprint on regional sea level rise. The method of seismic thermometry has the potential to substantially enhance the existing observing system, and it could be operated at very low cost. In addition, the project will contribute to the development of the new generation of scientists through the support of a post-doctoral scholar and a graduate student.


Preliminary application of the seismic ocean thermometry over a 2900 km long section in the equatorial East Indian Ocean using data from the period 2005 to 2016 identified temperature fluctuations on time scales of 12 months, 6 months, and about 10 days and inferred a decadal warming trend that significantly exceeds previous estimates. The proposed work would improve the currently preliminary methodology of seismic ocean thermometry, and it would provide improved constraints on the temperature variability and trends of the largescale deep ocean. The project will apply the method to two new regions: the Southern Ocean and the subtropical Northwest Pacific. The Southern Ocean is interesting because previous data coverage is particularly sparse, estimated trends are large, and the SOFAR channel expands towards the surface. The Northwest Pacific is interesting because it displays strong decadal variability and with the Kuroshio current system hosts a strong front and an energetic eddy field. These two regions are therefore ideal test beds, both to improve the methodology and to uncover interesting signals. Another advance will be to use hydrophone rather than seismic station data. This will improve the sensitivity and thus allow use of more abundant small earthquakes, and will allow observation of travel time changes at different frequencies, from which to infer information on the depth structure of the associated temperature changes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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