Ocean Observing System Design: Transatlantic Acoustic Propagation for Acoustic Thermometry

Abstract Ocean acoustic thermometry is simulated in the North Atlantic using two global ocean state estimates to assess its potential contributions to the Global Ocean Observing System. Acoustic data were simulated for transatlantic acoustic propagation from Cabo Verde to Bermuda, a 4183-km archetypal path across the Mid-Atlantic Ridge. Acoustic scattering by internal waves was simulated using a range-dependent, stochastic model added to the ocean state estimates. While the Ridge blocks acoustic propagation traveling deeper than about 3000 m, and internal-wave sound speed scintillations cause prohibitive scattering of later-arriving rays, it is apparent that O (10) acoustic ray arrivals on this path are likely resolvable. These rays generally cycled between 500 and 3000 m. Understanding the details of the acoustic scattering by the small-scale processes like internal waves, together with patches of warm and salty Mediterranean water, requires new observations. A simple inverse was applied to simulated travel time data derived from a 26-yr state estimate. The uncertainty of the tomographic estimates of temperature, averaged over basinwide range and between 500- and 3000-m depth, was about 3 m°C. A sparse, basin-scale array of such acoustic measurements would substantially reduce the uncertainty of basin-averaged temperature, while providing excellent temporal resolution and some depth resolution. The improvements to ocean resolution when tomographic information is combined with all other data in ocean state estimates, particularly in abyssal regions, have yet to be determined. Significance Statement The circulation of the North Atlantic has been adjusting to climate change at faster time scales and greater variation than anticipated. Observations of the ever-turbulent North Atlantic, by satellites, drifting floats, and ships, are perennially insufficient to precisely specify the evolving ocean state. This study highlights how transatlantic acoustics can provide basin-scale, hourly measurements of section-averaged temperature, complementary to existing measurements, with remarkably small uncertainty. Thermometry and other diverse measurements can be employed by numerical ocean models to obtain accurate estimates for the ocean state and its future evolution, much as the National Weather Service uses data and models to determine the atmospheric state and make predictions. This study reopens discussion of acoustical observations, first introduced 40 years ago.