What acoustic travel-times tell us about the ocean

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. Using an advanced numerical model, we can identify both the ocean dynamics that control the ocean state along a ray path and quantify the informational content of the ray travel-time observation. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea over a one year experiment. The travel-times are found to be sensitive to the internal tide generation prior to the sample time and to advective effects that alter density along the ray path. Temporal nonlinearity of these sensitivities suggest that prior knowledge of the ocean state is necessary to exploit the travel-time observations. After assimilating the travel-time observations, the contribution of the travel-time information to our estimation of the ocean state is quantified and evaluated to further identify what the travel-time observation reveals about the ocean.