Decadal observations of deep ocean temperature change passively probed with acoustic waves.

The oceans are filled with acoustic waves, which are trapped in a low-velocity layer at about 1 km water depth. The sound speed depends on the temperature. From hydroacoustic arrays, travel times can be obtained through cross-correlating transient signals. Hydroacoustic station H10 (Ascension Island) appeared capable of measuring deep ocean temperature change. A decrease in modeled and observed travel times of -0.002 s/yr was derived between two arrays, corresponding to a warming of 0.007 °C/yr, at about 900 m water depth over two decades. As such, acoustic waves provide an independent and passively acquired measure of the temperature change in the deep ocean.