Indirect observation to convert the sound speed has been carried out as a method of observing the ocean acoustic environment from water temperature and salinity observed in the mooring buoy and ship observations. However, there is a problem in that these have both high initial cost and running cost. Therefore, to address these issues, a study of the wide area ocean sound observation system using the observation data from a marine automatic observation float was performed. To capture in detail the changes in the ocean acoustic environment due to recent climate change, however, it is necessary to observe areas of the deep sea that can not be detected by Argo floats. Also, during the season when the Antarctic Ocean is frozen, continuous observation data of the deep sea cannot be acquired, and water temperature, salinity, and sound speed structure are not clear. As a result, JAMSTEC developed a new profiling float, called the “Deep NINJA” for deep-sea observations. The float was subjected to a yearlong monitoring of the Antarctic Ocean off the Adelie Coast in 2012. For the first time, it succeeded in monitoring long-term the sound speed profile to a depth of 4000 m in the Antarctic Ocean, and was able to capture a seasonal change in the surface area in the freezing and thawing seasons. In addition, by calculating sound speed from these data, simulations were performed assuming low-frequency sonar. The results obtained the ingredient that propagates while repeating a reflection in the extremely small layer of the sea surface neighborhood, and the ingredient that propagates while being reflected near a water depth of 100 m, which changes the sound speed gradient. From this, propagation loss was found to be smaller in winter than summer, and the possibility that a sound wave would propagate to a more distant place was demonstrated. This may affect the long-distance sound wave propagation of the echolocations of passive sonar and marine mammals.

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