Reciprocal sound transmission measurement of mean current and temperature variations in the central part (Aki-nada) of the Seto Inland Sea, Japan

A 30 km-range reciprocal sound transmission experiment was carried out on the line connecting Honshu and Shikoku (the first and fourth biggest main Japanese islands, respectively) in the central part (Aki-nada) of the Seto Inland Sea, Japan, during March-May 2010 to measure the mean current and temperature variations over the sea. The range-averaged current along the sound transmission line was estimated to have a mean and standard deviation of (3.8-4.4) +/- A (1.7-1.8) cm/s after converting the travel time difference data into currents, including a fortnightly tidal variation in the range of +/- 30 cm/s. The positive mean current implies slow water movement from the west to east through Aki-nada. The range-averaged speed of sound was estimated by converting from the mean travel time or one-way travel time into the speed of sound, and further converted into temperature for fixed values of salinity and depth, according to the standard speed of sound formula. Besides the precise measurement (to an accuracy of 0.01A degrees C) of semidiurnal and diurnal tidal variations and seasonal warming, the temperature data showed periodic variations with periods of 7.0 and 21.1 days that had never been observed in Aki-nada before. This study suggests that reciprocal sound transmission is a powerful technique for the long-term accurate measurement of mean current and temperature variations in coastal and inland seas.