Revisiting a coastal acoustic tomography experiment in Hiroshima Bay: Temporal variations in path-averaged currents and its relation to wind

摘要

Remote sensing is a desirable method for measuring ocean currents on various spatial and temporal scales. Coastal acoustic tomography (CAT) can measure current velocity in coastal and shallow seas, but the application of this method in stratified shallow estuaries has not been sufficiently tested. This study revisits the acoustic transmission data obtained from the 2013 Hiroshima Bay CAT experiment and investigates what information could be obtained from acoustic data when a relatively strong southward wind blew in the bay. We determined path-averaged currents of the first arrival rays and compared the temporal variations in this path-averaged current with those in the wind, water temperature, and detided sea level. When the southward wind was intensifying, the sea level at the northern coast of the bay decreased and then recovered when the southward wind subsided. These sea level variations must be accompanied by net volume transport variations. When sea level was recovering, the path-averaged current exhibited a positive bias, reflecting an increase of volume transport toward the inner part of the bay. In contrast, the path-averaged current did not show a noticeable variation when the sea level was decreasing although the surface layer current was expected to be intensified southward. This is because the first arrival ray did not propagate through the surface layer during the period, and therefore, the path-averaged current from the first arrival rays did not contain surface current velocity information. CAT is promising method for measuring variations in current velocity fields associated with winds; however, in the application to stratified and shallow estuaries such as Hiroshima Bay, it is crucial to consider acoustic ray-path variations when analyzing and interpreting travel time data time series.