Diurnal Fluctuation of Shallow-Water Acoustic Propagation in the Cold Dome off Northeastern Taiwan in Spring

The diurnal fluctuation of acoustic propagation in a cold dome region was studied by using the output of the high-resolution Finite Volume Coastal Ocean Model. The 3-D acoustic propagation in this region was studied first, and the results suggest that the diurnal fluctuation of the ocean environment results in the fluctuation of acoustic propagation. However, the variation pattern cannot be determined with only few outputs, and comprehensive analysis was needed. Then, a feature model for the thermal front on the edge of the cold dome was proposed. By analyzing the output of the ocean model, fluctuations were found to mainly occur in the two aspects of the feature model: first, horizontal excursion of the thermal front and second, vertical motion of the isothermal lines in the cold dome. The variation pattern of acoustic propagation over the two kinds of fluctuations was quantified. Results suggest that both kinds of fluctuations of model parameters result in great transmission loss (TL) variation in some cases. Typically, acoustic propagation with high frequency and deep source experience great influence from the fluctuations. The TL versus range usually experience influence from the fluctuations of the first kind from an approximately 40-km range and from the start for the second kind. The reason is that the vertical motion of the isothermal lines of the cold dome induces variation over the temperature profile from the start until the end, whereas the horizontal excursion of the thermal front brings influence mainly over the temperature profiles near the thermal front.