Subarctic frontal effects on long-range acoustic propagation in the North Pacific Ocean

Abstract

Acoustic propagation along the Hawaiian–Aleutian path at low frequency (65 Hz) for the acoustic thermometry of ocean climate (ATOC) has been simulated using the implicit finite difference parabolic equation (IFD-PE). This simulation is based upon the Semtner–Chervin model with one-half-degree resolution. The adiabaticity has been assessed, and it was found that significant mode coupling takes place at the sub-arctic front area (44.8 ° N). Due to the range-dependent double channel, two characteristic ranges were identified in this area: (i) the modal degeneration range RmD and (ii) the modal scattering range RmS. At range RmD, mode m and mode n (usually, n=m+1) are degenerated, the modal wave numbers of mode m and mode n become equal, and mode n then takes the shape of mode m. Therefore, there is no real repopulation (modal scattering), but a modal name change takes place at this range. The modal name change can cause a miscalculation of the adiabatic travel time. At range RmS, however, real repopulation does take place. Numerical simulations illustrate that RmD and RmS are separated by about 10 km at lower modes (m<5) and are merged at higher modes.