Scattering from the mixed layer base into the sound shadow

Long-range sound transmissions in the ocean are largely controlled by the SOFAR wave guide. The agreement between the measured and predicted arrival patterns is generally excellent, except for an observed extension downward by many hundreds of meters from the lower caustics into the deep sound shadow. The deepening of early arrivals is proposed to be associated with the sharp transition zone marking the lower boundary of the surface mixed layer. This mixed layer base (MLB) is distorted by internal waves. Multiple collisions of the ray-like acoustic transmission with the wavy MLB lead to a mean deepening of the lower caustics of 2γa−1σ2≈35m per collision, where γa=(1∕C)dC∕dz=1.13×10−2km−1 is the (fractional) abyssal adiabatic sound speed gradient and σ2=2×10−4 is the variance in MLB slope. There are typically 20 such collisions in a 1000km transmission. Monte Carlo numerical experiments yield statistics of ray inclination, range, travel time, and lower turning point. The resulting time front includes a deepening by several hundred meters. The acoustic signatures provide the possibility for monitoring upper ocean processes with abyssal acoustic arrays.