SINCE THE END of the Cold War, the US Navy has had an increasing interest in continental shelves and slopes as operational areas. To work in such areas requires a good understanding of ocean acoustics, coastal physical oceanography, and, in the modern era, autonomous underwater vehicle (AUV) operations. Each area presents challenges for both the scientist and the Navy. In physical oceanography, a complex interplay among winds, rivers, tides, and local bathymetry drives a nonstationary, shelfbreak front and the nonlinear internal wave (NLIW) field. These strongly affect acoustic systems but are not adequately understood. A key oceanographic challenge is to model the fully four-dimensional ocean from the largescale circulation down to fine scales, which include NLIW packets, internal tides, jets, and density fronts. Both Navy acoustics systems and Navy operations need the “local ocean weather” as well as the “ocean climate” as part of the routine forecast, but the former is not yet available.

In ocean acoustics, the Navy wishes to operate both at low frequencies (100–1000 Hz) and mid frequencies (1000–10,000 Hz), which poses questions on a variety of spatial and temporal scales. For low-frequency acoustics, it has become obvious that fully three-dimensional (spatial) oceanography is necessary for propagation prediction. A bit more surprisingly, it appears that fully three-dimensional (spatial) acoustics codes might be necessary as well, a big divergence from the two-dimensional slice between source and receiver that has been adequate for ocean acoustics to date. At medium frequency, the effects of NLIWs on sonar systems are predicted