Multifrequency acoustical volume backscattering patterns in the Arabian Sea - 265 kHz to 3 MHz

High-frequency acoustical volume backscattering was examined during three p eriods of the annual cycle of winds in the Arabian Sea-the Winter Northeast (NE) Monsoon, the Summer Southwest (SW) Monsoon, and the Fall Intermonsoon -using a multifrequency acoustic profiling system deployed on an undulating towed body. This system measured acoustical volume backscattering strength s at six frequencies ranging from 265 kHz to 3.0 MHz. Measurements were mad e from near the surface to 250 m depth along a ship track that paralleled t he coast of Oman, then proceeded away from the coast towards the middle of the Arabian Sea. At all frequencies, volume backscattering strength was hig hest near the surface, decreasing with increasing depth. Contrary to expect ation, backscattering was generally lower during the Summer SW Monsoon than during the Winter NE Monsoon. Also contrary to expectation, backscattering was not appreciably higher near the coast than it was offshore during the Summer SW Monsoon, although it was higher near the coast during the Winter NE Monsoon. Generally speaking, regional and seasonal differences were smal ler than expected, and much smaller than the fine-scale spatial and tempora l variability, particularly at frequencies below 1 MHz. There was a daily p attern of increased nighttime backscattering in the upper 100 m at 265 and 420 kHz. This pattern was less evident at 1.1 MHz and above. (C) 2000 Acous tical Society of America. [S0001 -4966(00)01901-9].