OCEAN MIXED LAYERS AND ACOUSTIC PULSE-PROPAGATION IN THE GREENLAND SEA

A simple one-dimensional ocean mixed layer model is used to study the effect of the transition between summer and winter conditions in the G reenland Sea on range-independent acoustic propagation. Acoustic norma l modes propagated through the evolving sound-speed profile simulate b roadband acoustic receptions from the Greenland Sea Tomography Experim ent. The resulting changes in arrival structure and travel time are co mpared with data recorded between two of the tomographic moorings. The starting state for the model is the average of measured summer temper ature and salinity profiles. At each time step the surface layer is mo dified by the removal of heat (modeling heat loss to the atmosphere) a nd the removal of fresh water (modeling evaporation minus precipitatio n). When necessary, static stability is maintained by mixing the surfa ce layer into deeper layers. The acoustic normal modes exhibit large c hanges in behavior as the profile changes. In both summer (seasonal th ermocline) and winter (adiabatic sound-speed profile) individual modes show minimal frequency dispersion. Intermediate profiles with a shall ow surface mixed layer give highly dispersive modes, delaying the fina l acoustic energy cutoff by several hundred milliseconds relative to t he summer and winter cases. This is the largest travel time signal obs erved in the data. The largest peak in the late continuous acoustic en ergy is due to minimally dispersed modes and corresponds to ray arriva ls with near horizontal receiver angles. The amplitude of the arrival is low when significant dispersion is present.