PERFORMANCE BOUNDS ON ACOUSTIC THERMOMETRY OF OCEAN CLIMATE IN THE PRESENCE OF MESOSCALE SOUND-SPEED VARIABILITY

The ability to measure climatic changes in ocean temperature is fundam entally limited by the presence of mesoscale variability. Because ocea n acoustic propagation can be used to measure the range-averaged tempe rature profile, long-range acoustic transmissions have been proposed a s a means of filtering out mesoscale variability in order to measure a global warming, related trend in mean temperature. In this paper, the Cramer-Rao lower bound (CRLB) on the estimation of the mean depth-dep endent temperature profile given a single acoustic transmission is eva luated to provide an indication of the highest accuracy which could be achieved by this experiment. The CRLB derived here applies to broadba nd vertical arrays of arbitrary length and thus extends previous work. Evaluation of the bound is performed using models of sound-speed vari ability derived from real Pacific environmental data. Results indicate that the performance of an acoustic thermometry system is limited by mesoscale variability above a threshold value of observation-time-sign al-to-noise ratio product and is acoustic noise limited below this thr eshold. Further, comparisons of the CRLB above this threshold suggest that for a 5000-km source-receiver separation, ATOC accuracy may vary from between 0.01 and 0.1 degrees C depending on the shape and uncerta inty of the change in mean temperature profile. (C) 1996 Acoustical So ciety of America.