North Pacific basin heat content variability determined from long-range acoustic transmissions

Abstract

ATOC signals transmitted from the Pioneer Seamount acoustic source have been received on hydrophone arrays located throughout the North Pacific Basin since the beginning of 1996. Propagation times can be used to infer average temperature (or heat content) variability because the speed of sound changes with temperature. The time series of resolved-ray travel times show both an annual cycle with amplitudes up to 0.5 s, and other higher frequency fluctuations of comparable magnitude caused by natural oceanic variability. The range-averaged temperature is derived from the low-pass filtered (<1 cpd) resolved-ray travel times by constructing a model for the structure and statistics of oceanic variability, fitting the model to the data using weighted least-squares techniques, and then calculating the range- and depth-averaged quantity including its uncertainty. Because many of the resolved rays reflect off the sea surface, the mixed layer in particular, and the upper ocean in general, must be correctly modeled. The variability of acoustically determined heat content is compared to that determined from the Levitus94 atlas, historical expendable bathythermograph (XBT) data; other concurrent data types are not yet available for direct comparison. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]