[1] The stratospheric infrasound duct, formed between Earth's surface and altitudes of $50 km, is sensitive to spatiotemporal variations in stratospheric wind speed and temperature. Infrasound recorded at long range from the 2010 summit eruption of Eyjafjallajökull volcano, Iceland, exhibits temporal variability correlated with diurnal stratospheric solar tidal wind speed variations. Between 18 and 28 April 2010, signal observations at stations BKNI, U.K. (range, 1745 km), and IS18, Greenland (range, 2285 km), exhibit prominent diurnal variations in infrasonic amplitude, bandwidth, back azimuth, and apparent speed, which we identify using the CLEAN spectral analysis algorithm for unevenly sampled time series. Results of 3-D acoustic ray tracing through operational atmospheric specifications indicate that tidal wind speed variations (with amplitudes of $20 m/s) can generate diurnal variations in the proportion of the acoustic wavefield propagating within the stratospheric acoustic duct. Range-dependent meteorology is required; propagation modeling using averaged meteorological profiles fails to predict the leakage of acoustic energy out of the stratospheric acoustic duct at times of low observed signal amplitudes. Ray tracing correctly predicts the phase of the observed signal amplitude and apparent speed variations. Diurnal variability in ducting, combined with diurnal variations in ambient noise at the sensors, can explain the observed signal bandwidth variations. Back azimuth variations (observed only along the Eyjafjallajökull to BKNI path) are not predicted by 3-D ray tracing. Tidal variations have implications for models of infrasound array network detection capability and for studies that utilize amplitude and bandwidth measurements to make inferences about the acoustic source.

[1] Hydroacoustic signals generated by drifting icebergs that crack, disintegrate, and collide were identified on two hydrophone arrays in the Indian Ocean. These hydrophone arrays are deployed in the Sound Fixing and Ranging channel, enabling the detection of small sources over ranges of several thousand kilometers due to the low attenuation. Source locations estimated from the signal bearings at the arrays are used to monitor two very large icebergs, C20 and B17B. Spatial and temporal correlation of the location estimates with satellite observations confirm that the icebergs can be hydroacoustically resolved. Hydroacoustic generation rates at both C20 and B17B are highest at times of observed breakup. For C20, which underwent continuous breakup, clusters of events to the southeast of the main iceberg show that hydroacoustic observations can identify trails of icebergs that calved from the main berg whose dimensions are less than that easily resolved by moderate resolution satellite monitoring.