A survey of acoustic techniques for monitoring El Niño.
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A survey of acoustic techniques for monitoring El Niño.

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    The challenge of understanding the El Niño-Southern Oscillation (ENSO) cycle in the equatorial Pacific Ocean is a test of our abilities to observe, model, and forecast the processes of global climate change. The only viable technology for monitoring the structure, dynamics, and energetics of the ocean interior on the space-time scales of the ENSO signal appears to be acoustic remote sensing. We, therefore, examine how the following acoustic techniques might be used to monitor ENSO-induced changes in the upper ocean: (1) ocean acoustic tomography; (2) a long-range acoustic thermometer; (3) passive monitoring of ambient acoustic noise level; (4) an occulation technique that depends on bottom absorption; and (5) space-time scintillation analysis. We computed the acoustic properties of ocean models based on the 1982-1983 ENSO event and found out how sensitive different acoustic measurables are to the temperature changes that accompany a strong El Niño. In the eastern Pacific, for example, the largest (and earliest) temperature increases occur between 40 and 100 m depth. For long-range (ducted) sound rays to pass through this region without being absorbed by the bottom, the ocean must be at least 4 km deep. Pulse tomography, applied to vertical ocean slices, could adequately sample the temperature and currents in the upper ocean if appropriate receiving arrays were used. The passive listening scheme could monitor changes in the ambient noise level with the onset of El Niño, but it raised many questions about the natural variability of the noise environment. An occulation scheme that uses a vertical receiving array could economically monitor changes in average thermocline depth. The long-range acoustic thermometer could monitor the heat content of the equatorial ocean, a likely ENSO precursor. The horizontal covariance of acoustic scintillations might be used to measure the structure of transverse currents crossing a long acoustic path. If problems in extending the theoretical model to longer ranges can be solved, scintillation analysis could be used to monitor subsurface equatorial currents that transport heat eastward along the equator. Each technique examined offers some remote-sensing potential, but each also poses problems to be solved before its relative advantages in cost, coverage, or convenience over in-situ methods are clear.
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