Acoustic Thermometry of Ocean Climate

Acoustic Thermometry of Ocean Climate
Name	Acoustic Thermometry of Ocean Climate
Acronym	ATOC
Classification	Oceanography, Climatology
Inventor	Walter Munk
Related	Heard Island Feasibility Test, Acoustic Doppler Current Profiler

Acoustic Thermometry of Ocean Climate. It is a method for measuring large-scale changes in the average temperature of the world's oceans by precisely timing the travel of low-frequency sound waves over trans-oceanic distances. The technique leverages the fact that the speed of sound in seawater increases with temperature, allowing integrated temperature measurements along entire acoustic paths. Pioneered by physical oceanographers, this approach provides a unique, basin-averaged perspective on ocean heat content, complementing traditional point measurements from ships and Argo floats.

Overview

The core concept involves transmitting precisely timed acoustic signals from a controlled source to distant receivers. These transmissions, often spanning thousands of kilometers across ocean basins, travel via stable underwater sound channels like the SOFAR channel. By measuring the travel time with extreme accuracy, scientists can infer the average temperature of the water column along the path, as sound speed is primarily a function of temperature, with secondary influences from salinity and pressure. This method integrates thermal information over vast scales, offering a powerful tool for detecting the subtle but critical warming signals associated with global warming and climate change.

Scientific principles

The fundamental relationship is governed by the equation of state for seawater, where sound speed increases approximately 4.5 meters per second per degree Celsius increase in temperature. Sound propagation is modeled using ray theory and normal mode analysis, accounting for refraction through ocean layers. The primary propagation path utilizes the SOFAR channel (Sound Fixing and Ranging), a horizontal waveguide where sound speed is at a minimum, allowing signals to travel immense distances with minimal loss. Precise timing of arrivals, often using Phase modulation or M-sequence coded signals, is critical. Corrections must be applied for effects from variable bathymetry, mesoscale eddies, and internal waves, which can scatter the signal.

Historical development

The theoretical foundation was laid in the late 1970s and 1980s, notably by Walter Munk of the Scripps Institution of Oceanography and Carl Wunsch of the Massachusetts Institute of Technology. A pivotal proof-of-concept was the 1991 Heard Island Feasibility Test, where signals transmitted near Heard Island were received at sites across the globe, including Bermuda and California. This success led to the formal establishment of the Acoustic Thermometry of Ocean Climate project in the early 1990s, a major collaboration between Scripps, the Applied Physics Laboratory, University of Washington, and other institutions. Early work faced public controversy and legal challenges over potential impacts on marine mammals, leading to extensive environmental impact studies.

Major projects and experiments

The primary ATOC project deployed permanent acoustic sources off Point Sur in California and north of Kauai in the Hawaiian Islands, with a network of receivers throughout the North Pacific Ocean. A parallel, larger-scale international effort was the 1996–2006 Acoustic Thermometry of Ocean Climate Basin-scale experiment. Other significant initiatives included the North Pacific Acoustic Laboratory (NPAL) experiments and the Thermometry of Ocean Climate (THETIS) program in the Mediterranean Sea. These projects often collaborated with programs like the World Ocean Circulation Experiment (WOCE) and utilized naval facilities such as the U.S. Naval Postgraduate School.

Data analysis and results

Analysis involves extracting precise travel times from complex received signals and inverting these times, using ocean acoustic tomography techniques, to estimate path-averaged temperatures. Data from the 1990s and early 2000s provided a multi-year time series revealing seasonal cycles and interannual variability linked to phenomena like the Pacific Decadal Oscillation and El Niño-Southern Oscillation (ENSO). A key finding was the detection of a gradual warming trend in the North Pacific, consistent with independent measurements from satellite altimetry (e.g., TOPEX/Poseidon) and the expanding Argo float array. The data also improved understanding of ocean acoustic propagation and basin-scale heat storage.

Significance and impact

The technique demonstrated the feasibility of using acoustics for large-scale, integrated climate monitoring, providing a valuable complement to the spatially dense but temporally sparse data from ship-based CTD casts. Its development spurred major advances in underwater acoustics, signal processing, and physical oceanography. The environmental concerns it raised led to extensive research on the effects of low-frequency sound on species like humpback whales and elephant seals, influencing policies and mitigation strategies for future marine acoustic research. While largely superseded for routine climate monitoring by the global Argo program, its legacy persists in concepts for future ocean observing systems and in the continued use of acoustic methods for measuring ocean currents and thermohaline circulation.

Category:Oceanography Category:Climatology Category:Underwater acoustics