NPAL Acoustic Thermometry

As a follow on to the Acoustic Thermometry of Ocean Climate (ATOC) project regular acoustic transmissions across the North Pacific Ocean have continued over the past six years. ATOC was originally funded by DARPA and the this work continues as part of the North Pacific Acoustic Laboratory (NPAL) project funded by the Office of Naval Research (ONR). The transmitted signals are received on U.S. Navy SOSUS and other listening arrays of hydrophones in the northeast Pacific. The travel times of the coded acoustic signals are a precise measure of the average temperature of the ocean. This measurement is valuable because the ocean is highly variable, so measurements at a single point, for example by a thermometer, are dominated by local temperature fluctuations which are usually unrelated to climate change.

Acoustic thermometry was accepted as part of the global ocean observing system during the OceanObs'99 Conference. The summary report [Koblinsky, C. and N. Smith, eds., Observing the Oceans in the 21st Century (Melbourne: GODAE Project Office and Bureau of Meteorology, 2000)] states (p. 28):

Acoustic tomography
The Conference was provided with a detailed account of the potential for acoustic tomography to make sustained contributions to the observing system [available from the link at left] and of the potential value of acoustic thermometry. Till this point, it was a technique that had not figured prominently in the planning for either GOOS/GCOS or for CLIVAR. In terms of the scientific approach to sampling the full depth of the ocean, globally, acoustic tomography offers some attraction since it provides long-path, integral (low wave-number) measurements of thermal variations to complement those from Argo and satellite altimetry.

The Conference concluded that acoustic tomography did represent a potentially valuable approach and that, initially, it should be implemented in the Arctic and at specific locations such as the Straits of Gibraltar. The Conference also encouraged an exploratory implementation in the North Atlantic in the presence of substantial profiling floats to test the complementarity and/or redundancy between tomography and other measurements. The Pacific Ocean plans also include the use of acoustic tomography in the North Pacific.

Changes in the climate state of the North Pacific are obvious in the acoustic time series. The central North Pacific warmed considerably during 1999-2001 and the annual cycle in ocean temperature was relatively weak. But in early 2003, the central Pacific Ocean rather suddenly cooled and developed a strong annual cycle. Similar changes were observed using TOPEX/POSEIDON satellite altimetry. Satellite altimetry is a somewhat less precise measure of ocean temperature (as a column of water warms, sea surface height rises), but altimetry gives the larger geographic context to the changes that are occurring on the acoustic paths. These climate changes are probably normal variations in the Earth's climate system. After 6 years, no obvious sign of "global warming" is apparent.

The time series apparently spans a cycle of the Pacific Decadal Oscillation (PDO - a phenomena that dominates Pacific Ocean climate variations), though the changes observed do not appear to be strictly related to the PDO. The climate state of the North Pacific Ocean has been shown to have a profound effect on the weather of North America.

This project is a contribution to CLIVAR (CLIVAR is derived from "CLImate VARiability and Predictablity")- an international research program to study climate changes and the impacts of changing climate on health, weather, agriculture, etc.

The ocean is highly variable, with changes occurring over yearly to decadal to century (and beyond) time scales. The difficulty in detecting these changes, distinguishing the signals occurring at many time and length scales, and distinguishing between anthropogenic and natural climate changes, are reasons why the long-range acoustic technique is an important tool for climate observation.