148th ASA Meeting, San Diego, CA

6+ Years of Acoustic Thermometry in the North Pacific Ocean:
A Cycle of the Pacific Decadal Oscillation

Brian D. Dushaw dushaw@apl.washington.edu
Applied Physics Laboratory
University of Washington
1013 NE 40th Street
Seattle, WA 98105

B. D. Cornuelle, M. Dzieciuch, W. H. Munk, P. F. Worcester
(University of California, La Jolla, CA 92093-0225)

J. A. Colosi
(Woods Hole Oceanographic Inst., Woods Hole, MA 02543)

B. D. Dushaw, B. M. Howe, J. A. Mercer, R. C. Spindel
(Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98105-6698)

Popular version of paper 4pUW1
Presented Thursday afternoon (1:30 pm), November 18, 2004
148th ASA Meeting, San Diego, CA

As part of the Acoustic Thermometry of Ocean Climate (ATOC) project - originally funded by DARPA and now continuing as part of the North Pacific Acoustic Laboratory (NPAL) project funded by the Office of Naval Research (ONR)- regular acoustic transmissions have been made across the North Pacific Ocean for the past six years. 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.

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.

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