Page 13 - Fall 2005

ACOUSTIC REMOTE SENSING OF OCEAN GYRES
Peter F. Worcester and Walter H. Munk
Scripps Institution of Oceanography, University of California, San Diego La Jolla, California 92093-0225
Robert C. Spindel
Applied Physics Laboratory, University of Washington Seattle, Washington 98105
“To understand the ocean, its dynamics, its role in climate, weather and other ocean- atmosphere phenomena, one must observe it on a basin- wide scale with adequate time and space resolution.”
This quote is from a paper pub- lished by Munk and Wunsch in 1982, titled “Observing the ocean in the 1990’s.”1 In that paper they described an ocean observation system consisting of two complementary com- ponents: ocean acoustic tomography and satellite observations of sea surface topography and wind stress. They noted, somewhat parenthetically, that tomography over basin-scale distances required “further engineering develop- ments.” It is now more than 20 years later. Where do we stand in the applica- tion of acoustic remote sensing meth- ods to observing the ocean on basin scales?
Acoustic tomography, satellite altimetry and scatterometry, and a third observational component men- tioned only briefly by Munk and Wunsch, freely drifting profilers, are today providing complementary basin- scale observations of the northeast Pacific Ocean. Large-scale, depth-aver- aged temperatures have been measured by long-range acoustic transmissions in the North Pacific Ocean for the past nine years. Acoustic sources located off central California and north of Kauai transmitted to receivers distributed throughout the North Pacific from 1996 through 1999 during the Acoustic Thermometry of Ocean Climate (ATOC) project. The Kauai transmis- sions resumed in early 2002 and are now continuing as part of the North Pacific Acoustic Laboratory (NPAL) project; a seven-year time series has
been obtained so far. Progress has at times seemed glacial, but the engineer- ing (and scientific) developments needed for basin-scale tomography are in fact occurring. The time series are now becoming long enough to be inter- esting.
Ocean acoustic tomography and ther- mometry
The ocean is largely transparent to sound, but opaque to electromagnetic radiation. Remote sensing of the ocean interior must therefore rely on sound. Ocean acoustic tomography measures ocean temperature and velocity within an ocean volume by transmitting sound through it.2 Travel time is a func-
tion of temperature (and to a much lesser extent, salinity) and water veloc- ity, and travel time measurements can provide information about the inter- vening ocean using inverse methods (Fig. 1). (Other less robust acoustic parameters, such as amplitude and phase, can, in principle, also be used.) The effects of temperature and velocity can be separated by using reciprocal transmissions in which sound is trans- mitted simultaneously in opposite directions. Sound traveling with a cur- rent travels slightly faster than sound traveling against a current. The differ- ence in travel time is sensitive to the current parallel to the acoustic path. Reciprocal tomography is particularly
(A)
Ocean surface
Sound channel axis
Seafloor
Sound wave speed 0 km
(B)
Ocean surface Source
Seafloor
Sound channel axis
4,000 km
1,000 km (620 mi)
2,000 km Distance
3,000 km
900 m
(3,000 ft) Receiver
3,000 m
(10,000 ft) 5,000 km
(3,100 mi)
seconds travel time)
travel time)
Slower
Faster
(C)
Transmitted pulse
Received signal
Received signal (3,399
(3,400
seconds
Received signal arrives earlier if the ocean has warmed.
Transmitted pulse
Fig. 1. (A) Sound speed as a function of depth. Sound speed increases with increasing temperature, salinity, and pressure. Near the surface temperature dominates, and sound speed decreases as the temperature decreases. At deeper depths temperature becomes nearly constant. Pressure then dominates, and sound speed increases as pres- sure increases. The result is a sound-speed minimum at about 1000 m depth in mid-latitudes, called the sound channel axis. (B) Low-frequency sound can propagate to long distances in the sound channel without interact- ing with the surface or seafloor. (C) If the ocean warms up between the source and receiver, travel time decreas- es because sound speed increases with increasing temperature. (From J. Howard, “Listening to the Ocean’s Temperature,” Scripps Institution of Oceanography Explorations 5, Fall 1998.)
Acoustic Remote Sensing of Ocean Gyres 11
Depth