Ocean Acoustic Tomography
133
Acoustic tomography is at its best in an integrating mode; integrating horizontally and integrating vertically. As such it does a good job in estimating the variation
in total heat content of a basin. The intensity of mesoscale variability greatly exceeds that of climate change, but the acoustic integration over 50 mesoscales reduces
mesoscale noise to acceptable levels.
In 1994 the THETIS-2 experiment
27 successfully estimated the seasonal heat
content of the western Mediterranean, combining for the first time satellite altimetry
with acoustic tomography. In my view the most successful application is the 1994
transmission conducted by Mikhalevsky
28 from a source north of Svalbard, Norway,
across the pole to a receiver near Pt. Barrow, Alaska. An ultralow-frequency source
(20 Hz) was dictated by the scattering loss from the ice cover. An earlier than expected
arrival of acoustic mode 2 (whose mode function peaks at the depths occupied by the
North Atlantic Intermediate Water) implied a warming by 0.4
◦ C. A subsequent 1999
transmission implied further warming by 0.5
◦ C. The results are roughly consistent
with independent CTD surveys conducted from submarines. The polar environment
provides three advantages to acoustic tomography (aside from being available in
winter when it is not accessible to standard oceanographic techniques): a low background of internal waves (the ultimate limit to precision), a dispersive sound channel
favorable to vertical resolution, and remoteness from public attention.
Parallel efforts by French, German, and Japanese oceanographers during this
period were less restricted by the biological consideration. An international working
group (SCOR 96) under the chairmanship of David Farmer met in France, India,
Denmark, Austria, and Japan.
29
NORTH PACIFIC ACOUSTIC LABORATORY (NPAL), 2000–
A summary of this period has been prepared by Worcester et al.
30 The ATOC Kauai
transmissions had lasted from October 1997 to October 1999. Worcester then began
the arduous process of authorization and the Kauai source finally resumed operation
in January 2002 as part of the NPAL program. The Kauai time series is therefore now
seven years long, albeit with a substantial gap.
From the early days we had practiced a joint analysis of satellite altimetry
and acoustic tomography. This combined the good horizontal resolution of altimetry with the depth resolution and good time resolution of tomography. What was
missing was a good model framework for the combined analysis. The ECCO Consortium (Estimating the Circulation and Climate of the Ocean) finally provided the
vertical resolution needed for calculating the acoustic propagation through the model
ocean (the forward problem), allowing straightforward comparison of measured and
predicted travel times. The model assimilates a variety of satellite and in situ data,
including TOPEX-POSEIDON altimetry, WOCE hydrography, XBT sections, and
Argo floats.
133
Acoustic tomography is at its best in an integrating mode; integrating horizontally and integrating vertically. As such it does a good job in estimating the variation
in total heat content of a basin. The intensity of mesoscale variability greatly exceeds that of climate change, but the acoustic integration over 50 mesoscales reduces
mesoscale noise to acceptable levels.
In 1994 the THETIS-2 experiment
27 successfully estimated the seasonal heat
content of the western Mediterranean, combining for the first time satellite altimetry
with acoustic tomography. In my view the most successful application is the 1994
transmission conducted by Mikhalevsky
28 from a source north of Svalbard, Norway,
across the pole to a receiver near Pt. Barrow, Alaska. An ultralow-frequency source
(20 Hz) was dictated by the scattering loss from the ice cover. An earlier than expected
arrival of acoustic mode 2 (whose mode function peaks at the depths occupied by the
North Atlantic Intermediate Water) implied a warming by 0.4
◦ C. A subsequent 1999
transmission implied further warming by 0.5
◦ C. The results are roughly consistent
with independent CTD surveys conducted from submarines. The polar environment
provides three advantages to acoustic tomography (aside from being available in
winter when it is not accessible to standard oceanographic techniques): a low background of internal waves (the ultimate limit to precision), a dispersive sound channel
favorable to vertical resolution, and remoteness from public attention.
Parallel efforts by French, German, and Japanese oceanographers during this
period were less restricted by the biological consideration. An international working
group (SCOR 96) under the chairmanship of David Farmer met in France, India,
Denmark, Austria, and Japan.
29
NORTH PACIFIC ACOUSTIC LABORATORY (NPAL), 2000–
A summary of this period has been prepared by Worcester et al.
30 The ATOC Kauai
transmissions had lasted from October 1997 to October 1999. Worcester then began
the arduous process of authorization and the Kauai source finally resumed operation
in January 2002 as part of the NPAL program. The Kauai time series is therefore now
seven years long, albeit with a substantial gap.
From the early days we had practiced a joint analysis of satellite altimetry
and acoustic tomography. This combined the good horizontal resolution of altimetry with the depth resolution and good time resolution of tomography. What was
missing was a good model framework for the combined analysis. The ECCO Consortium (Estimating the Circulation and Climate of the Ocean) finally provided the
vertical resolution needed for calculating the acoustic propagation through the model
ocean (the forward problem), allowing straightforward comparison of measured and
predicted travel times. The model assimilates a variety of satellite and in situ data,
including TOPEX-POSEIDON altimetry, WOCE hydrography, XBT sections, and
Argo floats.