Acoustic Thermometry in the Arctic Ocean Acoustic Thermometry in the Arctic Ocean … … following Ira to the Arctic following Ira to the Arctic Peter Mikhalevsky Peter Mikhalevsky Acoustic and Marine Systems Operation Acoustic and Marine Systems Operation Science Applications International Corporation Science Applications International Corporation Ira Dyer Symposium Ira Dyer Symposium Cambridge, MA June 14, 2007 Cambridge, MA June 14, 2007
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Acoustic Thermometry in the Arctic OceanAcoustic Thermometry in the Arctic Ocean……following Ira to the Arcticfollowing Ira to the Arctic
Peter MikhalevskyPeter MikhalevskyAcoustic and Marine Systems OperationAcoustic and Marine Systems Operation
Science Applications International CorporationScience Applications International Corporation
Ira Dyer SymposiumIra Dyer SymposiumCambridge, MA June 14, 2007Cambridge, MA June 14, 2007
OUTLINE• Acoustic thermometry and the Arctic Ocean – a little
background• ACOUS (Arctic Climate Observations using
Underwater Sound) – The Trans-Arctic Acoustic Propagation (TAP) Experiment
1994– Continuous acoustic section from Oct.1998 through Dec.
1999– Measurements at Ice Camp APLIS during SCICEX 1999
warming of AIW– Sections used to model acoustic response
• Ocean Observatories and Long term observations in the Arctic Ocean - update
Acoustic ThermometrySpeed of sound dependent on water
temperature• Acoustic Thermometry
– Average temperature along a propagation path derived from travel time changes between the source and receiver
– Reciprocal transmissions can resolve average net current along the path
– Arctic Ocean uniquely suited for acoustic thermometry due to good coupling of acoustic modes and major Arctic water masses
MAJOR ARCTIC OCEAN WATER MASSES ARE WELL SAMPLED BYACOUSTIC MODES/RAYS (Modes shown for 20 Hz)
ACOUSTIC THERMOMETRY in the ARCTIC OCEAN
Mikhalevsky, 2001
ATLANTIC INTERMEDIATE WATER (AIW) CIRCULATION in the ARCTIC OCEAN and Acoustic Thermometry Sections
APLIS
ACOUSSource
Nans
en B
asin
Fram
Bas
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Greenland
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Spitsbergen
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TAPTAP
SIMISIMI
TAP Acoustic Section: Ice deployed source to the Lincoln Sea ice camp and the ONR Sea Ice Mechanics Initiative (SIMI) ice camp in the Beaufort Sea April, 1994 – A feasibility test
ACOUS Acoustic Section: Moored source to the APLIS ice camp in the Chukchi Sea April, 1999 and to bottom moored vertical array in the Lincoln Sea Oct. 1998 to Dec. 1999
ARCTIC OCEAN influences Earth’ssurface heat balance and the thermohalinecirculation of the world’s oceans
Significant change in the Arctic Ocean in the last 20 years:• Temperature increase in AIW• ~40% loss in sea ice mass
Forecasts of ice-free summer in this century
Arctic Climate Observations using Underwater Sound (ACOUS)
• US/Russia bilateral program started in 1992• Use acoustic thermometry to measure Arctic
Ocean temperature and derive heat content• TAP Feasibility exp. in 1994 showed strong
coupling between travel times and AIW temp., observed basin scale AIW warming (~.4 °C avg. max)
• Source installed in Oct. 1998 with transmissions every 4 days to receive array in Lincoln Sea
• Reception of source signals at APLIS ice camp in April 1999 showed continued warming in AIW consistent with SCICEX CTD’s (~.5°C avg. max)
TAP Experiment CW and MLS Transmissions
Mikhalevsky, 1981Mikhalevsky, Baggeroer, Gavrilov, and Slavinsky, 1995Mikhalevsky, Gavrilov, and Baggeroer, 1999
RUSSIAN SOURCE 19.6 Hz,195 dB
CW signals show the exceptional stabilityof the Arctic acoustic channel and Ricianstatistics first observed in 1980 (but not at trans-basin range!)
Time measurement resolution of ~22msec by peak picking and ~0.5msec by phase in agreement with theory
255 digit MLS pulse compressed and beamformed
2 sec earlier arrivalof mode 2 ~.4˚C increase in AIW max
19941980’s
MEASUREDMODAL ARRIVALPATTERN 1962BEAUFORT SEADinapoli, Viccione, and Kutschale, 1978
MODAL ARRIVALPATTERN 1994TRANS-ARCTICPROPAGATIONEXPERIMENTMikhalevsky, Gavrilov, and Baggeroer, 1999
20 Hz
ACOUS SOURCE – Franz Victoria StraitACOUS RECEIVE ARRAY - Lincoln Sea
Oct. 10, 1998 to Dec. 8, 1999
ACOUS - LINCOLN SEA EXPERIMENT
ACOUS SOURCE Deployed OCT. 1998
ACOUS VLA Deployed OCT. 1998 Recovered MAR. 2001
20.5 Hz center frequencyQ~8, 195 dB source level
Both source and receivearray used rubidium standard for timing control
R/V Akademik Fyodorov
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0.05
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0.15
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0.25
0.3
Am
plitu
de
856 858 860 862 864 866 8680
0.05
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Travel time, s
Am
plitu
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Filtered modes of ACOUS signal N079
Mode 1
Mode 2
Mode 3
Mode 4
Mode 5
10 repetitions of 255 digit MLS sequence20.7 min total duration every 4 days, 107 transmissions
-2 0 2
0
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Temperature,0C
Dep
th, m
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Sound speed, m/s
a b
17 18 19 20 21 22 231445
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Gro
up v
eloc
ity, m
/s
17 18 19 20 21 22 231450
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1464
Frequency, Hz
Gro
up v
eloc
ity, m
/s
b
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1
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a
1
TYPICALEARLY 1990’s
WARM - 1994
TEMPERATURE, SOUND SPEED and MODAL GROUP VELOCITIES
TYPICAL
WARM
Mode 1 speeds up
Gavrilov and Mikhalevsky, 2002
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Time, day
Tim
e, s
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+ 0.3°C OVER300 KM OF THE PATH
DIFFERENCE IN ARRIVAL TIME OF MODE 1 and MODE 2
-2
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ture
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Range, km
Dep
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pera
ture
, 0 С
Temperature sectionusing climatology and profiles from theearly 1990’s
Temperature sectionusing profiles frommid 1990’s perturbedto fit late 1999 “warm”part of acoustic record
d e g .C
-1 .8-1 .4-1-0 .6-0 .20 .20 .611 .41 .82 .2
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0
0
5 0 0
1 0 0 0
D ep th , m
T e m p e ra tu re p ro file :S c ic e x9 5
-1 .6-1 .2-0 .8-0 .400 .40 .81 .21 .622 .4
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0
0
5 0 0
1 0 0 0
D ep th , m
S c ice x -9 8
-1 .6-1 .2-0 .8-0 .400 .40 .81 .21 .622 .4
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0
0
5 0 0
1 0 0 0
R a n g e , km (fro m 8 5 .0 3 N , 4 7 .1 E to 7 3 .6 5 N , 1 5 5 5 5 W )
D ep th , m
S c ice x -9 9
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
0
1 0 0 0
2 0 0 0
3 0 0 0
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B a th y m e t r y a lo n g th e p a thL o m o n o s o v R id g e
A lp h a R id g e
C h u k c h i R is e
TEMPERATURE SECTIONS FROM SCICEX 1995, 1998, AND 1999 ACROSSTHE ARCTIC BASIN WITH CORRESPONDING BATHYMETRY. WARMING INTHE ATLANTIC LAYER IS EVIDENT IN TOPOGRAPHICALLY GUIDED EXTENSIONSOF THE ATLANTIC WATER CIRCULATION.
Build in stages,Barrow Cabled Observatory,SEARCH, NPEO, and build on experience from MARS and NEPTUNE
International participationwith cable terminations inSvalbard, Greenland, andRussia (as well as US andCanada) will greatly reduceundersea cable costs withcost sharing for systeminstallation and operation
ARCTIC REGIONAL UNDERSEA OBSERVATORY for RESEARCH and ANALYSISAURORA
Conclusions and Future
• Acoustic modal travel times are an excellent proxy for the average temperature and heat content of the AIW in the Arctic Ocean– Monitoring of the average depth of the thermocline/upper
mixed layer also possible• Monitoring sea ice roughness/thickness by observing
changes in acoustic propagation loss and monitoring salinity above the halocline using acoustic halinometryare under investigation
• Network of undersea moorings as contemplated by the Ocean Observatories Initiative will include acoustic sources and receivers that can provide synoptic real-time coverage