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AD-A115 412 ROYAL AUSTRALIAN NAVY RESEARCH LAO EDGECLIFF F/6 8/10AIRBORNE EXPENDABLE BATHYTHERMOGRAGH SURVEYS, 1981. WESTERN TAS--ETC(U)MAR 82 P J1 MULHEARN
UNCLASSIFIED RARL-TM-2/82 l
2.8 1 2.52.2
1.25 III fL l1.
MICROCOPY RESOLUJTION TEST CHART
'-4 UNCLASSIFIEDAR Number 0026 97
.. AUSTRALIA .
DEPARTMENT OF DEFENCE
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION
R.A.N. RESEARCH LABORATORY
EDGECLIFF, N.S.W.
RANRL TECHNICAL MEMORANDUM
(EXTERNAL) No. 2/82
AIRBORNE EXPENDABLE BATHYTHERMOGRAGHSURVEYS, 1981, WESTERN TASMAN SEA
BY
P.J. MULHEARN
APPROVED DTICELECTE
FOR PUTBLI I ~LT -i~
COPY• N. 7 MARCH 1982
UNCLASSIFIED
i
DEPARTMENT OF DEFENCE
R.A.N. RESEARCH LABORATORY
AJ%
c Commonwealth of Australia (1982)
RANRL TECHNICAL MEMORANDUM (EXTERNAL) NO. 2/82
AIRBORNE EXPENDABLE BATHYTHERMOGRAPH
SURVEYS, 1981, WESTERN TASMAN SEA
P.J. MULHEARN
ABSTRACT
On four occasions in 1981 AXBT surveys were conducted in the western TasmanSea in support of the RAN's ocean analysis scheme. The results of thesesurveys show that the limited number of probes used (18 to 20 on eachoccasion) were able, in conjunction with other routinely available data,to delineate the main oceanographic features in the region. The depth ofthe sound speed maximum was more variable in winter than in summer, butthroughout the year, vertical sound-speed gradients below the maximum wereweakest within warm-core eddies and north of the Tasman Front.
POSTAL ADDRESS: The Superintendent, RAN Research Laboratory,P.O. Box 706, Darlinghurst, N.S.W. 2010.
iLi
(ii)
CONTENTS
PAGE NO.
1. INTRODUCTION 1
2. AXBT SYSTEM DESCRIPTION 1
3. AXBT FLIGHT PATTERN 1
4. OCEANOGRAPHIC BACKGROUND 2
5. RESULTS 2
6. SUMMARY AND CONCLUSIONS 3
REFERENCES 4
ACKNOWLEDGEMENTS 4
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1. INTRODUCTION
Oceanographic conditions in the western Tasman Sea are dominated
by the East Australian Current and its warm-core eddies, which are very
variable features in both space and time. Because of the strong thermal
contrasts caused by these features and the large variations in mixed-
layer depth associated with them, they have a marked effect on sonar
propagation. In recent years the R.A.N. Research Laboratory (RANRL) and
the meteorological/oceanographic officers at the Naval Air Station (NAS)
Nowra have been establishing a scheme for keeping track of the principal
features in the western Tasman, and this system is now operational
(Mulhearn,1981). Data are obtained fron infra-red satellite imagery,
RAN fleet XBTs, research cruises, satellite-tracked buoys and, from 1981,
airborne expendable bathythermographs (AXBT's). The AXBT surveys which
were performed every two months from March to September 1981, inclusive,
are discussed here. Previous AXBT surveys in this area are reported in
Lawrence (1980). Because of the limited number of probes available,
interpretation of the AXBT data often relies on information from the
additional sources discussed above.
2. AXBT SYSTEM DESCRIPTION
The AXBT's were dropped in a predetermined pattern by RAAF
Orion aircraft and the data sent back as sound-speed profiles with some
temperature information. The AXBT (AN/SSQ36 sonobuoy) is a cylindrical
package approximately 12.4 cm in diameter and 91 cm long. It can be
launched from a standard sonobuoy launch tube from which it parachutes
to the sea-surface, jettisons its parachute and deploys an antenna.
After approximately a minute a thermistor probe is released which sends
a signal by fine wire back to the surface float which transmits the
temperature profile by radio to the aircraft where the data is converted
to a graphical sound-speed profile and a digital temperature output.
Detailed descriptions of AXBT systems are in Sessions and others (1976)
and Sessions and Barnett (1979).
3. AXBT FLIGHT PATTERN
Generally AXBT's were taken every 0.50 lat. between 31.5 0 S,
1550 E and 37.50S, 151°E and from 31.5 0 S, 153.5 0E and 33.50S, 152 040'E,
2
for 13 March, 13 May, 9 July and 22 September 1981. For the last two
surveys drops were also made near 34.3 0S, 152.5 0E.
4. OCEANOGRAPHIC BACKGROUND
In December 1980 eddies Maria and Leo combined near Eden to
form eddy "Mareo" in which the well-mixed core of Maria overlay the
cooler core of Leo. This new eddy "Mareo" then migrated in a generally
north-west direction between March and September. Interaction with the
Tasman Front is apparent in July and the eddy appears to be north
of the front by September.
5. RESULTS
A. Survey of 13 March 81
Charts of sea-surface temperature (SST), temperature at depth
of 250 m (T2 50) and surface mixed-layer depth (LD) are shown in Figs. 1,
2 and 3. It can be seen that the SST data give a poor representation of
the deeper structure and that the mixed-layer depth is fairly uniform
throughout the area except around 32 S, 154 0 40'E. This survey was
taken towards the end of the summer heating period and the SST pattern
was not simply related to deeper features. The sound-speed cross-section
in Figure 4 shows weak gradients within the eddy and north of the Tasman
Front but strong gradients both vertically and horizontally at the Tasman
Front and on the southern edge of the eddy. (Units of ft. and ft/sec are
used here for convenience because of the way the data was obtained).
B. Survey of May 1981
In this survey eddy Mareo has not moved greatly, but a cyclonic
cold-core eddy has appeared to its north-east (such an eddy was observed
in March (Mulhearn, 1981b))and the Tasman Front appears to move south-
east from Sugarloaf Point to north of the cold eddy with a warm return
flow near 32.250S, 154.5 0E (see Fig. 6). Again SST is (Fig. 5) not
a good indication of deep structure, although surface mixeJ-layers -re
generally deeper than in March (Fig. 7). The sound-speed section in
Fig. 8 shows a deeper sound-speed maximum south of the Tasman Front,
with smaller vertical gradients in the warm-core eddy than in the cold-
core eddy immediately to its north. Within the East Australian Current
loop, vertical sound-speed gradients are again weak.
3
C. Survey of 9 July 1981
At this time the broad SST pattern (Fig. 9) was a much better
guide to deeper structure (Fig. 10) than it was earlier. It appears
that the mixed-layer structure (Fig. 11) is complicated by the inter-
action between the Tasman Front and eddy Mareo. The sound-speed cross-
section (Fig. 12) shows deep sound-speed maxima within the eddy and
north of the Tasman Front with weak sound-speed gradients in both these
areas.
D. Survey of 22 September 1981
On this occasion the eddy Mareo had moved well within the
Tasman Front, but weak anticyclonic and cyclonic features can be seen
further south (Fig. 14). Because the season is becoming warmer SST
(Fig. 13) is, once more, not a good indicator of deeper structure and
mixed-layer depths (Fig. 15) are becoming much shallower. A sound-
speed section could not be presented because of the loss of the profiles.
6. SUMMARY AND CONCLUSIONS
AXBT surveys made during 1981 in support of NAS Nowra's ocean-
analysis scheme are presented here. With normally available environmental
data they showed up the main oceanographic features quite well. They
demonstrate that when summer heating is important the presence of eddies
seems to have little effect on the depth of the sound-speed maximum but
that vertical sound-speed gradients away from fronts are much weaker than
those near them. The broad scale SST pattern appears to be a poor indicator
of deeper structure in summer, but infra-red satellite images having
better spatial resolution than ship or aircraft surveys, do indicate
that eddies and fronts are detectable throughout the year because of
advective effects. (Nilsson and others, 1981). More detailed data on
fronts may indicate that depths to sound-speed maxima are drastically
affected in their vicinity.
ii
4
REFERENCES
Lawrence, M.W. (1980). Oceanographic Surveys in the Tasman Sea using
airborne expendable bathythermographs. R.A.N.R.L. Tech.
Memo (Ext) 6/80.
Mulhearn, P.J. (1981). A Real time ocean analysis of the NSW coast.
Aust. Oceanog. Data Centre Bull. 15, July, 1981. pp 11-19.
Mulhearn, P.J. (1981)b. RANRL Cruise Leaders Report 5/81 (unpublished
manuscript).
Nilsson, C.S., Andrewo,J.C., Hornibrook, M., Latham, A.R., Speechley, G.
and Scully-Power, P. (1982). High resolution satellite
observations of mesoscale oceanography in the Tasman Sea
1978-79. Final Report Project HCM-051. RANRL Report 1/82.
Sessions, M.H., Barnett, T.P., and Wilson, W.S. (1976). The airborne
expendable bathythermograph. Deep-Sea Res. 23, 799.
Sessions, M.H. and Barnett, T.P. (1979). The airborne expendable
bathythermograph for oceanographic measurements. Near Surface
Ocean Experimental Technology Workshop. Nov. 1979.
ACKNOWLEDGEMENTS
Thanks are due to the aircrews of the Orion aircraft who took
these measurements and to the staff of the RAAF base at Edinburgh, S.A.
-. ...... . .
5
FIGURE CAPTIONS
Figure 1. Sea-surface temperature pattern 13 March 1981.
(contours are every 1C).
Figure 2. Temperature at 250 m depth 13 March 1981.
(contours every 1°C).
Figure 3. Surface mixed-layer depth 13 March 1981.
(contours 25 m).
Figure 4. Sound-speed cross-section from 31.5 0 S, 155 0 E to 37.5 0 S,
151 0 E. (Contours every 10 ft/sec).
...sound-speed maximum. (Points are labelled with last
2 digits of sound speed values for speeds > 5,000 ft/sec
and with last 3 digits for speeds < 5000 ft/sec).
Figure 5. S.S.T. pattern of 13 May 1981 (contours every 10C).
Figure 6. Temperature at 250 m of 13 May 1981 (contours every 1C ).
Figure 7. Surface mixed-layer depth of 13 May 1981 (contours every
25 m).
Figure 8. Sound-speed cross-section from 31.50S, 155 0 E to 37.50S,
151 0 E (contours every 10 ft/sec), 13 May 1981;
...sound-speed maximum. (Points are labelled with last
2 digits of sound-speed values).
Figure 9. SST - 9 July 1981 (contours every 1 C).
Figure 10. Temperature at 250 m of 9 July 1981 (contours every 1°0)..
Figure 11. Surface mixed-layer depth 9 July 1981 (contours every
50 m).
Figure 12. Sound-speed cross-section of 9 July 1981 from 31.5 0 S,
1550 E to 370S, 151 0 E. (Contours every 10 ft/sec);
... sound-speed maximum.
6
Figure 13. SST of 22 September 1981. (contours every 1°C).
Figure 14. Temperature at 250 m depth of 22 September 1981.
(contours every 1 Q.
Figure 15. Surface mixed-layer depth of 22 September 1981
(contours every 10 m).
t|
7
31*S SMOKY CAPE
x -9x249 214
320 x x25-8 24-3
SUGARLOAF Ptx x
24-6 24-6
33* x x23-7 248
SYDNEY 22-1 9261
35.JERVIS 2Sxf BAY
x22-3
X2286
X 21-6
151"E 1520 1530 1W ~ 155' 156*E
1 38 V
Fig. 1. Sea Surface Temnerature nattern 13 March 1981.
SMOKY C APC
3 2
SUGARLOAF Pt
11.5 0 2-3
330
12-0 l87
x TASMAN FRONT
SYDNEY, 11.8 12-6
340
35. (1-J ER VIS
t . BAY1
17-
37 11-
1519 1520 1530 1540 1550 156"E
38-SI
Fig. 2. Temperature at 250m depth. 13 March 1981.(Contotirs every 1C)
9 -I ...31S SMOKY CAPE I
32°
SUGARLOAF P9
29 57
33'28
XSYDNEY 38 X42
340 ...... .X35
350 X364
JERVS A
t 'BAY
X27
151"E 152" 1530 1541 156E
Fig. 3. Surface mixed-layer depth. 13 March 1981.(Contours 25m).
10-
M1 __ _ _ _ _ __ _ _2___ _ _ _ _
43 la is(akl
A9L v),0
V~C (A
CL
ra
imjV.
g0x0;9
so4*~
* to
ION,
31S SMOKY CAPE
- 21.9 .x X
32*>mmsi21-9 3*
SUGARLOAF Pt
x x21.9 214S
x x21-9 21-5
IL x x./SYDNEY.. -20-4 21u
340 SEA MUCH LIOHTER-- "' 21. BLUE HEREQg.1
t-J-ERVIS /BAY
- .VISIBLE LINE
~ LIGHTER BLUE TO SOUTH
x19.5 190
3701.
1510E 152* 1530 15' 155' 156E
389SI 1 1 1 1 1. 1 . 1 1 -
Fig. 5. SST pattern of 13 May 1981.(Contours every IC)
14 "
-12
310S SMOKY CAPE /T ' T;. -r-- ' + / *'-
163 K16-2lo "
SUGARLOAF Pt
-- x16.2 x1s.
- x 16"0IS52
/x
SYDNEY X163
340- "(15.2
TASMAN
-FRONT
350 IJERVIS I6
BAY
* ~14.2-
EDDY370 MARF0
X116
1516E 152* 1530 1540 155' 166*E
1380S I IIFig. 6. Temperature at 250m of 13 May 1981.
(Contours every 1*C)Ii
13
-S SMCK Y CAXP[E~-
33. X0 ISOSOUND TO 91m
99
350 XJERV IS 7
BAY
aX
122
91
3770
X49
1510E 1524 1530 1540 1559 IS60E
30S1 I .Flg.7. Surface mixed-layer depth of 13 May 1981.CContours
every 25M)
14
a S
II
-as
15
31*S SMOKY CAPE
26 21-3 21-77
32 x18-2 21,2
SUGARLOAF Pt
x -
1847
B~ Y
315.
370 1
1510E 1520 1530 154* 155r 1560E
38t 1 L
Fig. 9, SST - 9 July 1981 (Contours every 1C0
16............ T-
3 1S SMOKY CAPE j
x x \X2-
32 12x 1 .
SUGARLOAF Pt TSA169 13.8FRONT
33016.8 13*8
SYDNEY,. 17-6 1-
34Q 12
35.
BEVS~7 EDDY MAREO
~11.
rB
I1E 1820 1530 1540 155 1560E
1384S1 L LII
Fig. 10. Temperature at 250m of 9 July 1981 (Contours every 19C)
17 -W T~~31"S SMOKY CAPE I
329X X
SUGARLOAF Pt16I?
205 if, Isom
164 *122
SYDNEY. 214 2O~
340
137X274
91
35.
JE R VI lo o mBAY 183
169
loom
370
62
151'E 1520 1530 1540 l55e * 16E
Fig. 11. Surface mixed-layer depth 9 July 1981 (Contours every 5Orr#
a-
- 0 . .be1.
-400
. ..... ...................
#AVON1
19
3 1" SMOKY CAPE I
22-2 22-2 22-1
3022-7 20-0
SUGARLOAF Pt22-3 20-6
33* 22-1 20-4
421-7 20-2SYDNEY ~- m
20-6
350 7 0*4
J ERV ISt BAY
17-1
37. 15-6
- 17-S
15PE 1520 153* 154P 1556 156*E
3S*ISi
FIg. 13. SST of 22 September 1981 (Contours every 19Q
'20
3"S SMOKY CAPE
16.9 17.9 16-3
3216-1 18.9
SUGARLOAF Pt15. 18S*2
K 18
(17-2
SYDNEY. 01434. <~.iI~~EDDY MAREO
.JERVIS 13-1BAY
133.9
1510E 1520 1530 1540 1550 156*E
1369S I II
Fig. 14.. Temperature at 250 m depth of 22 Sept.81. (Contour every r'C).
21
310S SMOKY CAPE
5 8
320
SUGARLOAF Pt
33.
3'.9
35.
370 4'.
3530367
JERVIS 1640M 5'15'16
-B' YFi. 5.Sufae iedlaerdptho 2 Spt81 (ptorsv30m~i)
22
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23
DOCUMENT CONTROL DATA SHEET
1. a. ARNo. 1.b. Establishment No. 2. Document Date 3. Task No
002-697 RANRL-TM-2/82 1982 79/053
4. Title Airborne expendable bathythermograph 5. Security 6. No Pages
surveys, 1981, western Tasman Sea. a. document 25UNCLASSIFIEDb. title 7. No RefsUNCLASSIFIED 5c. abstract
UNCLASSIFIED
8. Author(s) 9. Downgrading Instructions
P. J. Mulhearn N/A
10. Corporate Author and Address 11. Authority (as appropriate)a. Sponsor b. Security
RAN Research Laboratory c. Downgrading d. Approval
P.O. Box 706DARLINGHURST NSW 2010 a.
b,cGHOSd. W.F. Hunter, SuperintendenRANRL , ,. ,,.-,-
12. Secondary Distribution (of this document)
Approved for Public Release
13. a. This document may be ANNOUNCED in catalogues and awareness servicesavailable to
Unlimited
13. b. Citation for other purposes (ie casual announcement) may be as for 13 a.
14. Descriptors 15. COSATI Group
Aerial Surveys 0802Bathythermograph Data 0810Oceanographic SurveysOceanographic Charts
16. Abstract. On four occasions in 1981 AXBT surveys were conducted in the western
Tasman Sea in support of the RAN's ocean-analysis scheme. The results
of these surveys show that the limited number of probes used (18 to 20
in each) were able, in conjunction with other routinely available data,
to delineate the main oceanographic features in the region. The depth
to the sound-speed maximum was more variable in winter than in summer,
but throughout the year, vertical sound-speed gradients were weakest
within warm-core eddies and north of the Tasman Front.
6 L __-A 1 ..-.- -.-. ... ..--.... "-- .......- - - - - - I I I