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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COPY NO.

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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