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The sound-velocity structure ofthe South Pacific OceanR. N. Denham a & A. C. Kibblewhite ba Naval Research Laboratory , HMNZ Dockyard ,Aucklandb Physics Department , University of AucklandPublished online: 30 Jan 2012.

To cite this article: R. N. Denham & A. C. Kibblewhite (1970) The sound-velocitystructure of the South Pacific Ocean, New Zealand Journal of Geology andGeophysics, 13:1, 39-51, DOI: 10.1080/00288306.1970.10428205

To link to this article: http://dx.doi.org/10.1080/00288306.1970.10428205

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No.1

THE SOUND-VELOCITY STRUCTURE OF THE SOUTH PACIFIC OCEAN

R.N. DENHAM

Naval Research Laboratory, HMNZ Dockyard, Auckland

and

A c KIBBLEWHITE

Physics Department, University of Auckland

(Received for publication 24 June 1969;

ABSTRACT

39

A general survey of the sound-velocity structure of the South Pacific Ocean ts presented. The type of variation of sound velocity with depth encountered depends on the area: antarctic, subantarctic, subtropical, and tropical velocity-depth profiles are defined. A major feature of the last three types of profile is the presence of a SOFAR velocity minimum between depths of 900 and 1,500 m. The actual depth of this minimum, together with other characteristics of the velocity structure, is affected by the South Pacific gyral and other anticyclonic eddies which are present in the Australasian area.

INTRODUCTION

A basic parameter which governs the characteristics of sound propagation in the ocean is the velocity of sound. It is well known that spatial and temporal variations in sound velocity have a marked effect on the quality of sound transmission; this topic is covered adequately by the texts of Officer (1958), Tolstoy and Clay (1966), and Urick (1967). Over the last decade, the New Zealand Naval Research Laboratory has been gathering sound velocity data in the South Pacific Ocean betweert'the meridians 150° E and 135° W, as shown in Fig. 1. Over the same period much additional informa­tion from other sources has become available for the whole South Pacific area. Although there still exist some large gaps in the area covered, it is felt that the data available at the present time can be summarised to give a meaningful description of the nature of the sound-velocity structure in the area lying between the Equator, latitude 70° S, longitude 150° E, and South America. In addition to doing this, an attempt is made to relate the characteristics of the observed sound-velocity structure to the general current circulation and water masses present in the South Pacific.

N.Z. Jl Geo/. Geophys. 13: 39-51

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40

150°

N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS

'=

165° E 180° w

. .

..

. .. . . . . .. .. ..... . . . • ·-::1'(:·:

... *' .... .. ,;. ··. "•

. ....

e •• •.!!

165°

.. : .... . . . .. .

150.

VoL. 13

135" 00

15°

30"

45" s

60°

FIG. !-Positions where measurements of sound-velocity have been made by Naval Research Laboratory (N.Z.).

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No.1 DENHAM & KIBBLEWHITE -SouND VELOCITY 41

MEASUREMENT oF SouND VELOCITY

Most of the sound-velocity data available are based on the derivation of sound velocities from temperature and salinity data measured by means of reversing-bottle casts. The sound velocity, V, is calculated by means of an empirical formula first used by Wilson (1960), which is usually expressed in the form

(1)

where Vis expressed in mjsec, VT is a function of temperature only, Vp is a function of pressure only, V8 is a function of salinity only, and VsTP is a datively small term which is a function of all three independent variables. The first term represents the velocity of sound in mjsec in water having a salinity of 35%0 , at zero pressure, and with a temperature of 0°c. If the temperature T is expressed in °C, the pressure P in kgjcm2 , and the salinity S in parts per thousand, a somewhat rough first order approximation of (1) is

V = 1449·1 + 4·57T + 1·4 (S- 35) + 0·16P. (2)

On the other hand, some of the Naval Research Laboratory data have been obtained by in situ measurements using a velocimeter based on the "sing-around" principle (Greenspan and Tschiegg, 1963). The commercial units used have an accuracy of about -t-0·3 mjsec, which is much the same as that of the indirect method using Wilson's formula. As well as giving a relatively quick, direct measurement of sound velocity, the velocimeter produces a continuous record of the sound velocity as a function of depth, whereas, for practical reasons, the usual reversing-bottle cast only gives spot readings at depths typically spaced no less than 10 m apart at shallow depths and some 200 m apart at a depth of 1 km.

A comparison has been made between sound velocities calculated from hydrological data and the values obtained directly with the velocimeter at the same stations between New Zealand and the Cook Islands in 1963 (Kibblewhite et a!., in prep.). It was found that the values computed from the temperature and salinity data using Wilson's formula were within 1 mjsec of those obtained directly. On the other hand, the values of sound velocity computed from the hydrological aa'ta using the tables of Matthews (1939) were usually 3 or 4 mjsec lower in magnitude. These results are in agreement with those of Hays (1961) who carried out a similar com­parison in the Mediterranean Sea, and confirm that Wilson's formula is applicable for the South Pacific area. Recent work (Carnvale et ai., 1968) indicates that Wilson's formula gives values that are 0·5 to 0·7 mjsec too high but no better formula exists at present and, in fact, Wilson's formula is adequate for most purposes and certainly for the general survey carried out here.

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42 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VoL. 13

VELOCITY-DEPTH PROFILES

The Antarctic Region

The antarctic region may be defined as the area south of the hydrological feature known as the Antarctic Convergence or Polar Front. This feature has been defined in various ways (Deacon, 1963) : the definition most appropriate here is that it represents the boundary where the permanent thermocline in the temperature-depth profile disappears The position of this boundary ranges from 60° S at the longitude of New Zealand, to about 55° S at around 140° W, to 60° S again at the longitude of South America. A detailed account of the hydrological structure of the antarctic region between 20° W and 170° W has recently been given by Gordon (1967).

Two typical velocity-depth profiles for this region are shown in Fig. 2, together with the temperature-depth profiles at the same stations. These stations, at the position 58° S, 160° W, were occupied in April 1958 during Cruise 3 of the research vessel Ob (IGY World Data Centre A, 1961 *; Klepikova, 1961) and in August 1964 during Cruise 14 of the USNS Eltanin (Jacobs, 1965). It is apparent that the velocity shows an essentially mono­tonic increase with depth apart from the shallow sound channel at around 150-200 m which exists in the April profile. This sound channel which is defined as the antarctic duct by Garner (1967) may be ascribed to the presence of Antarctic Winter Water between layers of comparatively warm Antarctic Summer Water and Pacific Deep Water. In August the layer of winter water of course extends to the surface and the antarctic duct is not present.

The Subantarctic Region

Immediately north of the Antarctic Convergence lies the subantarrctic region. The northern boundary of this region is defined by the Subtropical Convergence, which in the western parts of this area lies at about 40° S. Further to the east it is ill-defined and the subantarctic region extends north of 35° S.

The examples of velocity-depth profiles for this region are again pro­vided by Cruise 3 of the Ob and Cruise 14 of the Eltanin, the stations being in the vicinity of 50°S, 160°W (see Fig. 3). The April1958 profile, which shows a velocity minimum at 400 m, provides an example of the subantarctic duct defined by Gafher (1967). More important, however, is the deeper minimum which exists at around 1,000 m on both profiles. This minimum defines the axis of the SOFAR channel, which is found everywhere in the Southern Hemisphere north of the Antarctic Convergence. The mini­mum is a result of the presence of the permanent thermocline: at the depth of the soF AR minimum the rate of decrease of the term V T in ( 1) with increasing depth below the thermocline balances the increase of V P with increasing depth (see also ( 2) ) .

*The sound velocities given in this report are mcorrect and need to be recalculated.

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No.1 DENHAM & KIBBLEWHITE -SoUND VELOCITY 43

V E l 0 C I T V I N K M/SEC T E M PERATURE IN °C

o.--...:.l·f4.;:.4 __ r-....,.,-.._:..;~~r•::.6.......,;:-. ,---.:..,' ;;.•::.s __ r----!.i'·~s:.:,o _ __, o /.-------:~ 6

~~)t~+ ·t.,..

1•0

><-X APRil 1958 ·-·AUGUST 1964

~ ~ ~ ]

\.\ //

\ "!i \ \ 1

\ \

\ \

\ \

\ \

J

J X

I . I

3·0~-~----_. ___ _. ____ ~ ____ ._ ____ ~--~:_.\ __ ~ I j

FIG. 2-Sound-velocity and temperature--depth profiles at 58° C, 160° W taken m April 1968 and August 1964.

1·46 VELOCITY IN KMISFC TEMPERATURE IN "C

0 1·48 1·50 1·52 0 s 10 I '· ~/ _') ' \ /'/ . ,x

•• )l..x I/

~ ~ k II

/ \ I x

I I I X

' I I +

1·0 // I X

u ~

:;;

f " z -~

:r

\ f ,_ a. -..-, 0

2·0 '\ X

\ " APRIL 1958 I AUGUST 1964 ~\ II

\ r 3·0 \ I

FIG. 3-Sound-velocity and temperature--depth profiles at SOQ S, 160" w taken in April 1958 and August 1964.

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44 N.Z. JOURNAL OF GEOLOGY ANl:? GEOPHYSICS VoL. 13

The Subtropical Region

On proceeding north from the Subtropical Convergence warm, highly saline surface waters of tropical origin are encountered and as a result a marked increase in surface temperature occurs together with an intensification of the permanent thermocline. The subtropical region may be defined as the area north of the subtropical convergence over which the temperature at all depths down to 500 m increases with decreasing latitude along a meridian of longitude. In the western part of the South Pacific area, this region lies between roughly 15° S and 40° S but in the east it cannot be very well defined.

Fig. 4 shows a typical subtropical profile which was obtained by the Naval Research Laboratory in April 1965 at position 31° 24'S, 177° 08' W (Denham et al., in prep.), while Fig. 5 provides an example of seasonal variations of profiles in the New Zealand area. The data used in Fig. 5 were obtained by the Naval Research Laboratory during intensive surveys in the winter of 1964 and the summer of 1965 in the area to the north-east of New Zealand (Barker and Denham, in prep.).

It is apparent from Fig. 4 that the SOFAR channel is much better defined in subtropical waters than in the subantarctic region, a pronounced velocity minimum occurring at depths of 1,250-1,500 m in the New Zealand area. This is one result of the stronger permanent thermocline. Another conse­quence is that the stability in the thermocline layer will be much higher north of the Subtropical Convergence. In connection with this, recent velocimeter measurements made by the Naval Research Laboratory show that there appears to be much less fine structure apparent in velocity profiles taken in the subtropical region than in those taken south of the Subtropical Convergence. Other evidence of the effect of the Subtropical Convergence on the structure of thermocline turbulence has been obtained in the long-range acoustic propagation studies of Kibblewhite et al. ( 1965), and Kibblewhite and Denham ( 1966).

The main seasonal effect present in the subtropical region is, apart from the expected variations in surface sound velocity, the presence of a surface sound channel in winter months, as is shown by the July profile in Fig. 5. This is seen to result from the presence of a thick isothermal layer which extends to about 150m in the profile shown. The exact depth of this layer is found to be rather variable in both space and time and to depend on a number of factors as discussed by James (1966). In the summer months this layer is either very shallow or absent, the dominant upper feature then being a seasonal thermocline extending'"'typically from 50 m to 150m, as shown by the March profile in Fig. 5.

The Tropical Region

Towards the Equator, a further intensification of the permanent thermo­cline takes place: as well as the expected northwards increase in surface temperature there is also now a decrease in temperature at any given depth below about 200 m. For instance, a vertical section of temperature along the 160° W meridian shows that all isotherms of temperatures below 20°C

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No.1 DENHAM & KIBBLEWHITE- SOUND VELOCITY 45

VELOCITY IN KMISEC TEMPERATURE IN °C 0 1 · 4 a 1 ·50 1 ·52 o~--..,----\1or-:---"T""--~2'Wo--,

.-·-)'

, /

I 1·0 /

/ z \. I

\ I \

\. i

I 3·0L..-....L--..1..--..L...--'--....L--.J

Fro. 4-Sound-velocity and temperature-depth profiles at 31 o 24' S, 177° 08' W taken in April 1965.

:0

" z

()

VELOCITY IN KM/~EC

1 50 I 52

\-- ... ---"· ~ ........ ~ '/

lr /i

,-(/' .f/

;f !l

l I f >

HL--.....L......~.i...-l-.--.J~

0

~I

TEMPERATURE IN "C 10 20

- JULY 1964

•-x MARCH 1965

FIG. 5-Sound-velocity and temperature-depth profiles at 34° S, 176° E taken in July 1964 and March 1965.

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46 N.Z. JouRNAL oF GEoLOGY AND GEoPHYSics VoL. 13

reach a maximum depth between 20° and 30° S-see fig. 2 of Reid (1965). Thus it is possible to define a distinct tropical region in the western part of the area, extending north from about 15 ° S; in the east the distinction between the subtropical and tropical regions is less dear.

An example of a tropical profile (shown in Fig. 6) was calculated from the data obtained at a station at 7° 57'S, 165° 00' W occupied by HMAS Gascoyne in August 1963 (CSIRO, Australia, 1968). This profile shows the marked decrease in velocity from 100 to 400 m, characteristic of tropical profiles. In addition, the SOFAR velocity minimum is shallower than in the subtropical case, lying at around 900 m in this example.

MERIDIONAL SECTIONS

An instructive comparison of the velocity-depth vanatJon at different longitudes is provided by the vertical sections presented in Fig. 7. In Fig. 7 (a) the stations of the Ob Cruise 3 along 160° W are combined with the stations of the Gascoyne Cruise G3j63 along the 165° W meridian while Fig. 7 (b) combines the Ob Cruise 3 stations along 109° W with the stations occupied by Expedition "Downwind" of the r.v. Horizon (Scripps Institution of Oceanography, 1965) along roughly 110° W.

One of the most significant features of each section is the change in shape of the velocity isopleths at about 50° S latitude. This change is associ­ated with the transition from the antarctic to subantarctic profiles taking place across the Antarctic Convergence. At latitudes north of 35° S, the

z

J: .... ll.

"' CJ

VElOCITY IN KM/S~C TEMPERATURE IN "C

o1 _..;.1·f4,:;;.B_...,.._....:,1 rso~....,r--:0,1 1"'52'-----,--1!:,·5~4~--, 0 10 20 30

20

(

(

I \

---· /'

\ \

__ , ----- ,.

i I i I i

I

I

_,l

..........-·-· ,.,......

FIG. 6-Sound-velocity and temperature-depth profiles at 7° 57' S, 165 o 00' W taken in August 1963.

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No.1 DENHAM & KIBBLEWHITE- SOUND VELOCITY 41

SOFAR axis is well defined along both sections as shown by the crossed lines in Figs. 7 (a) and (b). In addition, the intensification of the thermo­cline at low latitudes is indicated by the convergence of all the near-surface isopleths of velocity greater than 1,500 mjsec as the Equator is approached

However, there are several important differences in the data from which the two sections shown have been prepared. Firstly, the SOFAR axis cannot be unambiguously defined south of latitude 35° S along the 110° W meridian whereas it is clearly discernible along the 165° W meridian as far as 50° S. Secondly, all isopleths above the SOFAR axis lie deeper along the 165° W section at mid-latitudes than they do along the 110° W section at the same latitudes. This is reflected in the position of the SOFAR axis which lies at about 1,300 m at 35° S, 165° W whereas it lies at a shallower depth at the same latitude on the 110° W meridian. Finally, the spacing of the isopleths above the SOFAR axis reflects the fact that the subtropical type of profile is much easier to discern in the west than it is in the east.

FIG. 7-Meridional sections of sound-velocity as a function of depth along (a) 160°-1650 W and (b) 110° W. The velocity is expressed m mjsec and the contour interval is 5 mjsec. The crossed lines show the position of SOFAR minimum while the dots indicate the density of sampling.

Geology-6

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48 N.Z. JouRNAL OF GEOLOGY AND GEOPHYSICS VoL. 13

The differences in character between the vertical sections at 160°-165° W and 110° W may be largely explained in terms of the circulation of the upper oceanic layers. East of New Zealand the main feature of the circulation is the presence of an anti-cyclonic gyral, the South Pacific gyral. This feature is probably more intense between 30° and 40° S near the 165° W meridian and much weaker further east (see for example figs. 18 and 23 of Reid (1965). As explained by Neumann and Pierson (1966), isopycnals, and therefore isothermals, will reach a maximum depth in the centre of an anticyclonic gyral as a result of geostrophic balance. Thus it may be expected that both the velocity isopleths above the SOFAR axis and the axis itself would lie deeper in mid-latitudes in the vicinity of the 165° W meridian. Furthermore, the transition between the tropical and subtropical types of profile is seen to be associated with this feature, taking place at its northern boundary.

VARIATION oF SoFAR-AXIS VELOCITY

The general pattern of the geographic variations in the SOFAR-axis velocity, or the minimum velocity where the axis cannot be sensibly defined, is shown in Fig. 8. A more detailed description for the New Zealand area is provided by Fig. 9 which also shows the 500, 1,000, and 2,000 m isobaths as traced from Lawrence (1967). Data for Figs. 8 and 9 have been compiled from many sources. In addition to the cruises referred to in tables 2 and 3 of Reid (1965), table 1 of Garner (1967), and earlier in the present paper, data have been obtained for the antarctic and subantarctic regions from Cruises 16 to 27 of the USNS Eltanin (Jacobs, 1966; Jacobs and Amos, 1967) and for the subtropical and tropical regions from Cruise G3/61 of the HMAS Gascoyne (CSIRO, Australia, 1967), Rotschi et at. (1967), and Heath (1968) as well as from our own stations shown in Fig. 1.

South of the Antarctic Convergence, the equivalent of the SOFAR channel is of course the antarctic duct (see Fig. 2). In this region, the minimum velocity shows marked variations, owing no doubt to the fact that the axis of the duct lies between the surface and 500 m; as a result, contour intervals of 10 and 5 mjsec have been adopted for velocities less than 1,480 m/sec in Figs. 8 and 9 respectively, and a liberal amount of smoothing has been employed in drawing these contours. Nevertheless, the positions of the 1,450 to 1,470 mjsec isopleths show a general trend, their latitude follow­ing that of the Antarctic polar front zone as plotted by Gordon (1967). Thus they lie furthest north between 150° and 140° W and furthest south around 180° W and again between 80° and 90° W. In addition the pro­nounced southward trend of the 1,450 mjsec isopleth between 140° and 130° W may be associated with the development of a secondary polar front zone between these meridians.

East of the 110° W meridian, the more or less smooth increase in axis velocity with decreasing latitude encountered south of the Antarctic Con­vergence is maintained as far as the Equator, as is the case for the North Pacific (Johnson and Norris, 1968). However, in the west the velocity

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No.1 DENHAM & KrBBLEWHITE- SouND VELOCITY 49

FIG. 8-SOFAR-velocity chart of the South Pacific Ocean The velocity is expressed m m/sec. For velocities less than 1,480 mjsec the contour interval is 10 m/sec and for velocities greater than 1,480 m/sec the interval is 2m/sec.

reaches a maximum at mid-latitudes and has a mtntmum between 10° and 20° S. Superimposed on this trend in the Australasian area are numerous regions of rather high velocity, values as high as 1,490 mjsec being found.

The deepening of isothermals which takes place towards the centre of an anti-cyclonic gyral would be expected to result in a maximum value of SOFAR-axis velocity in the centre of the gyral. This effect should lead to the presence of an area of maximum SOFAR-axis velocity at mid-latitudes near the 165° W meridian. Furthermore, in the Australasian area, the pre­vailing current systems are found to generate a complicated pattern of anticyclonic eddies (Barker and Kibblewhite, 1965; Hamon, 1965; Ridgway, in press; Heath, 1968) and the "islands" of high velocity observed could be largely a consequence of this situation. ...,

Another factor of importance is the effect of bathymetric features. Major perturbations in the circulation at intermediate depths in the New Zealand area are introduced by the Campbell Plateau and Chatham Rise (see Fig. 9). As a result an area of low velocity exists in the Bounty Trough (Garner, 1967) and areas of high velocity would be expected to exist north-west of the Chatham Rise.

Fig. 8 supplements and may be compared with the chart drawn up by Johnson and Norris (1968). Although Fig. 8 differs in detail from the map of Johnson and Norris the gross characteristics are essentially the same.

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50 N.Z. JouRNAL oF GEOLOGY AND GEOPHYSICS VoL. 13

CoNCLUSIONS

The sound-velocity depth profiles encountered in various regions of the South Pacific can be divided into four main types which have been classified as antarctic, subantarctic, subtropical, and tropical. A salient characteristic of the last three profiles is the presence of a SOFAR velocity minimum lying between depths of 900 and 1,500 m. An important factor governing the velocity structure in mid-latitudes is the South Pacific gyral, which is con­sidered to be stronger around 165° W. As a result a mid-latitude maximum in SOFAR-axis velocity and depth is found in the west of the South Pacific. The superimposed complex variation of SOFAR-axis velocity in the New Zealand area arises from the presence of anticyclonic eddies and perturba­tions induced in the circulation by topographical features in the New Zealand area.

ACKNOWLEDGI\1ENTS

This paper is published by permission of the Ministry of Defence (Navy). The authors wish to thank members of the Naval Research Laboratory staff who partici­pated in our own measurements and also Dr D. M. Garner and Mr N. M. Ridgway then of the New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, for their assistance with the machine computation of sound­velocities from hydrological data. In addition the authors would like to thank Dr Garner for his very helpful comments on the manuscript.

REFERENCES

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BARKER, P. H.; KIBBLEWHITE, A. C. 1965: Physical oceanographic data from the Tui cruise, 1962. N.Z. Jl Sci. 8: 604-34.

CARNY ALE, A.; BowEN, P.; BASILEO, M.; SPRENKE, ]. 1968: Absolute sound-velocity measurement in distilled water. f. acoust. Soc. Am. 44: 1098-102.

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HAYS, E. E. 1961 : Comparison of directly measured sound-velocities with values calculated from hydrographic data. f. acoust. Soc. Am. 33: 85-8.

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No.1 DENHAM & KIBBLEWHITE - SouND VELOCITY 51

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