REPORT ON THE OCEANOGRAPHIC INVESTIGATIONS CARRIED OUT AT

THE SOFALA BANK BY THE SOVIET TRAWLER "SEVASTOPOLSKY RYBAK"

IN SEPTEMBER~DECEMBER 1982

by

Antonio Jorge da Silva

INTRODUCTION

In September-December 1982 a fisheries research cruise was carried out at

the Sofala Bank (figure 1) by the SRTM "Sevastopolsky Rybak".

Oceanographic investigations have been conducted in association with the

fisheries research.

Three coverages were made of the Sofala Bank each lasting for slightly less

than three weeks. This will allow for three descriptions of the oceano­

g·raphic conditions in the area, with some reference being made to variabi­

lity. This paper is just a preliminary report of those investigations.

METHODS

The survey grids are shown in Fig·ures 2-4. No attempt was made to

indicate which observations were carried out at each station, as this would

make the figures unnecessarily complicated. Instead, the positions of the

stations used for the different horizontal distributions are indicated by

dots in Figures 5-12.

Nansen casts for temperature and salinity determinations were performed

at virtually all stations during the first and the third coverages. Water

bottles were placed at 0-2-3-5-10-20-30-50-75-100 m. Additionally,

bathythermograph soundings were carried out to the bottom at all stations

during the three coverages. Secchi depth and water colour (Forel Scale)

were determined at all stations performed during daytime in the first

coverage.

For sake of simplicity water colours were grouped in four categories:

1

2,3

4,5,6,7

8,9

deep blue

greenish blue

bluish green and green

yellowish green

Temperature and salinity profiles are shown in Figures 13-28. The

sections were numbered from south to north using the third coverage as a

basis. Each figure refers to one section and contains the three profiles

corresponding to the three coverages (e.g. Figure 16). In cases where

one or two profiles were missing the corresponding spaces were left blank

(e.g. Figures 13, 14 and 21). Surplus sections from the first coverage

were not considered.

RESULTS

During the approximately two months period separating the first from the

third coverage a temperature rise of about 2°C has occurred. In each

coverage the temperature tended to be some 2. 5°C higher at the north

than at the south (Figures 7, 9 and 11). No similar trend was found in

what concerns salinity, although important changes were observed from

one coverage to another (Figures 8, 10 and 12). It is quite remarkable

the correspondance between the salinity distribution of Figure 8 and the

distributions of optical properties of Figures 5 and 6. This suggests that

water of river origin can successfully be traced by means of colour sensors.

Important differences in the surface characteristics allow us to distinguish

three main areas in the Sofala Bank:

A - north of Pebane (latitude 17° 20'S)

B ·- between Pebane and roughly latitude 20°S

C - south of latitude 20°S

Area A was always characterised by the presence of salinities between

35, 1 and 35.3, values that are not typical of shelf water. During the first

coverage values above 35,2 were only observed north of latitude 17°S

(Figure 8) while, during the third coverage, they were found over the

whole shelf near Pebane (Figure 12). During the second coverag·e those

salinity values were instead found at the inner part of the shelf extending

to area B (Figure 10). Changes in the surface salinities from the first to

the third coverages were accompanied by the disappearence of an inshore

pocket of low temperature and its replacement by warmer water (Figures

7, 9 and 11).

Area B can be considered as characterised by a strong interaction of Low

Salinity Shelf Water (LSSW), mainly of Zambezi origin, with Oceanic Water

transported by the Mozambique Current 0 Strong horizontal salinity

gradients were therefore found in this area. Great variability was also one

of its characteristics 0

During the first coverage the lowest salinities were found to the north of

the Zambezi delta associated with a low temperature tongue extending

northwards (Figures 7 and 8). Salinities higher than 35 0 0 were only found

at the outer part of the shelf.

During the second coverage (Figures 9 and 10) only some remains of the

LSSW could be detected to the north of the Zambezi, near Quelimane.

Surface salinities higher than 35 0 0 were present over the whole shelf

north of the delta, while the lowest salinities were found to the south of

it 0 The salinity range and gradient were similar to those found during the

first coverage (Fig·ure 8). Similm· temperatures were detected at the inner

and outer parts of the shelf north of the delta, as well as over the whole

shelf due south of the Zambczi (Figure 9) 0 An area of lower temperature

was present at the mid part of the shelf north of the delta.

During the third coverage salinities greater thun 35. 1, characteristic of

Oceanic Water, were present over the whole shelf north of Quelimane

(Figure 12) o To the north of the Zambezi the temperature was fairly

constant around 28°C (Figure 11) 0 The lowest salinities were found near

the Zambezi mouth. Comparatively to the second coverage (Figure 10),

LSSW was apparently starting to propagate northwards (Figure 12). South

of the Zambezi the salinity distribution was similar to those of the two

previous coverages, except for an inshore pocket of higher salinity near

Beira. To the south of Quelimane the isohalines tended to be roughly

parallel to the isobaths at the outer part of the shelf. The temperature

distribution (Figure 11) shows a high temperature tongue extending

southwards at the inner and mid parts of the shelf and a low temperature

tongue extending northwards at the outer shelf.

Area C was characterised by the presence of high salinity water that is

known to be of shelf origin (BRINCA et al. 1981 a, 1981 b, 1983 a, 1983

b). This area is separated from Area B by a salinity front running more

or less zonally (Figures 8 and 12). A positive onshore salinity gradient

was observed during the first coverage (Figure 8), while Figure 12

reveals some kind of an eddy situation. A less intense meridional front

separated the High Salinity Shelf Water (HSSW) from the LSSW coming

from north.

Sections I~III (Figures 13-15) reveal that the HSSW was nearly

homogeneous from surface to bottom. Some unstability was, however,

detected at the surface during the third coverage (Figure 14-C). The

LSSW coming from north can also be traced in Figures 14 and 15.

Apparently, its influence in area D was greater during the first coverage

than during the third one.

During the third coverage the HSSW propagated northwards reaching the

area of Section V (Figures 16-C and 17-C). Traces of this water were in

fact found in stations 197, 201 and 204. At station 201 (Figure 17-C) it

was confined to the upper 3 m giving rise to an unstable situation. At

station 208 (Figure 18-C) it was still possible to trace the influence of

HSSW.

Contrasting with the nearly homogeneous vertical structure observed in

sections I-III, the other sections show vertical gradients over the shelf.

The temperature profiles reveal that water lifting occured at the outer

part of the shelf, which was probably a manifestation of a similar

movement over the non-investigated slope area. Water lifting was quite

clear in sections VIII and XII during the first coverage (Figures 20-A and

24·-A), sections VIII, XI, XII, XIII and XVI during- the second coverag·e

(Fig·ures 20-B, 23-B to 25~B and 28-B), and sections IV-IX, XI, XV and

XVI during· the third coverag·e (Figures 16-C to 21-C, 23-C, 2'1-C and

28-C). Particularly evident cases are those shown in Figures 19-C and

23-B to 25-B.

Salinity values greater than 35.3 can be seen in Fig-ure 22-C at dephts of

30-40 m. These values can only be due to lifting- of Subtropical Water from

about 150 m, where the core of the subsurface salinity maximum is usually

found offshore (SAETRE and JORGE DA SILVA, 1982). Figure 24-C shows

a similar situation where this water was lifted to the surface. In none of

these figures can one find any indications of water lifting in the

temperature profiles. This sugg-ests that this process occured before the

area was surveyed.

Salt wedges due to the influence of LSSW, particularly connected to the

Zambezi runoff, were observed mainly to the north of the delta. They

were quite clear during the first coverage in sections X and XII (Fig-ures

22-A and 24-A), changing into a distinct two-layer structure in section

XIII (Fig·ure 25-·A), and to an offshore lens of low salinity further north

in section XIV (Fig·ure 26-A). During the third coverage salt wedg·es were

found in sections VIII-X (Fig·ures 20-C to 22-C). Figure 19-A shows the

only case of a salt wedge found to the south of the Zambezi.

DISCUSSION

The results of the three coverages made during the present cruise may

reflect three different, successive stages of a variable dynamic process.

Each stage can be characterised by a certain degree of interaction of the

relatively warm and saline Oceanic Water, transported by the southg-oing

Mozambique Current, and the slightly cooler Low Salinity Shelf Water

(LSSW), mainly of Zambezi origin.

Lateral displacements of the core of the Mozambique Current may have

determined the routes of propag-ation of LSSW. During· the first coverage

only the shelf area north of Pebam} seemed to be directly influenced by

Oceanic Water, while the LSSW flew northwards and turned offshore

~ 10 -

between Quelimane and Pebane (Figures 7 and 8; see also Figures 24-A to

26-A).

During the second coverage the direct influence of Oceanic Water was

apparently exerted over the whole shelf area north of the Zambezi mouth,

and drastically reduced the northward propagation of LSSW (Figure 10).

This is seen to flow mainly southwards associated with temperatures

similar to those of the Oceanic Water (Figure 9). This association may be

regarded as an indication of the important role played by the Mozambique

Current in the transport of Zambezi water. Figures 23-B to 25-B provide

some evidence that the lower temperatures found over the mid part of the

shelf north of the Zambezi (Figure 9) were due to lifting of water at the

outer part of the shelf.

During the third coverage, while the whole shelf area to the north of

Quelimane showed surface values typical of Oceanic Water (Figures 11 and

12), low salinities were found to the north of the Zambezi apparently

associated with a tongue of low temperature that extended northwards,

The shape of the isotherms in Figure 11 suggests that such tongue was

probably the result of an intrusion of oceanic water onto the shelf area to

the south of the Zambezi. This might have forced part of the shelf water

northwards, while another part was forced southwards, as suggested by

the lower salinity tongue surrounding the area of HSSW, However, at the

inner part of the shelf south of the Zambezi the water was apparently

moving· southwards, as suggested by the shape of the isotherms and the

low surface salinity values.

The present results show that the southward transport of Zambezi water

is apparently a permanent feature. On the other hand, its northward

flow , although associated with lower salinities, seems to depend larg·ely on

the direct influence of Oceanic Water over the shelf area.

RECOMMENDATIONS FOR FUTURE RESEARCH

Having in mind all the results from the investigations carried out in the

Sofala Bank area (BRINCA et al., 1981 a, 1981 b, 1983 a, 1983 b),

attention should be payed to the following· points in future research:

- 11 -

i) Cruises covering· the whole area should always be

conducted in the same direction (preferably north- south),

particularly when surveys are made in sequence, as in the

present case;

ii) Water colour and Secchi depth should always be

determined aiming, inter allia, to obtain ground truth for

satellite data;

iii) Selected sections should be extended beyond the

slope and carried out in all research cruises (four sections are

probably enough, approximately along the directions of sections

II, VI, X and XVI);

iv) Short-term variability studies should be started, at

least along two of those selected sections;

v) Direct current measurements should be conducted over

the shelf in selected points along the sections where time

variability studies are carried out.

REFERENCES

BRINCA, L., SILVA, C. and SILVA, A. (1981 a) Relat6rio do cruzeiro

realizado no Banco de Sofala pelo arrastao "Muleve" em Julho/

Agosto 1979. Informacao No. 4, Instituto de Desenvolvimento

Pesqueiro, Maputo, 53 pp.

BRINCA, L., REY, F., SILVA, C. and SAETRE, R. (1981 b) A survey

on the marine fish resources of Mozambique, Oct. -Nov. 1980.

Reports on Surveys with the R/V Dr. Fridtjof Nansen, Insti­

tuto de Desenvolvimento Pesqueiro, Maputo, Institute of

Marine Research, Bergen, 58 pp.

BRINCA, L., BUDNITCHENKO, V.A., JORGE DA SILVA, A. and SILVA,

C. (1983 a) A report on a survey with the R/V "Ernst

Haeckel" in July-August 1980 Shallow-water shrimp and by

- 12 -

catch; Oceanography. Revista de Investigac;;o Pesqueira, No. N I

6, lnstituto de Investigacao Pesqueira, Maputo, 105 pp. - I

BRINCA, L.,

SAETRE,

Bank

JORGE DA SILVA, A., SOUSA, L., SOUSA,

R. (1983 b) A survey of the fish resources

M. I. and

at Sofala

the R/V

Pesqueira,

15 pp.

Mozambique, September 1982. Reports on

Dr. Fridtjof Nansen, Instituto de

Maputo, Institute of Marine Research,

Surveys with N

Investigacao I

Bergen, 70 +

SAETRE, R. and JORGE DA SILVA, A. (1982) Water masses and

circulation of the Mozambique Channel. Revista de ' <N Investigacao

Pesqueira No. 3, lnstituto de Desenvolvimento Pesqueiro, Maputo, 83

pp.

- 13 -

Figure 1. Sofa1a Bank - bathymetry (m) and main rivers.

•) - Gauge station

~ 14 ~

35' 36' 37' 38' 39° 40°

Figure 2. Station grid,22 Sep-8 Oct 1982

21

17.

18.

2 o·

21'

QUELIMANE

;:126

130 ·1J1,' • '· .127 m(

• ~1~5 /

1 ~9 .. } ~0 ,136 -~"/ '141

,/ •142

\ •144

l

- 15 -

· ... ~-· (.· '111

~ ·.m

Figure 3. Station grid, 8-25 Oct 1982

QUELl MANE

35" 36" 37°

Figure 4. Stat ion grid, 18 Nov- 4 Dec 1982

•. ' /i~~ ;''i'os·

38"

38"

.-··

40"

39" 40"

16'

water colours

ld [21 2-3

11'

~ '~- -7

~ 8-9

QUELIMANE 18'

35. 36° 39°

Figurt> 5. Water colour (Fore I), 22 Sep- 8 Oct 1982

17° SO m

QUELIMANE •

19°

BEIRA

2 o·

21'

35' 36' 37' 38' 39' 40°

Figure 6. Secchi depth, 22 Sep-8 Oct 1982

- l7 -

17.

O.UELIMANE 18'

19"

zo·

21"

35" 36" J7• 39" 40"

Figure 7. Surface:' temperature, 22 Sep-8 Oct 1982

.! I

21° I ,i

35" 36' 37" 38" J9• 40°

Figure 8. Surface salinity, 2 2 Sep- 8 Oct 1982

- 18 -

35. 37° Js• J9•

Figure 9. Surface temperature, 8-25 Oct 1982

17. s PEBANE

QUELIMANE 18"

19°

2 o·

21°

,J (

JG• 37" Ja• 39" 40°

Figure 10. Surface salinity, 8-25 Oct 1982

~ 19 -

0

17 °

QUELIMANE

IS'

19'

21°

35• 37' 38• 39° 40°

Figure 11. Surface temperature, 18 Nov- 4 Dec 1982

17. 5

QUELl MANE 18.

19°

2 o·

21'

35' 36' 37• 38• 39°

Figure 12. Surface salinity, 18 Nov-4 Dec 1982

c 177 178 179 180

0 ,.. .... -- ... ,

,. I I I

10 , , I I \ I

\ 25jS \ \

20 I I I I I I I l I I I

Ill I I QJ 30 I ... I -QJ

E 40

t •c

50

60

20 -

0 20 40 -'----'--'---'---'---L

181 0

26

10

20

30 \

', ... 40

50

60

km

Figure 13

Temperature and salinity profiles

at Section I

C- 18 NOV 1982

177 178 179 180 181

~

A 0

24 24 !:::-:::-

211 210

>24 Vl 10 ilJ '-.... C!J

E 20

c 184 185 186 187 183 182

0 I I I I I

' ....... «:::!:26.0 --25.5

10 < 26

20

Vl ilJ 30 '--CIJ

E

40

50

60

21 -

j_ 10 1,0 ~·J..~_.J__ I

0

10

20

0

10

20

30

40

50

60

kIll

24 23 22 21 20

Figure 14

Temperature and salinity profiles

at Section II

A - 23 SEP 1982

C -18-19 NOV 1982

184 185 186 187 183 182

,( ,[ I

,J, I I

- 22

0

A 25 26 27 28 29

0 I I ;J' I

10 Vl

24 OJ < 24 ... o; 20 E

30 t ·c

c 188 189 190 191 192 193 194

0 I I I ' I ' I ' > 26 I ' 10 I ' I I I I

26 25.5 25~5 I I I I

20 I I I ' I ' I

Vl I OJ I ... 30 I .... I OJ I E

40 t ·c _ .... ~..-25

50

20 40

0

10

20

30

0

10

20

30

40

50

km

25 26 27 28 29

I I I

\ 34.8 35.1 34!9

I 35.0 I I I I I

s I I I

Figure 15

Temperature and salinity profiles

at Section Ill

A - 23 SEP 1982

C - 19-20 NOV 1982

190 191 192 193 I

\ I I

3)1

I

35.3 35.2

>35.3

194 I

35t

<35.1

5

c 195 196 197 198 199 200

0

10 2~ 20

Ill 30

" ... -" E 40 t ·c

50

60

- 2] -

0 20 40

0

10

20

30

40

50

60

km

Figure 16

Temperature and salinity profiles

at Section IV

C - 20 NOV 1982

200

J, \

s

~ 24 ~ 0 20 40 km

A 33 35 37 33 35 37

0 I

,( I

,1,

I 0

10 10

20 20 Vl llJ ... o:; 30 30 E

40 40

8 137 138 139 140

0 I I

) I I

>25 10

20 Figure 17 25 Vl llJ ...

Temperature and salinity profiles at -llJ

E 30 Section v

40 A - 24 SEP 1982

B - 24 ocr 1982

50 c- 21 NOV 1982

c 201 203 205 207 205 2Q7

0 I I I

\}:::27 I 0 I

10 10

20 6"'

20

30 30 Vl t ·c llJ ... o:; 40 40 E

50 50

60 60

70 70

- 25 -

0 20

c 208 209 210

0 211 212 213 214

10

20

Ill w 30 ... ..... w E

40

50

60

40

0

10

20

30

40

50

60

km

Figure 18

Temperature and salinity profiles

at Section VI

C - 2 2 - 23 NOV 198 2

210 211 212 213

\ I

s I I I I

214

3'L \

I

A

VI <II .... .... <II E

8

VI <II .... .... <II

E

c

VI <II '-.... <II

E

43

20

30

40

128

217

50

45

/.,.-- .... , ' I \

I I

\

' ' I

\

' 24·~

1 >24 ' ' \

·c

129 130

220

t ·c

- 26 -0

47 0

10

' \ '

131

23

216 215

26

70

km

Figure 19

Temperature and salinity

5

profiles at

ection VII

27 SEP 1982

B - 20 OCT 1982

A -

C- 23-24 NOV 1982

217

35:{

~

A 52 51 so

0

/-~~ I

10 I I I I

241.5

20 I I

' I I Ill

30 (lJ ... .... (lJ

E 40

so

60

B 124 125 126 a

10

Ill 20 (lJ ._ .... (lJ

E 30

40

c 221 222 2 23 0

28

10

27

20

Ill (lJ '- 30 .... (lJ

E

40

so

60

- 27 -

0 20 40

0

10

20

30

40

so

60

0

10

20

30

40

so

60

km

52 51 so I . ~ \ ) 'S 35.2 34;8 3~.0 3~.1

s

-,35·2

Figur<~' 20

Temp<~'rature and salinity profiles a

Section VIII

A. - 28 SEP 1982

B - 19 OCT 1982

C - 2S NOV 1982

221 222 2 23

B 123 12 2 121

0 I

25~5 I

10 I \ '- ... ___

20 Vl C!J ... .... C!J

30 E

40

c 2 24 225 226 227

0 I I 0 28 I

10

20

30 Vl C!J ... .... C!J 40 E

50

60

70

228 I

27

6

- 28 - 0 20 40 km __.___,__....___.___.__.._

0

10

20

30

40

50

60

70

Figure 21

Temperature and salinity profiles

at Section IX

B- 18-19 OCT 1982

C - 26 NOV 1982

224 225 226 227 228

n ' <35\ s

A 58 59 60 61

0 I I

~25~ 10

20 VI «< b.

q; E 30

24

40

23 50

c 229 230 231 232

0 I I

~28 10

VI 20 «< ... - 26 «< E 30

40

- 29 -

0 20 40

0

10

20

30

40

50

0

10

20

30

40

km

58 59 60 61

35·2

Figure 22

Temperature and salinity profiles

at Section X

A - 29-30 SEP 1982

C - 27 NOV 1982

2 29 230

-\)4.~ ~8 5.0 '> <::::::::: _ _; 5 .,

-:~ 35.2

20 40 km

8 116 117 118

0 26

10 ~' 24 Figure 23

20 Vl Q! ,_

Temperature and salinity profiles .... ~ 30 E at Section XI

itO

B- 16-17 OCT 1982

50 c- 28 NOV 1982

c 233 234 235 236 233 234 235 236

0 0

~ 10 10

20 27

20 > 35.1

30 30 Vl Q! .... .... Q! 40 40 E s

50 50 25

60 24 60

23

70 70

A

Vl <lJ '-q; E

B

Vl <lJ '-.... <lJ

E

c

til <lJ '­....

0

10

20

30

40

0

10

20

30

40

50

60

70

0

10

E 20

30

- 31-0 20

74 75 76 77

t ·c

113 112

2

237 238 239 24 0

40 km

74 75 76 77 0

10

20

30

40

Figure 24

Temperature and sa 1 i nit y profiles

at Section XII

A - 3 OCT 1982

B - 15- 16 OCT 1982

c - 29 NOV 19 82

237 23 8 239 240 0

10

20

30

- 32 -

0 20

A 82 83

0

10 Ul <IJ ... "" E 20

30

B 108 109 110 111

0

10

20

Ul <IJ 30 .... ..,. <IJ E

40

50

60

70

c

Ul ,:l

<IJ .... +' <IJ

E 20

30

40

25

40

0

10

20

3

0

10

20

30

40

km

82 83 84

,.~34.0 ;::i?S2S

34.S

/ 3~.o ,

I

,' 35~1 I I s

Figure 25

Temperature and salinity profiles

at Section XIII

A - 4- 5 OCT 1982

B-14-15 OCT1982

C - 30 NOV 1982

241 242 243 244

B 107 106 105

0

10

Vl 20 Figure 26 OJ '-....

profiles OJ Temperature and salinity E 30

23 at Section XIV

40 A - 5-6 OCT 1982

B - 14 OCT 1982

c- DEC 1982

c 245 246 247 245 246 247

0 0

10 10

20 20

Vl OJ ... .... 30 30 OJ

E

40 40

- 34

0 20 40 km

B 102 103 104

0

10

Vl 20 Figure 27

"'' ... .... Temperature and salln1ty profiles <IJ

E 30 at Section XV

40

B - 13- 14 OCT 1982

50 20 c - 1- 2 DEC 1982

c 2 50 2 49 248 250 249 248

0 0 27~;

10 10

Vl 20 20

<IJ .... -<IJ

E 30 30

40 40

50 50

- 35 0 20 40 km

c 251 252 253 251 252 253

0 0

10 10

20 20

30 30 1/l <1J

.::: <1J 40 40 E s

50 50

60 60

70 70