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Translation Series No. 3820
•
IV The ocean environment of the world
by Keiji Nasu
Original title:
From:
IV
Sekai ni okeru kaiyo kankyo
Oceanic Environments and the Living Resources of the
World. p. 30-80, 1975
Translated by the Translation Bureau( JWC )
Multilingual Services Division
Department of the Secretary of State of Canada
Department of the Environment
Fisheries and Marine Service
Pacific Biological Station
Nanaimo, B.C.
1976
95 pages typescript
DEPARTMENT OF THE SECRETARY OF STATE
SECRÉTARIAT D'ÉTAT
TRANSLATION BUREAU
BUREAU DES TRADUCTIONS
MULTILINGUAL SERVICES
DIVISION DES SERVICES
DIVISION
MULTILINGUES
F
TRANSLATED FROM - TRADUCTION DE
n-)er-3rilPo
INTO - EN
Japanese
Enmlish
AUTHOR - AUTEUR
Keiji NASU
TITLE IN ENGLISH - TITRE ANGLAIS
IV The ocean environment of the world.
TITLE IN FOREIGN LANGUAGE (TRANSLITERATE FOREIGN CHARACTERS)
TITRE EN LANGUE ÉTRANGÉRE (TRANSCRIRE EN CARACTÈRES ROMAINS)
IV
Sekai ni okeru kaiyo kankyo.
REFERENCE IN FOREIGN LANGUAGE (NAME OF BOOK OR PUBLICATION) IN FULL. TRANSLITERATE FOREIGN CHARACTERS.
RÉFÉRENCE EN LANGUE ÉTRANGÉRE (NOM DU LIVRE OU PUBLICATION), AU COMPLET, TRANSCRIRE EN CARACTÈRES ROMAINS.
Sekai no kaiyo kankyo to shigen seibutsu.
REFERENCE IN ENGLISH RÉFÉRENCE EN ANGLAIS
The ocean environment and the living resources of the world.
Nihon. suisan shigen hog (..1)
kyokai/ Japanese Society for the
preservation of Maritime
resources.
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NUMÉROS DES PAGES DANS
L'ORIGINAL
30 to 80
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95
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DEPARTMENT OF THE SECRETARY OF STATE
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OCT - 4 1976
iiTaritime Research Series 27.
The ocean environment and the living resources of the worl.d.
Kei^ji NASU.
Japnnese Maritime Resources Protection Association.
P30
The ocean environment of the world.
IV.
1,
Tr?PFaci_fs.c Ocean.
The Pa.cific Ocean is the largest in the world. Its
area is equivalent to those of the Atlantic and lndian Oceans
together, being 165,246 x 1ob km2
This is about 50'r1o of the
whole of the World. Ocean, and about 35^- of the total area of
the globe.
Compared with the Atlantic and Indian Oceans it
is deep, the mean depth being 4,282 metres and at its point
p31
of greatest depth, the Vityaz I)eep which is the cieepest'in the
world, the depth is 11,034 metres. The volume is 707,5.55 x 106 km3,
or about 54o/,, of' a l.l the o cean water in the world.
UNEDITED Tf:AiVStAi"NOIJ
For ir);arnTettion ariiv
TRADUCTION N-,-)N REVJ.SL-E
Informatiom sr4ufcmr.nP
SOS-200-10-31
2
The surface currents are larg-ely to be attributed to
the trade winds which prevail in middle to low latitudes and to
the westerly winds which prevail in middle to high latitudes.
In the low latitude zone between the limits of about 25° North
and South, the main current flow is generally from east to west,
and in the middle to high latitudes the prevailing current
direction is from east to west. In each of the two hemispheres,
centred on the middle latitudes, large scale perpetual highpressure ring currents (anticyclonic gyres) are formed.
p32
The principal ocean currents to be mentioned are the
Kuroshio, the Oyashio, the West Wind Drift or the North Pacific
Current, the California Current, the North and South Equatorial
Currents, the Equatorià.l Counter Current, the Peru or Humboldt
Current, the East Australian Current, and the Mindanao Current.
In addition there are subsurface currents such as the Equatorial
Undercurrent or Cromwell Current, and below the California and
Peru Currents there are undercurrents directed towards the high
latitudes.
The depths of these currents are from 150 metres
to several hundred metres.
The volumes of these currents are shown* in
Table IV - 110. The flow directions on the surface are shown
in Figure IV - 1.
^
References are marked., but were not included in the
copy provided to the translator.
3
'Figure IV
-
la.
(Defant 1961).
Surface currents of the Pacific Ocean.
Duringothe msprwinte.
1.
2.
3.
4.
5.
6.
7.
North equatorial current.
South equatorial current.
Equatorial counter current.
Kuroshio.
Oyashio.
North Pacific current.
Alaskan stream.
8.
9.
10.
11.
12.
13.
California current.
Tsushima warm current.
Mindanao current.
Peru current.
East Autralian current.
Westerly wind drift current.
(Antarctic ring current).
150'
180'
150•
90'
120"
Figure IV - lb.
Surface currents at middle and low latitudes in
the Pacific and Indian Oceans.
(Defant 1961).
Durinp: northern hemisphere summer.
1. North equatorial current.
(south-west monsoon drift
in the Indian Ocean).
4.
Somali current.
5.
West Australian current.
2. South equatorial current.
6.
Mindanao current.
3. Equatorial counter current.
7. Peru current.
5
Table IV-1.
The volume of flow in the main currents in the Pacific Ocean.
Kuroshio
65 Sv
Oyashio
15
North Pacific Current
35
California Current
15
North Equatorial Current
45
Equatorial Counter Current
25
Equatorial Under Current
40
Peru Current
18
Mindanao Current
18 to 31
East Australian Current
30
Alaskan Stream
3 to 8
A typical Pacific Ocean current, the Kuroshio, has
its origin in the North Equatorial current which flows to the
west in the North Pacific in the region from 8° to 18°N.
When the extension of the North Equatorial current reaches
the eastern side of the northern Philippines, the change of
latitude brings the effects of the earth's rotation into play,
and the current is intensified on the western side of the
Pacific (Stommel 1948), becoming the so-called Western
Boundary current, and this flows into the Japanese region as
the Kuroshi.o . In the neiFhbourhood of the Ryukyu Islands the
6
Kuroshio splits, the main current going along the southern
shore of Honshu and flowing to the east from eastern Honshu
approximately following the 36° to 37 0 line. The split-off
current mixes into the water mass of the East China Sea, goes
northwards along the edge of the continental shelf, and flows
through the Korean Straits into the Sea of Japan as the
Tsushima warm current.
As the Kuroshio flows eastwards from the south shore
of Honshu, its width varies from place to place but is about
80 km, and the width of the region of high speed (3 to 5 knots)
is estimated to be about 50 km. The depth of flow of the Kuroshio
may reach more than 1000 m. The flow volume varies both locally
and seasonally, but to the 1000 decibar level it is calculated
to be about 65 Sv.
There is a number of studies in which
temperature is used to indicate the axis of flow of the Kuroshio.
This temperature has regional variations but the temperature at
the 200 metre level is generally taken to be 15 ° C which was
reported by Uda (1964) and is in accord with the overall results
of the investigations by Kawai (1970).
To the east of Honshu, the Kuroshio proceeds eastwards
through some remarkable meanders and becomes the Kuroshio
Extension which flows almost directly eastwards, its velocity
and thickness diminishing in ways of which much remains in
detail unknown.
Since 1965 there has been a remarkable abundance of
oceanographical research in the region of flow of the Kuroshio,
conducted principally by the International Cooperative Survey
P33
7
of Kuroshio (CSK) in which eleven North Pacific countries are
grouped with Japan.
For example, Yoshida et al (1967)
predicted a unique easterly flowing Subtropical Counter Current
on the basis of the theory relating ocean currents to the
distribution of wind stress. The existance of this current
was later confirmed in a report, based on CSK data, by Uda
and Hasunuma.
According to the report by Uda and Hasunuma, this
Subtropical Counter Current exists in the same location as
12
the Subtropical Convergence , and it continues to be a local
13
current throughout the year . At the 100 m level it flows
parallel to the 21 ° C to 24 ° C contours (centred around 22 ° C to
23 0 0 ), and it reaches at least as far as the neighbourhood of
160 °E.
The velocity of this current is 0.2 to 1.3 knots (with
an average of 0.7 knots, 35 cm/sec), the width of the current
is 60 to 180 km, the thickness of the current is estimated to
be shallower than 300 m, and, on the basis of data obtained in
the summer of 1965, the mean flow is calculated as 12 Sv, an
amount very close to that of the Tsushima warm current.
The oceanic currents in the neighbourhood of the
Subtropical Counter Current are shown in Figure IV - 2.
This
region of the ocean is believed to be the spawning area of
bluefin tuna (Thunnus thynnus) (Yabe et al, 1966) and of the
bonito (Katsuwonus pelamis) (Mori, 1972), and is also believed
to be the spawning area of the Japanese eel.
As can clearly
be seen in Figure IV - 2, the Subtropical Counter Current
displays considerable meandering
.
8
1m'
150'E
140'
130'
30'
y /^üR
A--
SUB+ROPI CAL
CONYERGENCE
..
(^\r ^`
^
`SUlf7ROPICA!
^ -^, ->
^-.--^^^-:i•^ORTH tQUATORIAL
!%d
COUNTER
CURRENT
CURRENT
110'
4-ANAO
CIIRRENT
+iIIND
EQUATORIAL COUNTER CURRENr
CURREHT
^
^
^.
` $pU1H Q:IAORIL
Figure IV
2.
Current motion in the neiahbourhood of the
Subtropical Counter Current.
(Uda, 19691.
as it flows from the Kyushu - Palau seamount to the vicinity
of Izu - Ogasawara Mariana seamount.
Moreover it is found
that from spring to summer there are regions of northwardmoving warm water which correspond to the northern extremities
of these meanders, and these regions are believed to be related
p34
to the northward migrations of bonito (Uda, 1969). Uda has
also shown that the Subtropical Counter Current and the Subtropical Convergence provide an environment with an extremely
large influence on the life habits of the bonito, particularly
in its early stages.
Off Sanriku, the Kuroshio converges with the polar
front which is partly formed by the Oyashio and partly by the
water masses of the Okhotsk Sea and the Behring Sea.
Suguira (1958)
states that each of these water masses contributes one third to
•
the Oyashio.
In the neighbourhood of 40 °N to 42 °N the Oyashio
is 200 m to 500 m thick.
In contrast with the Kuroshio, in
which the temperature, 12 ° C to 18 ° C, and the salinity, 34.5%.
to 35.0%0 are high, this water mass has low temperature, 4 ° C
to 5 ° C, and low salinity, 33.7%.
to 34.0% 0 , and it is
characterized as Intermediate Cold Water accompanying a
halocline 14 (Kajiura, 1949; Uda, 1963).
The average current velocity in the Oyashio is 0.7 knot,
the greatest velocity is 1.3 knots, and the width of the zone
of strong flow is 10 to 15 nautical miles.
It is of small
scale in comparison with the Kuroshio, and as shown in
10
Table IV - 1 the volume of flow is 15 Sv, about one quarter
of that of the Kuroshio. However it is highly productive of
life.
It abounds in zoo-phyto-plankton, and its water colour
and transparency are both low in comparison with the Kuroshio.
The colour of the Kuroshio for example, is normally greater
than 3 but that of the Oyashio is less than 4.
The California current which runs southwards
along the west coast of North America is the Eastern Boundary
Current of the Northern Hemisphere, comparable to the Canary
Current in the Atlantic. The surface velocity of the
California Current is generally low, being 0.5 knot (25 cm/sec),
and in the coastal region within 150 km of the shore, the
current direction is northward during the winter and southwards
during the summer. This region is also characterized by an
along-shore upwelling during the summer. In keeping with
Ekman's wind-stress theory of ocean currents15, when the
prevailing wind is from the north, as it is along the California
coast, and the sea coast is on the left (in the southern
hemisphere, when the coast is on the right), the shore current
is twisted by the wind and is deviated to the right (or, in
the southern hemisphere, it is deviated to the left). In other
words, it is turned in the offshore direction. Thus, along
the Californian coast in summer, the surface water is pushed
out to sea, and is replaced by a current of deep layer water
which wells up from the lower•layers. This upwelling has a
11
large influence on the ocean conditions in the region of the
California Current.
The temperature, for example, is lowered,
and the nutrient salt content is increased.
The lowering of
the surface temperature in the summer leads to the formation
of fog (the upwelling being the main cause of the famous San
Francisco fogs), and the upwelling has a large influence on
atmospheric conditions. The surface temperature in the
California Current region is in general in the range from 9 0 C
in the winter in the north, to 26 ° C in the summer, and the
salinity ranges from 32.5%0 in the north to 34.5%0 in the south.
The content of nutrient salts is from two to several times as
great as in the central part of the Pacific Ocean, and ranges
P36
from 0.5 ,u_g at/1 to 2 ,tg
A part of the California Current continues along the
coast of Central America to the low latitude region of the
Eastern Pacific. The Equatorial Counter Current which flows eastward
near the equator reaches its eastern end near the coast of
Mexico and then turns north and west to become the North
Equatorial Current.
This North Equatorial Current, which flows
to the west around 10 °N joins with the California current whose
flow direction turns westward off Baja California and becomes
part of the large-scale horizontal rotation of the North Pacifie.
Since the neighbourhood is typical of the regions for
purse seine tuna fishing there is much about its ocean condition
which is important.
12
p35
NAUTI'.AL
MILES
PER DAY
cm/sa.
Knots
+-0.1-0.3 5-15 2.b--1•b
.,--O.a-0.6 20-30 10-15
^--0.7--0.9 35-45 17-22
»g >24
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HDA RY
---•• CO NŸE R^E NCE
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rr^ `
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r-
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120'
110'
100'
90'
•^
Al
1 ^ ^ 1
t
ti ,^ Ô 2^
.
g0*
70
Figure IV --3a.
Surface current vectors in the Eastern Pacific (August).
Current boundaries.
Global ' Tropical Convergence.
13
NAUTICAL
MILES
sir
Knots
re,e'
• i 4 4 ,-\ •
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r •
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y_. 4- r •-• ..
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e.
•r• -•
...
,
e•
r
r
r
„. r.
, 0 17, r•,
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.-- -,
r
• --- ..„ _
S
K 4..
.- l .•% • '
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100'
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80'
70'
Figure IV - 3b.
Surface current vectors in the Eastern Pacific (February).
Current boundaries«
Global
Tropical Convergence.
The Tropical Counter Current which has already been
mentioned is an east bound current between the Northern Tropical
Current ( a west bound current) and the Southern Tropical
Current ( a west bound current), but there are seasonal
variations which become particularly notable in the Tropical
Counter Current,
The summer (August) and winter (February)
surface currents in the region of the Tropical Counter Current
are shown in Figure IV - 3a and Figure IV - 3b.
In the summer
the Tropical Counter Current can be traced as an east bound
current centred around 7 ° N to the east of 90 °W, but in the
winter the east bound current is much smaller in scale than in
summer and its eastern end is at about 120 °W.
In the summer, the Tropical Counter Current turns
northward off the central American continent and joins up with
the Northern Tropical Current, and as the current changes
direction anti-clockwise off the shore of Costa Rica this eddy
produces an upwelling which develops during and after the
summer and is known as the Costa Rica dome.
This not only
produces a tuna fishing ground but is also important
oceanographically.
Next, it is believed that one of the factors which
produce the purse seine tuna fishery in the eastern part of
the tropical Pacific is the thermocline, whose depth normally
agrees with that of the oxyline 16 and therefore it is not only
,
the temperature but the general oceanographic conditions whose
15
relation to the distribution of schools of fish needs to be
investigated.
Below the surface and centred around the equator there
is the west to east flowing Equatorial Under Current (or, as
already mentioned, the Cromwell Current if the name of the
discoverer is used). Its presence was confirmed in 1958. Its
maximum velocity is 2 to 3 knots, the depth of its core varies
with longitude but despite the variations it is in the vicinity
of 150 to 200 mp Its thickness may reach 200 m, but its width
is 300 km and as shown in Table IV - 1 its current flow may be
as much as 4OSv, and, after the Kuroshio it is the largest current
in the Pacific Ocean.
The westward flowing Northern Equatorial Current and
p37
Southern Equatorial Current form the current flow from east to
west in both the north and south hemispheres, but in the
northern hemisphere only it is limited by a boundary at about
5°N.
In the winter (February to March) the current has an
average velocity of 0.5 to 1.0 knot, its southern boundary is
from 5°N to 7°N, and its northern boundary is in the neighbourhood of:the northern tropic (23°30'N). The current velocity
increases as it goes west, and at about 7°N in the neighbourhood
of Samal Island in the Philippines,
south branches.
it splits into north and
The northern branch becomes the source of the
Kuroshio, the southern branch the source of the Mindanao Current,
and it also contributes to the Equatorial Counter Current.
16
p38
There is thus a thermocline in the reedon of the Northern
Equatorial Current.
This thermocline gets generally shallower
in going from west to east, and its depth is reported to
influence the distribution of tuna (Suda et al., 1 9 6 9).
The prevailing wind in the equatorial region is the
trade wind from the east (or the tropical east wind).
However,
on the equator the deflecting force due to the earth's rotation
(the Coriolis force) is zero, and according to Ekman's theory,
the current produced by the wind is in the same direction as
the wind. The surface layer water to the north of the equator,
(in the Northern hemisphere) is deflected to the north, and that
to the south of the equator (in the Southern hemisphere) is
deflected to the south.
The resulting divergence of the surface
17
water in the equatorial region produces upwelling currents, and
the deep layer waters, rich in nutrient salts, come close to
the surface and form a potent source of living matter.
Thermoclines and oxyclines are also formed, and the distributions
of many oceanographic variables around the equator are shown
in Figure IV - 4.
The oceanic environment in the low latitude region of
the Western Pacific is greatly influenced by the equatorial
current system (Wyrtki, 1961).
It is also influenced by
meteorological conditions and in general the motion of the
current assumes the same direction as the monsoon.
The
develonlent in February is due to the North East monsoon, and
17
P37
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100
ec)
.'N
,e-
-,_
2.0 *--------
200
2.0r ,
300
0
(men
0 L1
4
4
E
----7•••:\_,
, Ie 42 É
"Sji\--‘
1
0.2 /
100 -- 0.4 )
•
3...7
4-
•
-
- 4 2.o
.111111111.
1
300
-I
/ n
(pg-atoms P/ 1 )
1,
-
'NW
yi
1-5-2--
-->
100 -•---5
4--
--->
200
10
15
20
2
-
300
(pg-atoms Si/
)
Fiure IV -4.
Pacific Ocean cross-sections from 10 °S to 20 o N, showing the
distribution of water temperature (A), salinity (B), estimated
current velocity (C), dissolved oxygen (D), phosphates (E) and
silicates (F).
(Sverdrup et al., 1946).
18
a large part of the surface water of the southward flowing
current in the South China Sea flows into the Java Sea.
Under
the influence of the North West wind, the surface water which
has flowed into the Java Sea reaches the Flores Sea and the
Banda Sea, and a part of this goes into the Indian Ocean and
the Timor Sea. In August the development of the Southern
Equatorial Current causes a Pacific Ocean water mass to flow
into the South China Sea from the neighbourhood of the
Philippines,
the current direction becoming generally North East.
On the eastern side of the Philippines,
as already
stated, the westward flowing North Equatorial Current near Samal
Island becomes thaKuroshio or the Mindanao current.
This clear
water current joining the North Equatorial Current and the
Equatorial Counter Current has a flow volume of 18 to 31 Sv
(Masuzawa, 1969).
A squid fishery has recently been developed in the
vicinity of New Zealand, with about 150 Japanese fishing
vessels engaged in fishing in the 1972/-1973 season.
Attention
is also being paid to the potential resources of both bottom
fish and surface fish in this part of the ocean. It is therefore necessary to discuss ocean conditions in the area
surrounding New Zealand.
New Zealand consists of relatively long and narrow
islands stretching in the North - South direction from about
35 ° S to 47 °S.
The oceanic structure in its neighbourhood is
19
greatly influenced to the north by the subtropical water
masses originating in the low latitudes, and in the south by
the subantarctic water masses originating in high latitudes.
The currents in the region, especially on the east side, are
believed to be greatly influenced by the topography of the
ocean bottom.
p39
As shown in Figure IV - 5, the topography of the ocean
bottom around New Zealand is more complicated on the east side
than on the west side. On the west side, the 200 m contour
follows the coast line from the North Island to the South Island.
The distance of this contour from the coast is greatest near
Cape Egmont and decreases towards the southern tip of the South
Island.
There is a wider region of continental shelf near New
Zealand between Cape Egmont and Cape Farewell. To the west of
this there is also a wide ocean plateau with a depth of less
than 1000 m, whereas the slope from the south west part of the
South Island is steep. On the eastern side, taking the 1000 m
depth contour for example, the largest distinctive feature is.
the Chatham Rise, which stretches as a long narrow strip out
to 173°W. Chatham Island is found on this rise at 176°W.
To the south of the South Island, the continental shelf
with a depth less than 200 m reaches to Snares Island and
further south still there is a wide region of ocean plateau
with a depth less than 1000 m. This ocean plateau shows
greatly undulating topography,. including Auckland Island, the
Campbell Plateau around Campbell Island and the Pukaki Bank
where the depth is less than 200 m.
20
leVE
170.
›.*
CHALLENGER
CHATHAM R ISE
AV
CHATHAM ISLAND
STEWART ISLAND
,
SNARES
BouNly Gm.
ISLAND
BOUNTY ISLAND
z•BOUNTY R ISE
AUCKLAND ISLANDS
PUKAK I
4:Je
CAMPBELL. ISLAND
Figure IV
-
5.
The topography of the sea floor
near New Zealand.
DA MK
21
To the east of the Pukaki Bank there is the Bounty
Plateau which reaches to the Bounty Archipelago, where the
depth of 2000 m is reached. Between the Bounty Plateau and
the Chatham Rise there is a valley running from east to west
in which the depth is greater than 2000 m,(the Bounty Deep).
Figure IV - 6 shows diagramatically the water mass
structure around New Zealand. On the east and west sides of
the islands of New Zealand, an oceanic front is formed between
the Subtropical water mass and the Subantarctic water mass
which has àlready been mentioned, and this front produces the
Subtropical Convergence.
This oceanic structure is very similar
to the ocean front formed by the Kuroshio.and the Oyashio at
Sanriku, and to the ocean front in the Japan Sea formed by the
water mass of the cold Lyman current and the water mass from
from the warm Tsushima current. The fisheries of high value
in the regions around New Zealand are centred around the
Subtropical Convergence, and once again there is high
productivity in the Subantarctic water mass. This Subantarctic
water mass is greatly influenced by the Antarctic water mass.
The ocean along the shore of the Antarctic continent
to the south of 65 °S flows from east to west in the East Wind
Drift because of the prevailing eastwards winds, and to the
north of this, in the high latitude zone from 65°S to 40°S the
West Wind Drift is produced by the prevailing westerly wind.
In the interior of the West Wind Drift, at about S4-°S to 62°S
p40
22
SUBTROPI;AL CONVERGENCE-}
ANTARCTIC CONVERGENCE'..
Figure IV
-
6.
Diagram of water mass structure around New Zealand.
1. Bottom level Antarctic water.
2. Deep level warm water.
3. Antarctic intermediate
water.
23
but varying with longitude, is formed the Antarctic Convergence,
in which the water temperature, the salinity and other chemical
and biological factors change rapidly.
The surface temperature
at this convergence is in general about 4.0 0 0 to 4.5 0 0 .
The
ocean to the south of the Antarctic Convergence is called the
Antarctic Ocean.
In latitudes lower than the Antarctic Convergence,
that is to say in the region to the north of the Subantarctic
region, the temperature of the water mass is relatively high.
The boundary of this mass (the Subtropical Convergence) is
generally found around 35°S to 47 °S, with variations depending
on longitude. At the boundary of the Subtropical Convergence,
the subtropical water mass with a surface temperature above 16 0 0
and salinity above 35%.can clearly be distinguished from the
subantarctic water which is below 13 ° C and below 34%0 .
Deacon
(1937) found that the surface temperature at the Subtropical
Convergence was frequently 11.5 ° C in winter and 14.5 ° C in summer,
with the salinity 34.7% .
The surface currents around New Zealand are shown in
Figure IV - 7. The principal currents are the Tasman Current,
the East Cape Current, the Southland Current, the Canterbury
Current and the West Wind Drift flowing from West to East in
the Antarctic region.
p41
p4-l
170'
Tus^«>r C rreN^ -
v<h-. a^ ^', --'^ ti7
r^.^^r
-^--
^.s,^
1kurly lalandS
-v=;_- Snnr.i Iflar4
C y
I1 uNtit
,n•^
°
0
.R:f^HOdQ
-OJtS,^Iin+•d5
Figure IV - 7.
Surface currents around New Zealand.
25
The Tasman Current which flows to the east in the
Tasman Sea originates from the East Australia Current which
flows southwards along the East Coast of Australia. Its main
flow is in the subtropical region, and to the west of the west
coast of the South Island of New Zealand it splits into a
north flowing and a south flowing current. Flowing northwards
off the west shore of the New Zealand South Island, it reaches
to about 41°C, and the ocean structure at its northern edge is
one of the conditions which are suitable for forming a squid
fishery.
The south flowing branch flows around the southern
tip of the South Island, turns north along the East Coast, and
becomes part of the Southland Current. The southern boundary
of the branch which flows south from the western shore of the
South Island is not well known, but within widely varying
limits its location is probably related to that of the
Subtropical Convergence.
The direction of the East Cape Current which flows to
the south along the east coast of the New Zealand North Island
is turned to the east over the Chatham Rise by the Subtropical
Convergence and an ocean structure similar to that produced by
the Kuroshio at Sanriku is established.
The Southland current formed by the part of the
Tasman Current which has flowed to the north round the southern
tip of the South Island, is turned to the east a little to the
north of Dunedin, and flows into the Subtropical Converuence.
The force of this current and of the East Cape Current is
thought to be affected by the Chatham Rise, and the eastern
component of the flow is strengthened by the confluence with
the East Cape Current (Burling, 1961).
The low temperature and low salinity water mass which
stretches from Dunedin to the Banks Peninsula forms the
Canterbury Current which flows northward along the east coast
of the South Island. A northward continuation of the Canterbury
Current flows into the Cook Strait. In the area of the
p42
Canterbury current, particularly in the Canterbury Bight, there
are regions of the upwelling which is thought to be important
as an ocean condition for the production of a squid fishery.
Centred around the Antarctic Convergence, the Westerly
Winds which prevail over a wide area produce the West Wind
Drift extending from the Subtropical Convergence to the northern
edge of the area near the Antarctic continent where the
prevailing winds are easterly.
This Antarctic Surface Water,
which has been produced near the Antarctic Continent, and the
eastward flowing West Wind Drift flow in a direction which has
a northerly component and flow into the low latitudes. The
West Wind Drift thus shows convergence with the Subtropical
Water Mass at the Subtropical Convergence, and is an important
factor in the formation of fisheries, not only in the vicinity
of New Zealand, but in the southern hemisphere in general.
27
The water temperature around New Zealand is in general
highest in January and February, and lowest in July to
September.
Figure IV - 8 and Figure IV - 9 show the mean
distribution of surface temperature and salinity from December
to February.
The temperature and the salinity show, on the whole,
1)43
the same patterns.
To the East of New Zealand and to the
south of 45 °S the trend of the contours is from south-west to
north-east under the influence of the continuation of the
Tasman Current and of the West Wind Drift.
Considerable changes of temperature and salinity are
to be found over the Chatham Rise and to the south of the
Snares Islands, the values being from 12.0 ° C to 14.5 ° C and
from 34.6%0 to 34.8%0 .
This can be taken as the Subtropical
Convergence. Figure IV - 8 and Figure IV - 9 do not show the
upwellings that occur between the north east coast of the Banks
Peninsula and Dunedin and produce regions of low temperature
• (below 14 ° C) and high salinity (more than 34.6%0 ).
The up-
welling is found to change in response to the local wind
direction (Burling, 1971).
The Peru Current in the southern hemisphere is the
cold current portion of the anti-clockwise rotation of the SouthEast Pacific.
Its source is in general in the Subantarctic region
and it flows along the west coast of South America, turning to
the north from about 40 ° S. In . areas very.close to the coast
corresponds roughly to the upwelling regions.
it
28
35 .
165'E
170'
175'
180'
.,,, ..".....,...
..„.
--..\...
"-17.
..
401-
it)
)
-
,...„e ;,..,.
• v-
..c_
14.5
....-•-)
200
, \.....
19.5.-.
,.- -r.7-irf..:.7....: 14.°
__...-12. 0
o
19.0
..-3-. ....- .:
11.0
'..;..-7
0.
.--"' .....---- 15
...
(
11.5'.
e pih.e.5 „.,
180'
35.3
f 95
.
:
..,...„.
--•*--.......7.....:... ..........., c...70;
40'
175'
170'
165E
35'
...7.-,7._..E..1 9
• ".::':•.:2' 9 '
t9.07
.........;,,f
13.0/ . r •
•
-er\.,.
/ (9.0
,./ : i /
-,," - IL 0 ,--)
-14.0' ,)
_ ii.o
.
34 .834 .85.34-.•9
"
24
.5
....f/
. ..„34.4 34 . à3
:7;34.3 34.3a
45 • 1.7
. 5
0. 0
R
....--9.0
,-.
i \
.8
50'
34.7
34.8 7-2
34.6
34.35.
_.- 34.3
e
0 fej
6
,
3
----34.7
1 2
L.34.27
34.5 34.4
9.0" ...-.Y
1 4.35
34.39 "
8.5/ '8.2 %
/if34 .30
55'
55'
Figure IV - 8.
The horizontal distribution of
o
surface temperature ( C)
of the ocean around
New Zealand in the summer.
Figure IV - 9.
The horizontal distribution of
surface salinity
(% )
of the ocean around
New Zealand in the summer.
29
The general names for these currents are Peru Coastal
Current for the northward flowing current close to the continent
and Peru Oceanic Current for the northward.flowing current
offshore.
The alternative name Humboldt Current is also used
for the two together but Gunther (1936) restricted the use of
Humboldt Current to the Coastal Current.
The Peru Coastal Current and the Peru Oceanic Current
are separated by the intermittently southward flowing Peru
Counter Current (Figure 1V - 10). This Counter Current, which
is also called the Pacific Equatorial Water, rises to the
surface in places (Wyrtki, 1963) but in general it is a
Subsurface Current, extending a distance from 500 to 180 km
offshore, and most noticeable from November to March. Before
November, the Peru Counter Current does not rise to the surface,
and since it is weak the Peru Oceanic and Coastal Currents are
not divided and form a single current whose strength is greatest
at that season.
Along the coast of South America the isothermal lines
are in general parallel to the coast line. However, looking
at them in detail, there is a tongue of warm water stretching
to the South East around 5°S to 10°S. In winter (July to
August) the temperature of this warm water is above 20°C, and
in summer (February to March), it is above 25°F (Figure IV-11).
There is low water temperature water on both sides of
this re^ion of warm water, and the animal life near the warm
p44
30
Figure IV - 10.
Outline diagram of the surface currents
along the west coast of South America.
(Summer).
,3'
w^;tL wr-a Drifr^
^ . .^^. -
^
c ^,-
.
^^l/`
F1^+wrcfl^ Cawveryence
E6'
74'N
s0'
"-I
N !,,.SO B:enco 1931JUl.Y-AUB
zla
(Gunth^r; 1935)
2U' ^r Punta Apia
%
20'9'
L
11'\Pimeniel
ot
o
i _ 4ç.ièverr
\ Chi:^.hoie
Huarney
Supe
\ \\\^,Callao
^ f
--l
35,
is'
,o.
Figure IV - 11.
Horizontal distribution of
surface water temperature (OC) in
winter (July to August) and summer (February to I•.,arch).
31
water is very different, especially in winter.
The high
temperature water comes right to the coast at Cape Blanco, but
the cold water flows north 25 nautical miles offshore, and the
discontinuity between the cold and warm water forms the
northern boundary of the Peru Coastal Current. There are also
narrow regions of cold along the coast of South America, whose
presence can be attributed to upwelling.
The variations of
temperature on going out from the continent to the ocean depend
on the locality and the season, and are in general greatly
influenced by local winds. . The regions of upwelling shown by
the temperature distribution appear to the north of 30 °N.
Other facts about the Peru Current include its
production by the turn to the north of the West Wind Drift in
high latitudes and its turn to the west off Peru to form part
of the South Tropical Current.
The average current velocity
is in general low, averaging 10 to 12 nautical miles per day
along the coast and 3.5 miles per day at 100 to 130 miles offshore.
The western edge of the Peru Current flows north and
is influenced by the South East Trade wind since it flows
parallel to the coast, the surface water is, in accordance with
Ekman's theory, deviated away from the coast, being replaced
p4
by upwelling cold water. The upwelling regions developed in
the Peru Current by the upwelling currents are found 50 to
100 nautical miles off Chile and 150 to 250 nautical miles off
Peru (Gunther, 1936).
32
At the northern extremity of the Peru Current, a zigzag ocean front is formed off Peru at 2 °S to 3 °S between the
cold northward flowing water of the Peru Current and the
warm southward flowing water.
At the southern end of the Peru Current, the West Wind
Drift flowing from West to East splits off the coast of
southern Chile into the northward flowing Peru Current and the
southward flowing Cape Horn Current. Thus the northward
flowing water mass off Chile can be considered as originating
entirely in the West Wind Drift.
Gunther (1936) takes the southern boundary of the
region of upwelling which develops off Chile as the southern
boundary of the Peru Coastal Current, and according to
observations made from the ship William Scoresby, this is
located near Cape Carranza (about 36 ° S).
Mossman (1909) found a meteorological distinction
between the low pressure circulation and the high pressure
circulation at 41 ° S.
To the south of this the prevailing wind
is west-south-west, and to the north the prevailing wind
changes with the season. From October to March the south wind
prevails, and from April to September the north wind. This
region of seasonal winds off the Chilean coast reaches to the
neighbourhood of Caldera at about 25S.
°
33
The ocean phenomenon called El Nino also occurs off
Peru around Christmas in the confluence of the northern edge
of the Peru Current and the southward flow of warm water, and
the local fishermen connect it with the naming of the infant
Christ (El Nino means"the infant"). Its southern boundary
normally reaches 2°S to 3°S. However,•bn rare occasions and
under the influence of abnormal variations of the atmospheric
circulation, the warm tropical water suddenly flows south to
Peru.
In these abnorma.l El NinQ years, it may reach as far
south as the neighbourhood of Callao at about 12°S.
Schott gives the persistence of El Nino shown in
Table IV - 2.
Table IV -- 2.
The persistence of El Nino in 1925.
Place
(offshore)
Southern
boundary
of warm
water.
Dates of
appearance
Persistence
Lobitos
4°20'S
20 Jan to 6 April
76 days
Puerto Chicana
7°40'C
30 Jan to 2 April
63 days
15 days
Callao
12°220' S
12 Yar to 27 Mar
Pisco
13°40' S
16 Mar to
24 Mar
8 days
34
In years of abnormal development of El Ni5o the
unusual atmospheric and ocean phenomena are accompanied by
great disaster.
p46
The mixture of the southward flowing warm
water with the Peru Coastal Current results in high water
temperature, and large amounts of the animal life of the ocean,
the plankton, the fish (especially the anchovy and the squid
are killed. Their remains drift into parts of the offshore
areas, and the large quantity of hydrogen sulphide produced
pollutes the air and the offshore water. The paint of ships
in harbour is darkened by this large amount of hydrogen
sulphide, and it is therefore known as "the Callao painter", or,
in local speech "Aguaja".
In years of abnormal El Nirio, the Tropical Rain Belt
moves southwards, and brings heavy rains into the dry lands.
For example, at Trujillo at 8 °S where the average rainfall in
March for eight years had been only 4.4 mm,
during the abnormal El Niiio year 1925.
395 mm were recorded
The recorded years of
abnormal El Nirio include 1891, 1925, 1941, 1957 to 1958, 1965
to
1966 and 1972
to
1973.
35
2.
The Atlantic Ocean.
The Atlantic Ocean is second to the Pacific Ocean in
extent.
Its area is 82.441 x 106 km2, and, if the Mediterranean
Sea is excluded its average depth is 3962 m, the greatest depth
being 9199 in.
The area of the continental shelf is 14.180 x
106 km2. The principal currents are the Gulf Stream, the North
Atlantic Current, the Labrador Current, the Canary Current,
the Benguela Current, the Brazil Current and the Falkland
Current.
The surface flow directions of these currents are
shown in Figure IV.- 12a and Figure IV - 12b, and the volumes
of flow in Table IV - 3.
Table IV - 3.
The volume of flow of the principal
currents in the Atlantic Ocean.
Gulf Stream
Brazil Current
100 Sv
20 Sv
Labrador Current
6 Sv
Benguela Current
15 Sv
Canary Current
16 Sv
North Atlantic Current
38 Sv
Florida Current
26 Sv
Antilles Current
2 Sv
Irminger Current
3 Sv
36
Figure IV - 12a.
Surface currents of the Atlantic Ocean.
(during the northern hemisphere winter)
(Defant, 1961).
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Northern Equatorial Current
Southern Equatorial Current
Equatorial Counter Current
Gulf Stream
North Atlantic Current
Irmin9:er Current
Norway Current
East Greenland Current
West Greenland Current
Labrador Current
11 ,
12.
13.
14.
15.
16.
17
18.
19.
Florida Current
Antilles Current
Canary Current
Guinea Current
Benguela Current
Guyana Current
Brazil Current
Falkland Current
West Wind Drift
(Antarctic Ring Current)
37
Figure IV - 12b.
Surface currents of the middle and low latitudes
in the Atlantic Ocean.
(During the northern hemisphere summer).
38
The oceanographic characteristics of the extremely
important fishing grounds in the North Atlantic Ocean are
determined by the warm flow of the Gulf Stream and the cold
flow of the Labrador Current (see Figure IV-12a and Figure IV - 12b).
The Gulf Stream flows north-east alone the North American
Continent, and is produced by the confluence of the Florida
Current and the Antilles Current, which itself has its origin
in the North Equatorial Current. It is a typical western
boundary current like the Kuroshio in the Pacific Ocean.
In
its zone of strongest flow off the North American coast its
velocity is 4 to
5 knots (200 to 250 cm/sec) and its thickness
is 1500 to 2000 m.
p48
The structure of the water masses surrounding the Gulf
Stream is presented in Figure IV - 13, and is made up of the
Shelf Water, the Slope Water, the Gulf Stream and the Sargasso'
Sea. The temperature and salinity of the Shelf Water are low,
and it flows to the south west along the east coast of North
America as a counter current to the Gulf Stream.
When it
reaches the south of Cape Hatteras, a part of it joins the Gulf
Stream and is turned to the East. The Slope Water exhibits
values of temperature and salinity higher than those of the
Shelf Water, but they are lower than in the Gulf Stream. The
Slope Water lies between the Shelf Water and the Gulf Stream,
and forms a long elliptic counter-clockwise rotating loop current.
39
•
X:
BLUE-F IN
Y:
YELLOW -4. 1 N
Z:
ALBACORE
,FLOR 10A CLIRRE NT
Figure IV - 13.
Horizontal and cross-section structure of the
water masses surrounding the Gulf Stream.
6V W
•
r
,
.,-}
60'
'
.
•
/
cjNJ
s
-•
10
Figure IV - 14.
Distribution of isotherms at the 200-metre level
to the south of Nova Scotia ( ° 0).
4o
The Gulf Stream exhibits a conspicuously meandering
motion, and this phenomenon is an important ocean condition
in the production of fisheries, particularly for tuna.
p49
According to a report by Fuglister and Voorhis (1965) the
axis of flow can in general be followed as the 15°C isotherm
at the 200 m level. The remarkable meanders of the Gulf
Stream can be understood from the. distribution of the water
temperature at the 200 m level near Nova Scotia shown in
Figure IV - 14. This meandering develops from the eastward
turn from Cape Hatteras, and it produces complex ocean fronts
with accompanying, eddies, so that Fuglister (1951) has called18
the motion of the Gulf Stream a"1VIultiple Current".
Figure IV - 15 shows the currents near Nova Scotia,
where there could be a squid fishery. It is clear from the
Figure that the topography of Newfoundland and Nova Scotia,
and of the sea floor in their neighbourhood results in an
extremely complicated flow pattern. There are many differences
between the summer and winter pattern, and in the summer in
particular the local eddies, which are considered to be one of
the conditions for forming fisheries for squid and other species,
are numerous.
As the Gulf Stream reaches the southern part of the
Grand Banlcs which stretch southwards from Newfoundland, the
width of the current increases and it flows to the North East,
becomin^ the I,4ortl: Atlantic Current.
To the east of the
Figure IV - 15.
Surface currents near
Nova Scotia.
(Above, winter.
Below, summer).
Mid Atlantic Ridge, which runs through the middle of the
Atlantic from the northern to southern hemisphere, it forks,
and some, which turns from north to east, forms the
Canary Current.
42
The part which flows North to the North-East,.becornes
the Norway Current and reaches the West Coast of Norway, and is
the main reason for the distribution of tuna (Thunnus thynnus)
up to around 70°N. The current which flows north from the D'iid
Atlantic Ridge turns to the west from the south of Iceland and
forms the Irminger Current, and most of it goes southwards
along the east coast of Greenland and flows into the East
Greenland Current.
p50
The East Greenland Current which has its origin in the
Arctic Ocean transports the low temperature water from off the
East Coast of Greenland and the continental shelf into the ICNAF
areai9.
A front forms between the East Greenland Current and the
Irminger Current, and a part of the East Greenland Current joins
with the Irminger Current to form the West Greenland Current.
p52
The West Greenland Current joins with the current
flowing southward from Baffin Bay and becomes the Labrador
Current.
Stream.
The Arctic front is formed between it and the Gulf
This Arctic front is an important oceanographic
condition in the production, off Newfoundland, of one of the
world's three greatest fisheries, and in addition, the vertical
mixing which is produced in the ICNAF area by the air cooling20
in winter renders it greatly productive of life. One part of
the Labrador Current detours around Newfoundland and flows into
the Gulf of Saint Lawrence, and Sato (1974) has described the
relation between ocean conditions and the distribution of
squid in this region.
The Greenland region and the ICNAF area are greatly
affected by the drift ice transported from the north during
the winter.
An understanding of ice conditions has become
necessary for the operation of fisheries, especially those
which have been found in recent years on the east coast of
Greenland.
The areas of Greenland and of the North-west Atlantic
Ocean to the north of 70 ° N are covered with close pack ice
throughout the year, but in areas to the south of 70 ° N, as
in other areas of drift ice, the ice conditions vary not only
seasonally but also from year to year in dependence on
variable meteorological conditions. The seasonal distributions
of drift ice are shown in Figure IV - 16.
The drift ice in the North West Atlantic arrives
earliest off East Greenland.
This drift ice mostly originates
in the Arctic Ocean, but some is also formed locally around
Greenland.
The area of drift ice in the North West Atlantic
Ocean is largest in spring, when the sea around Labrador to
the north of Newfoundland and both the East and West coastal
areas of Greenland are beset by the ice. In the summer the
regions of drift ice naturally diminish.
The southern tip of
the ice in the Labrador Sea retreats to about 50 ° N, and there
is no ice to be found on thé west coast of Greenland, except
to the east and south and on the south west shore. In the fall
the area of ice is smallest, being limited to the east coast
44
7
„
\
.•
•
e
'••
•
-
-•
• g. ••
erS7
; e..:
:„
•
•
\,
• . • \
I -.
..!
:
IntS
;C'e., $e
,a.
pr.le ice)
pfe ice)
WCSe
1.
1,3 )
Olece.WtaMta)
\ j.:-.. ,.
,. rne..4.:.,
, • r..„
7
33:‘
(07e1
•
ra- 1
1
YI
i:
'''''..'it-J-.=
C.-‘ MAC A
-
tx
\,
-
glt
MU
10.21
_
tat
ice)
_
pfn ix)
::e)
El r
Figure IV - 16.
The seasonal distribution of drift ice in
the North-West Atlantic.
(NFÉD is Newfoundland).
45
of Greenland, and there is none at all in the Labrador region
or the whole of the Greenland west coast. Then, on the east
coast of Greenland, the ice starts to move south at the
beginning of September, and tongue-shaped areas reach Kap Farvel
at the southern tip of Greenland in December or January, with
the ice covering the whole of the east coast. This ice may
reach from the southern tip of Greenland to Frederickshaab on
the west coast, or even to Gothaab.
Not only the seasonal variation of ice distribution
but also the variations from year to year must of course be
•eonsidered to have important significance for the fluctuations
of fishery resources and operations. The amounts of drift ice
brought down from the Arctic Ocean to the area from Norway to
Greenland have increased in recent years, a fact which
p53
Dinsmore (1971) has attributed principally to air current
phenomena in the Arc-tic regions.
The variations from year to year in the distribution
of drift ice near Iceland are shown in Figure IV - 17.
Investigations of the ice distributions from 1950 to 1968
given in this Figure shows that in recent years the drift ice
has extended eastwards, becomina- most conspicuous in 1968, and
no comparable pattern can be found at least as far back as 1918.
Figure IV - 18 shows a comparison of the distribution of drift
ice in I,:ay 1968 with the average annual values of the distribution
for 1911 to 1950. According to Figure IV.- 17 and Figure IV - 18,
46
3
4
5
6 tiONTH
Figure IV - 17.
The year to year variations of the distribution
of drift ice in March to May in the vicinity
of Iceland (shown as dark •ortions).
(Dinsmore, 1971).
47
p54
Figure IV
- 18.
The distribution of drift ice in Mav 1968
solid line)
and in the average year (dashed line).
Oblique lines:
Areas extending beyond those of the average year.
Dark portion:
Areas of retreat from the average year.
48
the year 1968, in comparison with the average year, was one
of a remarkable southward drift of ice, related to a remarkable
southern flow of the East Greenland Current, and the resulting
p54
oceanographic characteristics are low temperature and low
salinity (Dinsmore, 1971).
The currents in the North East Atlantic are greatly
controlled by the North Atlantic Current and its branches, and
the warm weather conditions in Northern Europe stretching from
England to the coast of Norway are principally due to this current.
The extreme end of the North Atlantic Current, after
going northwards past the Faeroe Islands to,the west coast of
Norway, splits into two, one of which goes north to the west
of Spitzbergen while the other flows into the Arctic Ocean along
the.north coast of Norway, and this portion warms the western
and southern parts of the Barents Sea.
The North Atlantic Current flows to the north along
the west coast of England and passes to the north of the
Shetland Islands, where a branch flows southwards along the east
coast of En,-land. Another branch of the North Atlantic Current
flows along the south coast of England through the English
Channel, and both of these branches have important effects on
ocean conditions in the North Sea.
The southern branch of the North Atlantic Current
flows southwards along the coast of North West Europe, and
becomes the Canary Current flowing from Portugal to the
49
North West coast of Africa. The volume of this current is
p 55
16 Sv, and its surface velocity is estimated to be less than
10 cm/sec (Sverdrup, 1942). Variations in wind result in
variations of current direction and velocity, but after
reaching the west coast of the African continent, this current
turns generally west and becomes tributary to the Northern
Equatorial Current.
As the Canary Current flows past the
shores of Portugal and North West Spain to the North West
coast of Africa, it produces regions of upwelling, which are
important factors in developments along the Portuguese coast.
One portion of the Canary Current flows south along the
western African coast, and keeping to the shore of the continent
turns generally to the east during the northern hemisphere
surnmer to become the Guinea Current. However, the Guinea is
also present during the winter.
The Sargasso Sea is the centre of the large clockwise
gyre which forms in the middle latitude regions of the North
Atlantic, and in this region the evaporation is large in
comparison with the rainfall, so that a region of high salinity
is formed.
This high salinity water region diffuses from the
Sargasso Sea into the whole of the tropical Atlantic, and is
called the Subtropical Underwater (Defant, 1936).
Numerous measurements have been made in the Atlantic
areas close to the Equator since the confirmation of the
presence of an Equatorial Undercurrent like that in the Pacific.
50
It is of smaller scale than in the Pacific, and the greatest
velocity, which is observed at depths less than 100 m, is
80 cm/sec.
The depth of greatest velocity is generally less
than in the Pacific, rising to less than 25m.
The axis of this
Equatorial Undercurrent is found in the lower layer of the
westward South Equatorial Current, but there are north and
south excursions, and the range in longitude is not more than
from 38 °W to 7 °E.
Metcalf et al. (1962) believed that the origin of this
Equatorial Undercurrent in the Atlantic Ocean was in the Guyana
Current which flows north and sinks after reaching the Equator.
This belief was opposed by Chanaichenco et al (1965) who
referred to the high salinity of the North Atlantic surface
water near the northern tropic.
However the results of recent
observations in both the Pacific (Tsuchiya, 1968) and the
Atlantic (Metcalfe, 1967) of the distribution of dissolved
oxygen, have fully confirmed that these undercurren-boriginate
fYsom the water masses in the southern hemisphere.
In the
Atlantic Ocean the southern hemisphere water mass which forms
the undercurrent flows to the north east after flowing along
the north east coast of Brazil.
Marine resources such as pelagic and middle level fish
are being developed throughout the whole of the Guinea Current
which flows into the Gulf of Guinea.
An oceanographic
peculiarity of the Gulf of Guinea is the thermocline which is
51
formed between the Tropical Surface.Water (TSW) on the surface
P56
and the South Atlantic Central Water (SACW) in the lower layers.
The thermocline in the Gulf of Guinea is present, with regional
variations, throu^_xhout the year.
Thé temperatures at its upper
and lower boundaries are above 25°C and below 19°C. The
thermocline is at shallow depths offshore, (in particular it is
only 12 to 14 m deep near the coasts of Senegal and Liberia),
and at greater depths out to sea (Williams, 1966). As is shown
in Figure IV - 19, the Gulf of Guinea can be divided into five
regions (Williams, 1966).
The Tropical Surface Water (TSW) in the Gulf of Guinea
has high temperature (above 24°C) and low salinity (less than
35% )• In the Northern Transition Zone (NTZ) and the Central
Upwelling Zone (CUZ) of this water mass, there are seasonal
variations in the regional distribution because of the ebb and
flow of the water masses with temperature below 24°C and salinity
more than 35J .
The Northern Transition Zone (NTZ) and the Southern
Transition Zone (STZ) are influenced by the seasonal north and
south displacements of the Tropical Surface Water (TSW), and
p57
in the Central Upwelling Zone (CUZ) the lower temperature and
salinity water forms upwellings from the latter part of June
to October. There is also upwelling
from January to March off the Ivory Coast.
In the Western
Transition Zone and the Eastern Transition Zone, the temperature
and the salinity vary because of rainfall and of the influx of
water from the land.
52
10'E
0'
r-
10'
20"W
-
15'
AFRICA
1 0'
5.
'9APE
(.1NtZ)- /
ALMA
(ETZ)
- -
o•
'GULF OF
s'
'
•
(slz)
_
• UINEA
•
_ _
NTZ
NORTHERN TRANSITION ZONE.
JANUARY TO FEBRUARY, MEAN LIMIT OF COLO WATER (TEMPERATURE BELOW 24 ° C).
WTZ
WESTERN TROPICAL
CLIZ
CENTRAL UPWELLING ZoNE.
ETZ
EASTERN TROPICAL ZONE, TEMPERATURE ABOVE 24 °C.
STZ •
SOUTHERN TRANSITION ZONE.
JULY TO AmJsr, MEAN LIMIT OF COLD, HIGH SALINITY WATER.
(TEMPERATURE BELOW 2 4 ° 0, SALINITY ABOVE 3 5%0 ).
ZONE, TEMPERATURE ABOVE 24 00.
Figure IV - 19.
Water masses and their boundaries
in the Gulf of Guinea.
(Williams, 1966).
53
Attention has recently been given to the potential
resources of Cephalouods in the Caribbean Sea. In the
tropical areas of the Western Atlantic, the ocean conditions
in the upper layers are greatly influenced by the Northern and
Southern Equatorial Currents, and those in the lower layers by
Antarctic water. As is evident from the temperature and
salinity structure of this area shown in Figure IV - 20, a
thermocline is formed between the surface water in which the
upper layer characteristics are those of the tropical zone, and
the lower layer at 100 to 200 m. The salinity structure shows
that these layers can be regarded as Surface Water (0 to 50 m)
and Subtropical Underwater (50 to 200 m). Below these water
masses there is at 400 to 600 m a lower layer of low temperature
water with low dissolved oxygen content (less than 3.0 cc/litre),
and the lowest layers of low temperature water can further be
divided into Subantarctic Intermediate Water (700 to 800 m) and
North Atlantic Deep Water (1800 to 2500 m). Regarding the
vertical distribution of salinity in these layers shown in
Figure IV - 20 as a Core, the salinity in such a core is very
small in the surface layer, very large in the Subtropical
Underwater layer, very small in the Subantarctic Intermediate
Water and again very large in the North Atlantic Deep Water.
The salinity in the surface layer is normally influenced
by evaporation, by precipitation, and by the influx of land water.
54
Along, the coast of South America there are also, during the
winter, regions of salinity greater than 36/o which are to be
attributed to upwelling.
Because of the large amounts of
precipitation and land water during the Caribbean summer, the
salinity of the Caribbean Sea may be lower than in winter by
0.5%,in_.the south and by more than 1.0/o in the north. The
layer of highest salinity shown in the vertical distribution
of salinity in Figure IV - 20 is'extremely thin, but it is
physically in equilibrium.
This high salinity layer is at
shallow depths (50 to 100 m) in the southern area, and deeper
(200 m) in the northern area.
The Subantarctic Intermediate Water originates in the
Antarctic Ocean, and its depth increases as it moves northward.
In the southern part of the Caribbean Sea it is 600 to 700 m,
and in the northern part 800 to 850 m. The layer of low
salinity is thickest to the south, and. the north its presence
becomes difficult to discern.
The changes in the surface temperature in the Carribean
are smaller than the changes in salinity, the difference between
summer and winter being in general 3°C. This depends on the
influx and efflux of heat in the Central Caribbean, and it has
been established2l that the sea absorbs 6.26 x 101$ calories
per day from the air, with a range of variation from this
average of 0.5 x 1018 calories per day (Gordon, 1966).
p58
55
The currents in the Gulf of Mexico and the Caribbean
Sea are shown in Figure IV - 21.
The current velocities in the
Caribbean are mostly small, and in the layers less than 1000 m
deep the direction is from east to west. The Caribbean is
isolated from the surrounding ocean, and the flow at 2000 m
is complex.
The water flowing into the Caribbean originates in the
Guiana Current which flows north west along the coast of South
America up to the Lesser Antilles. Most of the Guiana Current
flows into the Caribbean through the middle of the islands, in
particular through the straits to the north and south of
St. Lucia. The remainder flows into the North Equatorial Current,
and after flowing past the east and north parts of the Caribbean
turns towards the Bahamas.
The water mass which originates in the Guiana Current
passes the Grenada Basin and the Aves Ridge, and about 200 to
300 km from the South American coast becomes the Caribbean
Current.
The axis of this current goes westward between the
Aruba Gap and the Colombia Basin and veers northward to the
west of the Colombia Basin. This axis arrives at the Jamaica
Ridge to the north of the Colombia Basin and from about 85 °W to
P59
90 °W it flows along the Cayman Basin, turning to the north
again at the Yucatan Strait and flowing out of the Caribbean
into the Atlantic Ocean.
56
2.0
3.0
0
4.0
(;,E xV I)
51.C1.5 2 ^0
^•.0(cc/! )
b_0
Figure IV - 20.
Vertical distribution of
temperature, salinity,
dissolved oxygen and nutrient
1500
salts in the Caribbean Sea.
:oool--'_G00
?-c0^_
TEMP
ERA
sooo 1^^Tl1RE ïE
3500: ___
.^.. i.. _
SALIN
ITY
^Xy .
EM
_-._-_. .. .
Figure IV - 21.
Current flow in the Gulf
of Nïexico and the
' Caribbean Sea.
Arrows:
Figures:
Flow directions
Dynamic depth
anomalies.
57
The surface current velocity depends on the trade winds and
varies in response to its seasonal changes. The greatest
observed velocities of the Caribbean Surface Water are in the
latter half of the winter (with an average of 0.76 knots) and
in the first half of the summer (average 0.80 knots) and the
average -throughout the year is 0,70 knots (36 cm/sec).
The
greatest velocity found on the axis of flow is 2.7 knots.
The
principal axis of flow obtained by geostrophic current
calculation is not deeper than 300 to 400 m, its velocity is
4 0 to 60 cm/sec on the surface and 10 cm/sec at 3 0. 0 m. At 1000 m
to 1500m there is absolutely no motion.
An eastward flowing counter current is formed along
the shore lines, passing the Caribbean shores of South America,
Cuba and Haiti.
The presenc_e of counter currents can also be
established in the Colombia Basin, and in the western parts of
the Cayman and Yucatan Straits.
The oceanographic conditions of main importance for
fisheries in the Caribbean are the upwelling currents which
occur along the coasts from Venezuela to Colombia. These
upwellings resemble the upwellings which occur along the coast
from California to Peru, in that they are to be attributed to
the prevailing North East Tradewind which is just parallel to
the coast of Venezuela (Fukuoka, 1974).
58
In the South Atlantic, as in the South Pacific and
the Indian Oceans, an anticyclonic gyre is formed under the
combined influence of the wind and the Coriolis force due to
the earth's rotation, and the Benguela Current forms the
eastern part of this anticyclonic gyre.
This current goes northwards between 15 °S and 35 o S
along the west coast of South Africa, and there are low
temperature upweliings within 100 nautical miles of the shore.
The temperature of the surface. water in the region of
the Benguela Current is generally 8 o C lower than the temperature
of the shore water in the same latitude.
The regions of upwelling are to be attributed to the
prevalence of wind from the south along the west coast of
Africa, and the speed of the Benguela Current is comparatively
low, the geostrophically required velocity being less than
25 cm/sec.
The continuation of the Benguela Current becomes
the South Equatorial Current.
The South Equatorial Current, flowing to the west in
the South Atlantic near but to the south of the Equator, forks
off the shore of South America into north and south flowing
branches. The southward branch becomes the Brazil Current, and
has a remarkably high salinity.
At about 35 °S to 40 °S it runs
into the Falkland Current flowing northward from the Subantarctic,
and produces the Subtropical Convergence, which is indicated
by a summer surface water temperature of 14.5 ° C.
The Brazil
p60
59
current after producing the convergence turns to the east and
becomes the South Atlantic Current or South Ocean Current.
The Brazil Current is weak, its
surface velocity being about
one to two knots. Its southern extremity is furthest to the
south during the southern Hemisphere summer.
The change in
location results in a change of temperature from summer to
winter of about 4 ° C, and this change must have at least some
effect on troll fisheries.
The Patagonia area of the Brazil
and Falkland Currents is already attracting attention as one
of the regions with most scope in the world for the development
of maritime resources.
The Falkland Current, flowing north from the Falkland
Islands, develops in the surface water of the subantarctic.
This originates in the eastward flowing West Wind Drift in the
Southi East Pacific, which comes south from the southern part
of the Pacific coast of South America, detours around Cape Horn
and flows to the east as the Cape Horn Current, with a surface
velocity of 1.7 knots.
The Cape Horn Current forks to the east of Staten
Island.
The main portion becomes what should be called the
Falkland Current (but is also called the Malvinas Current) and
flows north between the Pataeonia continent and the Falkland
Islands. The other branch turns to the North East and joins
up with the eastward flowing West Wind Drift in the South Atlantic.
6o
In the Patagonia area the temperature and the salinity
of the Falkland Current are both low,
and,
in contrast to the
Brazil Current which has a high colour index, (this index means
a small amount of blueness) the colour index shows little
greenness.
The current velocity away from the east side of the
Falkland Islands is high, and isolated low temperature regions
are formed in this area. Klaehn (1911) has suggested that they
are produced by vortices accompanying turbulent flow22, but it
has also been suggested by Deacon (1937) that they are connected
to topographic features of the steeply sloping continental shelf.
The velocity of the Falkland Current along the Patagonian
shelf is low, and in the coastal areas it has a southward tendency.
Consequently the water along the Patagonian coast has a higher
temperature than the water mass found over the continental shelf.
This high temperature water mass is not directly related to the
subtropical water mass of the Brazil Current, and is thought to
be formed by warming of the subantarctic water mass of the Falkland
Current.
It has been called 01d Shelf Water (Deacon,
1937).
A detailed fishery and oceanographic survey of the
Patagonian area was made in December 1969 and January 1970 by
the research ship Kaiyo Maru
Fisheries AFency.
(2539 gross tons) of the Japanese
The results were reported in
"Report for 1969 of the research ship Kaiyo I,iiaru Argentina and Patagonia".
published in 1971 by the Japanese Fisheries Agency. A summary
of this report is presented here.
p.61
61
Bottom material.
(1)
In the areas investigated by Kaiyo Ptiiaru, the bottom
consisted in aeneral of medium sand or fine sand, but it was
locally mixed with mud and seashells. There were also places
in the mud or gravel bottoms where there was no medium or fine
sand.
The distribution of these bottom materials is shown in
Figure IV - 22.
The sandy reqions.
Sandy regions were observed principally in connection
with the La Plata estuary, the north of the Los Estados Islands
p62
the Scotia side of the Burdwood Bank and the Platform on the
East of San Matias. In these regions the benthic marine life
was extremely prolific23.
The gravel regions.
The principal gravel regions were the Burdwood Bank,
the south west of the Malvinas Islands, the mouth of San Matias
Bay, and the region from Cape Blanco to the eastern tip of
Fuego Island. It is proposed that a principal cause of the
occurrence of gravel in the offshore regions is the gathering
by surface traction of large brown algae with air bladders,
but it is also thou,-ht that other possibilities are the drift
of ice formed by frost in the high latitude coastal regions
and the iceberc:,,s which float away from the Antarctic Continent.
To the North West of the P.Ialvinas Islands and to the
North East of Fuego Island there are wide areas of gravel
62
Figure IV - 22.
Bottom materials in the
Patagonian region.
(Japan Fisheries Agency, 1971).
Figure IV
- 23.
Surface currents* in the Patagoni'an
region measured by GEK.
(Japan Fisheries Agency, 1971).
Arrows:
Figures:
-
Current directions.
Velocities (knots).
* But see text page 63.
Translator.
63
formed by the eddies in the cold current which stagnate while
carrying the floating matter already mentioned.
Mud bottoms.
The principal regions of occurrence of mud bottom
are around the Malvinas Islands, off Mar del Plata and in the
San Jorge Gulf.
Since these regions of mud bottom are in
general those of estuaries, eddies and ocean fronts, or of
uDwelling near the continental shelf, they are relatively
highly productive of life.
(2)
Current flow.
Figure IV - 23 shows the results of observation by
GEK
24 at depths greater than 100 m*.
Apart from the flow of
the Brazil Current at 55 °W from 41 ° S to 43 ° S, in almost the
whole region the prevailing flow is towards the north, and south
of 52 ° S the eastward flowing current which is part of the West
Wind Drift was found.
The Cape Horn current flowing eastwards to the south of
Los Estados Island attains the large velocity of 2.47 knots
(the direction being 74 ° ), and it flows strongly to the east
without turning north.
North East of Fuego Island there are
clockwise rotating eddies, and regions of large scale eddies
are found from the North East of the Malvinas Islands to
Cape Blanco.
There are normally tidal currents on the continental
shelf.
Off the Argentine coast, the flood tide flows northwards
and the ebb flows southwards, whereas round the Malvinas Islands
*
Sic, but the caption to Figure IV-23 says "surface currents".
Translator.
64
they flow east and west. The tidal range25 suddenly increases
in Bahia Blanca where it is reported that there is an anticlockwise eddy on the flood tide and a clockwise eddy on the ebb.
Using the Mar del Plata as reference, the tidal ranges
may be summarized as follows. From the boundary at 58°50'W
p63
it rapidly increases to 3 times as much,at Comodoro Rivadavia
it is 4 times, at San Julian 5.5 times, and at Santa Cruz the
maximum value of 8.2 times is attained. Near the entry to the
Straits of Magellan it is 7 times, at the Rio Grande 4.3 times,
on the east of Fuego Island 2.7 times and on the I4alvinas Islands
1.2 •times.
The average distribution of temperature and salinity
at the 10 m level and on the bottom are shown for reference in
Figures IV - 24, 25, 26, and 27.
From this description of the currents it is evident
that the main feature of the South Atlantic current pattern in
a large scale anticlockwise gyre. On the east, this gyre
touches the Benguela Current, on the north the South Equatorial
Current, on the west the Brazil Current and on the south the
West Wind Drift, and a converr2^ence is formed at its centre
at about 30°S.
In the area to the south of the southern extremity of
Africa, a converp•ence is formed between the West Wind Drift
which has the characteristics of the subantarctic water mass,
and the Aaulhas Current which flows southwards alon-- the east
coast of Africa.
65
(
a ^^
LJ
^•..
Cl
Figure IV - 24.
TemUerature distribution at
Tempera.ture 'distribution at
the bottom, Patagonia.
the 10 m level, Patamonia.
(Japan Fisheries Agenc.y 1270-
(Japan Fisheries Agency 1971).
I
5^•^
Finure IV - 27.
Finure IV - 26.
Chlorinity distribution at
Chlorinity distribution at
the 10 m level, Patagonia.
the bottom, Patn-nnia.
(JaDan Fisheries Amency 1971)
0
(Japan Fisheries Agency 1971)
.
66
p.64
3.
The Indian Ocean.
The seasonal variations of the winds result in large
seasonal variations of ocean conditions in the Indian Ocean.
In
order to describe the structure of this ocean, it is therefore
p.65
necessary to grasp the meteorological processes in outline.
As can be seen from the average distribution of the
isobars for January shown in Figure IV - 28, the equator lies
within the 1009 to 1012 mb isobars, and is in a calm region with
light winds.
However there is a zone centred around the
metéorolo2ical equator at 10 °S which forms a dividing line, with
different meteorological phenomena to its north and south. To the
north of the meteorological equator there is the North East Trade
Wind (or North East Monsoon)
is the North West wind.
and to the south the prevailing wind
From the west of Sumatra to the Malay
Archipelago this North West wind is called the North West Monsoon.
The lowest pressure in the region just south of the
meteorological equator is less than 1009 mb, and occurs in Africa,
Australia and Madagascar because of the heating of the continents
The highest pressure is in
26
at about 35 o S,
the subtropical cairn zone (the Horse latitudes)
during the southern hemisDhere summer.
and the hiollest
pressures, above 1020 mb, are observed in the
centre of the Indian Ocean and near the Paul - New Amsterdam
Islands.
To the south of this high pressure region the pressure
falls till the subpolar trough, where the pressure is less than
990 mb, is formed at about 64 ° 30' S.
p66
67
JANUARY
JJLY
Figure IV - 28.
Average isobar distribution in the Indian Ocean.
(Mintz and Dean,
quoted from Fairbridge, 1966).
68
Because of this pressure distribution, the North East
Monsoon is found to the north of the meteorological equator,
mostly in the northern hemisphere.
In the southern hemisphere
the South East Trade Wind prevails over the whole area as far
as 35° S, with the exception of Australia, and to the south of
35S the prevailing Nest winds
are
known as the "roaring forties".
In the Arabian Sea the prevailing North
East Monsoon reaches its greatest strength in January, and in
the zone off the Somali coast the average wind force is greater
than 4 on the Beaufort scale (Figure IV - 29).
The winds in
the Arabian Sea have, in general, the following directions.
1,
The Gulf of Oman: outwards from the Gulf.
2.
Southern Arabia:
3.
The Gulf of Aden: Into the Gulf.
4.
Iran - Pakistan:
Offshore
5.
Indian coast:
North East or North West, which is
North East, which is parallel to the coast.
p67
offshore or parallel to the coast.
The pressure pattern to the north of 10 °S in July
is just the reverse of that in January.
The region from the
Arabian peninsula to the Asian continent which showed high
pressures of more than 1020 mb in January becomes a 1ow
pressure areà with pressures below 1005 mb. From this low
pressure area southwards to the South Horse latitudes (about
30 ° S), the pressure increases. Because of this pressure
distribution, the Trade Winds from the south prevail in the
69
Fi,gure IV - 29.
Mean winds in the Indian Ocean in January.
(Wooster et al., 1967).
Numbers:
Wind force on Beaufort Scale.
Solid lines:
Wind direction.
Dotted lines:
Lines of constant wind force.
70
Indian Ocean from the southern to the northern hemisphere,
and in particular the South West Pi,onsoon results in upwelling
currents off the coast of Somalia-where it has greatest force.
(Figure IV - 30). The South West Monsoon is most greatly
developed from June to August, and for 10'to 20% of the time
the wind strength is Beaufort 7 or greater.
During the period of transition from the North East
Monsoon to the North West Nionsoon, in April, the distinctive
features of the prevailing South West wind direction off the
coast of Arabia can be summarized as follows.
1. Gulf of Oman:
South west to.South east, that is to say,
in general towards the interior of the
Gulf.
2. South Arabia:
South west, parallel to the coast.
3. Gulf of Aden:
South west or South.
4. Somalian Gulf:
South west, that is to say, parallel to
the coast.
5. Iran to Pakistan: Onshore.
6. Indian coast:
West, onshore.
The South west monsoon be^7ins to weaken in October,
and a. wind from the north beRins to develop with the approach
of the winter season of the North East h-:onsoon which is
stron5Test in January.
The seasonal variations during the
year can be summarized as follows.
71
Figure IV - 30.
Mean winds in the Indian Ocean in July.
(Wooster et al., 1 967).
Numbers:
Wind force on Beaufort Scale.
Solid lines: Wind direction.
Dotted lines: Lines of constant wind force.
72
1.
December to February.
North East monsoon prevails, with maximum development
in January.
2.
March to May.
A season of changing wind direction, the North East
monsoon begins to weaken in March, and the South West
monsoon begins to develop in May.
3.
June to August.
South West monsoon prevail's, with maximum
development in July.
4.
September to November.
A season of changine wind direction, the South West
monsoon begins to weaken in September, and the North
East monsoon begins to develop in November.
There is great influence from the monsoons on the
currents in the northern part of the Indian Ocean, particularly'
in the Arabian Sea, and the principal currents in the area are
the South West Monsoon drift in the summer and the North East
p68
monsoon drift in the winter.
The principal remaining currents
in the sou lthern hemisphere are the Mozambique Current, the
Agulhas Current, the West Australian Current and the West Wind
Drift.
These currents are shown in Figure IV - lb (for the
northern hemisphere summer), and in Figure IV - 31 (for the
northern hemisphere winter).
73
]U•
]0•
` ^^^'';s "=
' `=',-,-
•
`'
30•
•{^fir^i+`^^i{rii^^ii ^^,^^^ii-,•,^
^ Fi ^/^^jJ
';li^^•`^
^\^>^1^^`'J/'-"
^^1'1^_^^^`=^^
-
^
Z_i--^•^
-,:
,o•.^-.^...fi.^i,-{I
E30• 60• 90'110•1W E
Fi.aure IV -- 31.
Surface Currents in the Indian Ocean. (Defant 1961).
Durinff the northern hemisDhere winter.
1.
North East Monsoon Drift
5.
Mozambique Current
2.
South Equatorial Current
6.
Agulhas Current
3.
Equatorial Counter Current
7.
West Australian Current
4.
North East Mtonsoon Drift
8.
West Wind Drift
(Antarctic Gyre).
714.
A large scale anticloàkwise gyre is formed in the
southern hemisphere, and its southern edge. is in contact with
the West Wind Drift which extends from about 40 °S to 50 °S,
forming thereby the Sub-tropical Convergence. The eastern
edge of the anticlockwise gyre adjoins the South Equatorial
Current.
In the region of the Equatorial Current, the current
p69
direction is in general to the west during the northern
hemisphere winter (apart from the Equatorial Counter Current)
and to the east during the summer.
The South Equatorial Current flows to the west and
splits on arrival at the east side of Madagascar, one part
turning somewhat to the north of west and the other flowing
south.
The portion' which turns north and west, after arriving
at the northern part of Madagascar, turns west again to the
east side of the African Continent, where a part goes north
to become the Equatorial Counter Current, and the remainder
goes south to become the Mozambique Current.
The Agulhas
Current is formed from the Mozambique Current and the portion
of the Southern Equatorial Current which has flowed south
along the south of Madagascar.
It has already been mentioned that the monsoons cause
large seasonal variations in the surface currents to the north
of 10 0 S.
During the North East Lionsoon season from November
to March, the North Equatorial Current and the North East
Monsoon Drift develop in the westerly and south westerly directions.
75
The boundary between these currents and the eastward flowing
Equatorial Counter Current moves from 3°N to 4°N in November,
to 2°S to 40S in February and remains there until the South
West monsoon prevails at the beginning of April.
During the South West Monsoon season from April to
October, the Somali Current flowing to the North East develops
at about 10°S off the Somali coast. This current forms a cold
water zone, as shown in the distributions of surface water
temperature in January and July given for reference in
Figure IV - 32.
p70
The Somali Current is known to be a typical Western
Boundary Current like the Kuroshio and the Gulf Stream which
have already been discussed. The average velocity of flow of
the Somali Current in April at about 6°N has been found to be
86 cm/sec (1.7 knots) and when it is most highly developed in
July the mean velocity is 264 cm/sec (5.1 knots). The maximum
velocity was observed to be 360 cm/sec (7 knots or more,
(Wooster et al., 1967). The width of the Somali Current is
reported by Stommel and Wooster
(1967) to be 80 km at 4°S and
200 km at 8°N. The inshore Somali Current which develops
during the winter from October to Aiarch as a North East
Monsoon drift has a lower velocity than the Somali Current,
and to the south of 6°N from December to February the mean
current velocity is 100 cm/sec (about 2 knots).
76
40E
• •
30 •7.N: a )
zeL
se■
, ,--r-i-i-r-- i
l'-',.."........,,
\1..8
'
70
•
èt
1i ■ i i i é i .
I
.
,1 ■
a
80'
à r_i
S,‘,._.
b-•:„..,/-1/4.2j -----,---::::
zwiu›i'
J—,- -
j
2
-!--
15."
//
2 ---
/i
10•
• /
/
/
./
21 \
E•
• '', .....
/
...
0.
0 •'.
1_1. e..1:fr_L_L.
•
'
■
WE
29 .
[
ei
20.F\
2 '›i
.•-,__/
15
2
2P
,i
7:-N..e,..5„..-.-•,-.: ----i ,2,9 ,
: (.-.----- `21.r
2
lo«-_- 30,-.
i,„
\
I
..
f
I
•
>28
e
4
L
Fi,çrure IV . - 32.
Distribution of surface
temperature in the Arabian Sea.
(Wooster et al., 1 967).
a:
January.
bs
July.
77
As already mentipned, part of the South Equatorial
Current flows south along the East coast of Africa. The
velocity of the Mozambique Current flowing southward in the
P7
Mozambique Channel between Madagascar and the African Continent
is large, and it is also called the Agulhas Current.
The
Agulhas Current is the South West portion of the large scale
anticlockwise gyre in the southern Indian Ocean, and flows
south west along the coast of South Africa between 25 °S and 40 ° S.
As can be seen in Figure IV - 33, part of it flows round the
Cape of Good Hope into the Atlantic, but most becomes a
returning flow and turns from east to north east, the southern
portion joining the West Wind Drift in accord with the general
westerly winds of the southern Pacific.
The Agulhas Current
maintains its direction but its velocity is found to vary
seasonally from a minimum of 20 cm/sec to a maximum of 60 cm/sec.
Off the east coast of South Africa, from 25 ° S to 32 °S a long
elliptical eddy, persisting throughout the year, is formed, and
its current velocity reaches 10 cm/sec.
The seasonal
variations of the Agulhas Current are attributable to the
changes of the South Equatorial Current and of the West Wind Drift.
The Equatorial Undercurrent found in the Pacific and
the Atlantic Oceans has also been found in the Indian Ocean
by the observations of Taft and Knauss (1967).
According to
observations up to the present time, the core of the Undercurrent
was at 100 m depth from 53 °E to 92 °E during March and April 1963,
78 .
Fi,--ure IV - 33a•
Currents off the shores of South Africa.
E 20'
-rS
30' ; ^\12
1
30•
90'
'T-f-r
/ 22
.20^
32-1L, s--
;0
---
Fi-,ure IV - 33b.
Surface water temperatures in
the seas around South Africa.
Left: January.
Right: July.
6
79
and a zone of strong current was observed at 70 m from 58 ° E to
67 ° 30'E during March to April 1964.
When investigated during
the summer, no undercurrent could be found. It thus appears
from the observed results that the Equatorial Undercurrent in
the Indian Ocean can be thought of as influenced by the North
East monsoon in the winter and by the South West monsoon in
the summer, but the relation is as yet unknown.
In order to forecast the future development of the
potential resources of the South East Indian Ocean, it is
necessary to discuss the ocean environment. With regard to
the surface layer of the South East Indian Ocean, Rochford (1962)
found from north-south cross-sections of the salinity that there
are considerable surface discontinuities in the salinity at
15 °S to 22 °S and at 36 °S to 40 ° S.
These discontinuities are
formed in the Transition Zone between the Tropical Zone and
the Subtropical Zone, and in the Transition Zone between the
Subtropical Zone and Subantarctic Zone. The oceanographic
properties of these zones are listed in Table IV - 4 and the
representative distributions in Figure IV - 34.
The oceanographic characteristics of these zones can
be summarized as follows.
(i)
Monsoon Current Reeion.
Water masses of high temperature and low salinity
formed to the_north of the equator and to the west of Sumatra
which flow to the south east in July with the monsoon drift
(Wyrtki, 1957).
80
OCEANOGRAPHIC
AREA
EMPERAT.iRE
REGION
.:.11ANTITIES
SAL INI TY
PHOSPHATES
(SEASON OF OBSERVATION).
(WU)
28.9
MONSOON DRIFT
Tnorics
28.7
EQUATORIAL COUNTER CURRENT
(JULY TO SEPTEMBER)
SOUTH EQUATOR IAL r;URRENT
SOUTH EQUATORIAL LOW SALINITY CORE
--
TROPICAL ra-s-a---m-5-ncAL.
DIVERGENCE
( ° '0). _
_
•
(n g at '/)
31.10
_
0.12
34.56
0.21
26---27.5
34. 48-31. 55
0. 20-4. 30
26 ,--, 27
34. 26 ,-31. 31
0. 20 ,-0. 25
24-25. 5
(JULY TO SEPTEMBER
_
31. 90
0. 15 ,-4. 20
_
SOUTH EAST I ND IAN OCEAN, HIGH
SAL INITY SURFACE
SUBTROPICS
20 ,-21
36. 00.--,36. 10
0. 10 ,-4. 15
21. 5-22. 3
36. 30-36. 50
0. 20-4. 06
35.40
0. 20 ,-4. 30
<35.00
>OEM
(FEBRUARY TO MARCH)
AUSTRAL IAN R IGHT,
INITY
HIGH SAL
SUBTROPICAL ...- SUBANTARCT 1:::
sO I VE RGE NCE .
16-17.5
(FEBRUARY TO MARCH
<15.5
IC
(Fcnnut nv in Ai anti\
.
Table IV - 4.
Oceanoraphic characteristics of the surface layers
of the South East Indian Ocean (Rochford, 1962).
120*
1 00'
140'
E
o'
c:à
10'
I sal-',"fle:
,,.,e«."- -:,,,,› .- C.
• ,1,...s:
- .i..„.--,..,.:.
....<,
`...... .., • .-_.;-•".:/.--(:•::i4i4.,-•,, • ,•
-
_
-s-:::z•
:.-. •
------
West
qing
tit,
Fest,A•sfura
L'; ,e;t.ng
...‹.
C: -.I•e of T,...P.e.31-
7.-,e
AUS -; F.ALIA
of
,s•s:;en
:,, ,. .7-4, 'I , F ir•f
.7 -I'...: , VI.
20'
..
.....
e ••C
•.•
CE,!•e cl
l'c
41:e
v",:•••••sj
%.:• tr
'S enlYf•-•-.T.
i.MD•g, P>if.''if 1
r:-.7j D. 0. F,:,- 43‘,...; ,
Ca,
4.0„-1/1 3
e•4 r•-•//i,
>.• -... •-"'.
,
- -
„.;
• .1f,,,,...C73.
Eli
'-'›;. ).2;:•.ui,- ia10A
- --_,...._________‘,
-
•
3J
".• "
I Il i
Tr,.r.sit•en Zz•re
11
11 1
I
Figure IV - 34.
Pattern of ocean environ.'ents in the South East Indian Ocean,
based on Table IV - 4. (Rocbford, 1962).
SUBANTRC
81
(ii)
Equatorial Counter Current Remion.
A relatively high salinity Equatorial Counter Current
is believed to flow eastwards in July, south of but near to
p73
the equator (Wyrtki, 1957). This water mass is relatively
rich in nutrients in the surface layers.
(iii)
South Equatorial Divergence Re^zion.
This region is characterized by very high salinity,
and is fbund to reach from 95°E to 110°E. The eastern portion
is a small scale divergence, in which at the 50 m level there
is a definite region of low dissolved oxygen content, less than
4.0 cc/litre, and the salinity is high throughout the divergence.
(iv)
South Equatorial Low Salinity Region.
This region is the core of the South Equatorial
Current, and is characterized by very low salinity on the
surface and at the 50 m level. In general, the surface is
rich in phosphates. There are régions of high salinity to the
north and to the south of this region of low salinity, showing
that the South Equatorial Current-originates in a water mass
of low sa.linit,y.
During the North East monsoon season, this
low salinity water is in the extremely low salinity Western
Arafura Sea and the Eastern Banda Sea (Wyrtki, 1961).
(v)
Tropical - Subtronical Transition Zone.
The centre of this water mass is at the 34.90/,,
isohaline, and is found at 18°S to 19°S. It is the region of
maximum values in the horizontal distribution of phosphates at
the 50 m level.
82
(vi)
South-East Indian Ocean Surface Salinity kaximum.
Highly saline water with salinity more than 36.00%0
is formed in summer to the west of 100 °E.
This high salinity
water spreads westwards and between 95 o E and 115 o E northern
boundary is at the
35% isohaline and its southern boundary
at the 35.8%. isohaline.
This high salinity water corresponds
to the subtropical zone.
(vii).
Australian Bight Salinity Maximum.
A water mass of high salinity with little nutrient
salts is formed in the Australian Bight.
Wyrtki (1962)
suggested that its high salinity originates from the tremendous
evaporation.
(viii)
Subtropical-Subantarctic Transition Zone.
The centre of this region is located generally along
the 35.4%0 isohaline from 36 °S to 38 °S.
To the south of this
region the Phosphate content increases rapidly.
(ix)
Subantarctic Zone.
As is clear from Table IV -
4, the northern boundary
of this water mass is the 35%0 isohaline, and to the south of
Australia it occurs at lower latitudes than in other areas.
83
The monsoon generally develops upwellings in the Indian
Ocean, and the Somali region can be regarded as typical.
Durin,a, the season when the South West m-nsoon
p74
produces a northwards flowing current, upwelling currents are
formed in the Somali coastal zone. These upwellings are
evident as regions of temperature below 25°C in the distribution
of surface water temperature for July shown in Figure IV - 32b.
The region of upwelling extends from Cape Guardafui to Socotra
Island and along the Somali coast. The extent of the region
of upwelling chan.ges from month to month as shown in Table IV --5,
and it evidently varies in response to the rise and decay of
the monsoon.
Table IV - 5.
Monthly variation of the extent of the
upwelling region formed off Somalia.
(Cushing 1969).
Month
Distance from Cape Guardafui
Area
to the upwelling region.
94 x 103 km2
May
375 km
June
625
112 x 103
July
625
112 x 103
August
375
94 x 10 3
Sentember
375
94 x 10
October
200
61 x 10 3
3
84
Upwelling currents produced by the South West monsoon
are also formed along the coast of South Arabia, but these
upwelling currents are not to be seen at the entrance to the
Persian Gulf in the interior of the Gulf of Oman (Schaefer and
Robinson, 1967).
There are also upwelling currents on the
Malabar coast of India, and during the season when the
prevailing winds are those of the South West monsoon, the
production of algae reaches its peak (Subramanyan and Sarma, 1965).
In the Bay of Bengal upwelling currents are formed
from February to March off Waltair (La Fond, 1954), and they
enlarge as far as Calcutta in June.
Upwelling currents are
also found off the whole of the Orissa coast during the South
West monsoon (Cushing, 1969).
Upwellings also occur from December to January in the
Andaman Sea between Burma and the Andaman Islands, and on the
west shore of the Gulf of Siam (La Fond, 1954., Wyrtki, 1961).
*
Other upwelling currents in the Indian Ocean are
formed by the monsoon in North West Australia and in the East
of the Arafura Sea.
While the South East trade wind prevails
on the North West coast of Australia, the quantity of phosphates
in the surface water over the continental shelf between 18 ° 30'S
and 11 ° 30'S increases (from 0.1 to 0.3Ajat/litre) and the
amount of dissolved oxygen decreases (to less than 4.50 m1/1).
From this Wyrtki (1962) deduces the presence of upwelling.
During the prevalence of the South East trade winds, the
85
quantity of phosphate on the surface of the Eastern Arafura
Sea also increases (from 0.34 to 0.66 Ag at/litre) and the
amount of dissolved oxygen decreases (from 3.57 to 2.25 m1/1)
and this also shows the presence of upwelling.
In the
surrounding regions the quantity of phosphates increases
during the upwelling by a factor of 6.
Wyrtki (1971) has also deduced the existence of
p . 75
upwelling currents in the southern part of the area between
Java and Sumatra from the mean distribution of temperature
and of nutrient salts at the 100 m level, and this also occurs
during the South East monsoon.
4.
The Antarctic Ocean.
Since the Antarctic Ocean adjoins the Pacific, Atlantic
and Indian Oceans, a strict determination of its boundary is
difficult.
However in general the name Antarctic Ocean is
applied to the area of Antarctic Surface water characterized
by low temperature, below 4.0 ° C, and low salinity, below 34%0 ,
and lying to the south of the Antarctic Convergence
formed by
the warmer but somewhat lower salinity
Subantarctic Surface Water.
86
The Antarctic Convergence is centred on the Antarctic
Continent and develops in the Pacific, Atlantic and Indian
Ocean.
As shown in Figure IV - 35, its latitude varies
considerably with longitude.
p76
In the Indian Ocean and the Atlantic Ocean, the
Antarctic Convergence is located at latitudes of 50°S or lower,
but in the Pacific it is in higher latitudes: 55°S to 60°S.
On the average, the Antarctic Ocean is the whole of the area
south of about 52°C. Moreover, although the Antarctic Convergence
remains in the same latitudes, displacement of about 100 nautical
miles in the period of a month have been observed (Ishino, 1967).
The area of the Antarctic Ocean is estimated by
Bogusla,w to be 32 x 106. km2, with 5 x 106 km2 covered by sea
ice during the summer and 20 x 106 km2 during the winter. Thus
the area of open water in the Antarctic Ocean varies seasonally
with the distribution of drift ice. The month to month average
location of the ice is shown in Figure IV - 36, and it is
evident that in general the northward extension of the drift
ice in the Antarctic Ocean is greatest in the Weddell Sea.
From Table IV - 6, the area of the ice is least in Mlarch, and
P77
the lar:^est value, in September, is 18.8 to 20.6 x 106 km2.
Comparison of these fi^ures with the values of 17 to 20 x 106 km2
obtained from the Nimbus satellite shows that they are reliable
as to order of magnitude (Kusunoki, 1971). Notable tongue-shaped
regions of ice pointing east in summer to the east of the Weddell
Sea and centred around 63°S can be attributed to the Weddell drift.
87
%V II,' F.
FiRure IV - 35.
Surface currents in the Antarctic Ocean,
WD:
Northern boundary of the Weddell Drift.
EWD: Northern boundary of the East Wind Drift.
1.
East Wind Drift.
2.
West Wind Drift.
3.
Average summer ice line.
88
0'
EA^
]50' w
60"
SO'
n
1
`1
Figure IV - 36.
Location, by ronth, of the average ice line
in the Antarctic Ocean (Kusunoki, 1971).
(a)s
Winter (April to September)
(b):
Summer (October to Pr;arch).
89
Table IV - 6.
•
Area of Ice in the Antarctic Ocean. (Kunsunoki, 1971).
(Units 10 6km2 ).
.
1
Mo NIH
.
L. 1. Est.:in
E. Vowinckel
SUBANTACTIC
314
516
I I I
1
1 1
7
819
1
10 1 11 1 12
I.
112
31 17. 81 15. 91 11. 4
2111. 01 13. 9115.
6. S..1 4.21 2. 61 6J
5.91 5. 91 8.1110. 51 1 6. 1118. 61 19. 520. 6119. 4116. 5112.0
8.
11
I
SUBTROPICAL
ANTARCTIC
ANTARCTIC
CONVERGENCE
CONVERGENCE
DIVERGENCE
Low
COLD ANTARCTIC
S
SURFACE WATER
sALIHrry LAYER
4--- , 1
tr
/,-)•, e
—
—
vi,
—
h
SUBANTARCTI8
WARM LAYER
..—
,.
SURFACE WATER
HIGH
INTERMEDIATE WATER
/.e./
SAL I N1TY LAYER
ANTARCTIC CONTINENT
(Y
HU
71.0
CIRCUMPOLAR DEEP WATER
DEPTH
(ME TR ES )•
200
25e
e'r
3000
-4--
v/eee /7
DEEP WATER
re
Low
OXYGEN
LAYER
7i<
s
.
ANTARCTIC
BOTTOM WATER
Figure IV - 37.
Diagrammatic vertical cross-section of the circulation
in the Antarctic Ocean and in the Subantarctic.
9o
As can be seen from the dia.7rammatic vertical crosssection shown in Fi,--ure IV - 37 of the circulation in the
Antarctic Ocean and in the Subantarctic, the structure of
water masses in the Antarctic Ocean is comparatively simple.
The Antarctic surface water extends down from the surface to
about 250 m, (though in fact it becomes shallower to the north), and
as
is clearly shown in Figure IV - 38, it is characterized by
low temperature and low salinity, especially close to the
surface in the summer because of the melting of the ice (salinity
less than 34/. , temperature in the core less than -1.0°C.
Mossby (1934) found that the properties of the water
at the 50 to 250 m level remained essentially the same in the
P78
winter and called it Antarctic Winter Water.
The current direction in the Antarctic Ocean is
principally from West to East, but some of it moves northwards.
This northward flowing current sinks at the Antarctic Convergence
to become the Antarctic Intermediate Water which continues
northward at about the 1000 m level and reaches the Northern
hemisphere.
Below the Antarctic Surface Water there is a layer of
Antarctic CircLUnpolar Water, which is characterized near its
upper boundary by hiq;h temperature and low oxygen. It is also
called. the Deep Warm Water, and its presence can easily be27
confirmed by bath,y-tl:ermoF-raph measurements of the temperature
inversion.
Near the unper boundary of the Deep Water there is
91
1000
2000
(e)
Sal
CC
1.•
.
UN
x.
4000
0000
'
moo
2000
U.1
W •
Z.•
3000
,
a.
,
I 4000
0i 00
Figure IV - 38.
(a):
Temperature cross-section near longitude 0 °
from the ice edge to Capetown in the 8th and
9th series of Antarctic investigations ( ° C).
(b):
The salinity cross-section in the same place
(Watanabe, 1971),
(%.
).
92
partial mixinq with the Antarctic Surface Water. At the high
latitude dividing line between the East Wind Drift and the West
Wind Drift, some of the East Wind Drift is deviated to the
p79
left (so that some of it goes southwards) in accordance with
Ekman's wind transport theory, and some of the West Wind Drift
is deviated to the right (so that some of it goes northwards)
and the result is a divergence known as the Antarctic Divergence.
The upwelling currents produced in this Antarctic Divergence
are the principal reason for the phyto-plankton growth there
during the summer, and this again is considered to be connected
with the distribution of krill.
The lower boundary of the deep warm water is generally
characterized by salinity of more than 34.7/o . The depth of
the high salinity region is about 1000 m at the Antarctic
Divergence, and near the Antarctic Convergence the depth reaches
2300 m. This water reaches to the subantarctic, flows from
west to east encircling the Antarctic Continent, and becomes
the common Deep Water_of the Pacific, Atlantic and Indian Oceans.
In all areas close to the Antarctic Continent, the
tremendous lowering of temperature during the winter results
in cooling of the water and in the formation of ice. The
expulsion of salt from the freezing water produces water of
hi^;h d-nsity (the generally observed density is
27.89) and
this water sinks, mixing with the Antarctic Circumpolar Water
and becoming the low temperature Antarctic Bottom Water with
temDerature below 1°C.
This sinking is noticeable in the
?3
Weddell Sea where the observed temperature at depths greater
than 4000 m is -0.4 ° C, the salinity 34.66%0 and the Cri-is 27.86.
These values are to be explained as the result of mixing of
surface water from the continental shelf which has sunk after
the formation of ice with the Circumpolar Water in which the
temperature is 0.5 ° C, the salinity 34.68%0 and rsre = 27.84.
As can be seen in Figure IV - 37, Subantarctic Upper
Water with temperature above 8 o C and relatively low salinity
is found in the subantarctic, where Antarctic Intermediate Water
with low temperature and low salinity flows northwards at
lower levels.
The deep water below 1500 m contains an upper part
where the salinity is above 34.80%
and a lower part which is
the extension of the Antarctic Circumpolar Water, so these
lower layers are divided between Intermediate Water and Upper
Deep Water, Lower Deep Water and Bottom Water.
The Upper Deep Water has high temperature and salinity
and since the Lower Deep Water is in contact with the
Circumpolar
water, the temperature and salinity have somewhat low values.
At depths less than 100 m in the Antarctic Ocean in
the winter, the water produced has a temperature close to the
freezing point and a salinity of 34%4, to 34.5%0 .
In the summer,
the salinity is lowered by the melting of the ice, and the
temperature rises somewhat because of the heat absorbed by the melt
water.
These values of temperature and salinity are important
•
rnhenes
Penh.
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la mer
IV
TRANSLATION 3820
SERIES NO(S)
2 of 2
94
factors in the melting of pack ice and icebergs, and in the
neighbourhood of pck ice and icebergs there are normally
variations.
p80
In the vicinity of the Antarctic Continent, the East
Wind Drift is produced by the prevailing east wind which
results from the outflow of the Polar Cap High Pressure region
(see Figure IV -- 35). At about 90°W its northern boundary is
around 70°S and as it goes to the west it trends slightly to
the north, so that when it reaches the Weddell Sea it is found
at 65°S.
On the northern side of the East Wind Drift, the
prevalance of the West Wind to the north as far as the subtropical
convergence produces the West Wind Drift flowing from West to
East.
According to recent investigations, this West Wind Drift
flows to a depth of 2000 m, and its flow volume is 150 Sv. To
the south of Australia it is 180'Sv, and to the south of
Africa it is calculated to be 190 Sv (Kort, 1962).
Kort (1962) finds that the West Wind Drift has local
meanders due to the continents and to the shape of the sea
floor.
On corAn}g to a sea peak it is generally deviated to
the left (northwards) and after passing the sea peak it tends
to turn to the right ( southwards ).
A larf^e scale clockwise gyre is forried in the Weddell
Sea, and its extension, as the Weddell Drift, turns to the
north east and flows parallel to the Palmer Peninsula. Its
95
eastern end reaches to about 20°E to 30°E, and greatly controls
the distribution of drift ice in the lower latitude regions of
the Atlantic part of the Antarctic Ocean.
In the subantarctic regions of the ocean, the direction_
of flow of the surface water has a southerly component, but
there is much which is still unknown about its structure. As
already discussed, near the Antarctic Convergence there are
rapid variations of temperature and salinity in the North-south
direction, and apart from these properties the nutrient salts
such as phosphates and silicates also increase rapidly to the
south of the Antarctic convergence, and the Antarctic Ocean
becomes a highly productive region. One cause to which this
is attributed is the upwelling, already mentioned, of Deep
Warm Water into the Antarctic Ocean, and on the north side and
the south side of the Antarctic Convergence, there are rapid
biological changes in the species and the quantities which
are present.
For example the zooplankton Euphausia superba which
has in recent years become the objective of the development of
a maritime resource, occurs only to the south of the Antarctic
Convergence.
litre,
Also, considering the amount of phytoplankton per
Hasle,(1956) found 10 times as many diatoms in the
Antarctic Ocean as in the subantarctic. In this way, the
Antarctic Conver^ence is found to be a region of severe
discontinuity, not only physically and chemically, but also
biolo-lically.