AN ABSTRP.CT OF THE THESIS ...r
Nelson R. Silva-Sandoval
for the denree of
Oceanography
in
Title:
presented on
laster of Science
Aur'ust b, l)77
'JATER MASS STRUCTURE AND CIRCULATION OFF
SOUTHERN CHILE
Redacted for privacy
Abstract
Redacted for privacy
A detailed analysis of the water mass structure and geostrophic
circulation off Southern Chile (L3°
63° S, !)l° t! to the Chilean Cer'st;
(hereafter called study area) was performed.
1.
It was found that:
Potential temperature-salinity relations indicate four d.is-
tinct structure types north of the Polar Front Zone in the study area,
involving seven identifiable water masses.
These water masses arc:
Subantarctic Water (SAAW), Antarctic Water (flAW), Equatorial Subsurface
Water (ESSW), Western Pacific Subsurface Water (WPSSt!) (a new rerional
water mass identified in this study), Antarctic Intermediate Water
(MIW), Pacific Deep Water (PDW), and Antarctic Bottom Water (M3Y).
2.
A scattergram of dissolved oxygen-salinity maximum pairs
identifies two distinct subsurface salinity maxima.
The subsurface
salinity maximum associated with dissolved oxyoen minimum, establishes
the presence of the ESSW as a wedge that penetrates to at least
The other subsurface salinity maximum which is nonassociated
°
S.
'ith dis-
solved oxygen minimum, is traced from at least 162° W to as close to the
Chilean Coast as 800 W at 43° S.
In this study, this water mass is
called the Western Pacific Subsurface Water (WPSSW).
3.
Acceleration potential on the 26.8k sigma-8 surface relative.
to 3000 db, combined with the analysis of 8-S structures distribution
and vertical sections of geostrophic velocity relative to 3000 db, show
that the Peru-Chile Undercurrent extends as far south as48° S.
4.
Significant linear correlation between surface phosphate or
nitrate vs. salinity for t.he study area where the salinity ranges be-
tween 33 - 34
0/
show that the relatively low nutrient content of the
coastal waters (<300 km) mainly results from mixing oceanic waters with
waters of the Archipelago system of Southern Chile.
explains only 60 - 70
Two point mixing
of the nutrient variability; thus, biological
and/or chemical processes may also be important causes of therelatively
low nutrient concentrationsof the coastal waters.
Water Mass Structure and Circulation off Southern Chile
by
Nelson R. Silva-Sandoval
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed August 4, 1977
Comencement June 1978
APPROVED:
Redacted for privacy
Redacted for privacy
in Charge of Major
Redacted for privacy
ting Den o?JSchóo1 of Oceanography
Redacted for privacy
Date thesis is presented
Typed by Cheryl M. Schurg for
August 4, 1977
Nelson R. SilváSandoval
ACKNOWLEDGEMErTS
Heth and care are more appreciated if they are offered in a foreign
country.
Dr. Steve ileshyba was always rich in them, willing to be with
me during my good as well as my bad moments.
my work were cheerfully given to me.
was my home.
There is no doubt that
His advice and comments on
His office was my office, his home
I
am in debt to him forever and
I
want here to give him my acknowledgement "desde el fondo de ml corazon."
Several other professors from the School of Oceanography were always iflflng to help me during my attendance at Oregon State University.
I
want to thank Dr.
!ayne Burt, Dr. Louis Gordon, Dr. Victor Neal, and
Dr. Robert Smith.
I
want my acknowledgement to be extended to my sponsor, Organiza-
tion of American States, without whose help my studies at Oregon State
University would not have become a reality.
I
also want to acknowledge
the Universidad Catolica de 'falparaiso-Chile which allowed me to take a
leave of absence, making possible my attendance at Oregon State University.
Finally,
my thesis.
I
want to thank 1rs. Cheryl Schurg for her care in typing
She was the hero of the last minute rush.
To my wife Rosi
To my son Narcelo and my daughter Erika
To my country "Chfle Lindo"
TABLE OF CONTENTS
Page
I.
II.
INTRODUCTION
1
BACKGROUND
3
2.1
2.2
III.
3.2
Cruises and data
Methods of data analysis
3.2.1
Water masses
3.2.2 Distribution of properties on slgma8 surfaces
3.2.3 Geostrophic analysis
3.2.14
Choice of level of no motion
14
5
6
8
10
10
10
11
11
14.1
General
13
4.2
4.3
Subantarctic Water
Antarctic Water
Equatorial Subsurface Water
24
14.14
14.6
Antarctic Intermediate Water
13
214
33
36
WATER MASS STRUCTURE
1i7
General considerations
5.2 Water structures present in the rejion
47
5.1
VI.
14
DESCRIPTION OF WATER MASSES IN THE STUDY AREA
15 Western Pacific Subsurface Water
V.
3
DATA AND METHODS
3.1
IV.
Water masses and water mass struc ture
Geost rophic circulation
2.2.1
West Wind Drift
Division at the Chilean
Coast
2.2,2 Peru-Chile Undercurrent
2.2.3
Flow at intermediate, deep and bottom levels
147
GEOSTROPHIC CIRCULATION
6.1
6.2
6.3
6,4
General
Flow on
Flow on
Flow on
considerations
the sea surface
the 26.814 sigma-8 surface
the 27.12 sigma-e surface
56
56
58
61
TABLE OF CONTEFITS (Continued)
Page
65 Flow In vertical sections
The coastal flow
Identification of the subsurface coastal flow
with the Peru-Chile Undercurrent
6.5.3 Geostrophic flow south of 48° S
Geostrophic circulation and water mass structure
6.5.1
6.5.2
6.6
VII.
CONCLUDING REI1ARKS
BIBLIOGRAPHY
63
63
70
73
74
78
80
LIST OF TABLES
Table
Page
Water mass constituents of e-s structures present
in the area north of the Polar Front Zone.
LIST OF FIGURES
Page
Figure
The study area, showing locations of SCORPIO (s)
1967 and PIQUERO (P) 1969 oceanographic stations.
Letters Pt - F define vertical sections.
2
3
4
5
6
7
8
e-s diagrams of representative stations showing
the different water masses present. In the study
Depth of the base of seasonal thermocline
area.
is indicated by stars.
10
15
Salinity distribution on the sea surface, from the
SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
17
Dissolved oxygen distribution on the sea surface,
from the SCORPIO (A) 1967 and PIQUERO (0) 1969 Expeditions.
18
Phosphate distribution on the sea surface, from
the SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
19
flitrate distribution on the sea surface, from the
SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
20
Regression of surface phosphate or nitrate with
salinity for surface waters of salinity range
34°/, from the SCORPIO 1967 andPIQUERO 1969
Regression equation and correlation
Expeditions.
coefficient r2 are given.
22
Silicate distribution on the sea surface, from the
SCORPIO (A) 1967 and P1QUERO (c,) 1969 Expeditions.
23
Bathymetry in meters of the 26.84 sigma-8 surface,
from SCORPIO (A) 1967 and PIQUERO (°) 1969 Expeditions.
11
12
14
Temperature distribution on the sea surface, from
the SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions; PFZ marks the Polar Front Zone.
33
9
9
Potential temperature distribution on the 26.84
sigma-8 surface, from SCORPIO (A) 1967 and PIQUERO
(o) 1969 Expeditions.
25
26
Salinity dIstribution on the 26.814 sigma-8 surface,
from SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
28
LIST OF FIGURES (Continued)
Figure
13
14
Page
Dissolved oxygen distribution on the 26.81i sigma-B
surface, from SCORPIO (A) 1967 and PIQUERO (°) 1969
Expeditions,
Phosphate distribution on the 26.84 sigma-B surface,
from SCORPIO (A) 1967 and PIQUERO (°) 1969 Expeditions.
15
17
18
19
20
21
30
Nitrate distribution on the 26.84 sigma-B surface,
from SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
16
29
31
Silicate distribution on the 26.84 sigma-B surface,
from SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions,
32
Dissolved oxygen vs. subsurface salinity maximum
showing the separate groups of O2_Smax pairs corresponding to two different water masses, from
stations north of 55° S of the SCORPIO (5) 1967
and PIQUERO (P) 1969 Expeditions.
34
Superposition of 9-S diagrams along 43° S from
169° W to the Chilean Coast from selected stations..
of SCORPIO 1967, Stars indicate the depth of the
base of the seasonal thermocline.
35
Bathymetry in meters of the 27.12 sigma-B surface,
from the SCORPIO (A) 1967 and PIQUERO (e) 1969
Expeditions.
37
Potential temperature distribution on the 27.12
sigma-B surface, from the SCORPIO (A) 1967 and
PIQUERO (o) 1969 Expeditions.
38
Salinity distribution on the 27.12 sigma-B surface, from the SCORPIO (A) 1967 and PIQUERO (0)
1969 Expeditions.
39
22
Dissolved oxygen distribution on the 27.12 sigma-B
surface, from the SCORPIO (A) 1967 and PIQUERO ()
1969 Expeditions.
23
Phosphate distribution on the 27.12 sigma-B surface, from the SCORPIO (A) 1967 and PIQUERO (°)
1969 Expeditions.
42
LIST OF FIGURES (Continued)
Figure
Page
24
Nitrate distribution on the 27.12 sigma-8 surface,
from the SCORPIO (A) 1967 and PIQUERO () 1969
Expeditions,
25
Silicate distribution on the 27.12 sigma-8 surface,
from the SCORPIO (A) 1967 and PIQUERO (°) 1969 Expeditions.
26
27
28
29
30
31
32
33
34
Representative stations showing StructureType
44
I
from the SCORPIO (s) 1967 and PIQUERO (P) 1969
Expeditions.
Stars indicate depths of the base of
the seasonal thermocline.
47
Representative stations showing Structure Type II
from the SCORPIO (s) 1967 Expedition.
Stars indicate depth of the base of the seasonal thermocline.
48
Representative stations showing Structure Type III
from the PIQUERO (P) 1969 Expedition.
Stars indicate depth of the base of the seasonal thermocline.
49
Representative stations showing Structure Type IV
from the PIQUERO (P) 1969 Expedition. Stars mdi'cate depth of the base of the seasonal thermocline.
50
Distribution of 8-S Structure Types north of the
Polar Front Zone for the SCORPIO 1967 and PIQUERO
1969 stations in the study area.
51
0-S diagram of the SCORPIO station 5-72 showing the
presence of two weak subsurface salinitymaxima
at 251 and 399 m.
The star marks the base of the
seasonal thermocline.
55
DynamIc topography of the sea surface relative to
3000 db, from SCORPIO (A) 1967 and PIQUERO (o)
1969 Expeditions,
57
Acceleration potential on the 26.84 sigma-8 surface relative to 3000 db, from SCORPIO (A) 1967 and
PIQUERO (o) 1969 Expeditions.
60
Acceleration potential on the 27.12 sigma-B surface
relative to 3000 db, from SCORPIO (A) 1967 and
PIQUERO (a) 1969 Expeditions.
62
LIST OF FIGURES (Continued)
Figure
35
Page
Geostrophic velocity relative to 3000 db at section
Positive values
A, from SCORPIO 1967 Expedition.
indicate northward flow; negative values indicate
Solid horizontal lines represent
southward flow.
the potential density at which the cores of ESSW
(26.84), WPSSW (27.06) and AAIW (27.12) are found;
dashed line is the base of the seasonal thermocline
(BsT),
36
37
38
39
40
61+
Geostrophic velocity relative to 3000 db at section
B, from PIQUE.RO 1969 Expedition. Positive values
indicat.e northward flow; negative values indicate
Solid horizontal lines represent
southward flow.
the potential density at which the cores of ESSW
(26.84) and AAIW (27.12) are found; dashed line is
the base of the seasonal thermocline (BST).
65
Geostrophic velocity relative to 3000 db at section
C, from PIQUERO 1969 Expedition. Positive values
indicate northward flow; negative values indicate
Solid horizontal lines represent the
southward flow.
potential density at which the core of AAIt4 (27.12)
is found; dashed line is the base of the seasonal
thermocline (BST).
66
Geostrophic velocity relative to 3000 db at section
D, from PIQUERO 1969 Expedition. Positive values
indicate northward flow; negative values indicate
Solid horizontal lines represent
southward flow,
the potential density at which the cores of ESSW
(26,81+) and AAId (27.12) are found; dashed line is
the base of the seasonal therrnocline (BST).
67
Geostrophic velocity relative to 3000 db at section
E, from PIQUERO 1969 ExpeditIon. Positive values
indicate eastward flow; negative values indicate
Solid horizontal lines represent the
westward flow.
potential density at which the cores of ESSW (26.84)
and AAIW (27.12) are found; dashed line is the base
of the seasonal thermocline (BST).
68
Geostrophic velocity relative to 3000 db at section
Positive values
F, from PIQUERO 1969 Expedition.
indicate eastward flow; negative values indicate
westward flow. Solid horizontal lines represent the
potential density at which the cores of ESSW (26.81+)
and AAIW (27.12) are found; dashed line is the base
of the seasonal thermocline (BST).
69
LIST OF FIGURES (Continued)
Figure
41
Page
Composite of oceanographic sections showing approximate positions and geostrophic velocities of the core
of the Peru-Chile Undercurrent based on STEP
1960
(Wooster and Gilmartin, 1961), MARCHILE VIII 1972
(Sievers and Silva, 1975), SCORPIO 1967 (this study)
and PIQUERO 1969 (this study) Expeditions. Respective reference levels are given in the text.
71
Geostrophic velocity profile relative to 2500 db for
PIQUERO 1969 stations pairs P-149
P50 and P-50
P-51.
Positive values indicate northward flow, negative values indicate southward flow.
72
8-S dia,ram of station P-19. The segment between 714
and 1535 m resembles a structure type IV, while the
whole structure corresponds to structure type III.
Star marks the base of the seasonal thermocline.
76
I
42
43,
WATER MASS STRUCTURE AND CIRCULATION OFF SOUTHERN CHILE
I.
INTRODUCTION
The details of water mass structure anc! circulation of the southernmost extreme of the Eastern Pacific Ocean are poorly known.
This area,
off Chile, between 350 S and 550 S and east of 900 W, has been included
in several studies of circulation and water masses (Gunther, 1936; Reid,
1961; Reid and Arthur, 1975; Warren, 1970, and Wyrtki, 1975).
Only the
gross characteristics of the regional waters were described because of
the large scale geographic ranges of these papers.
More specifically,
little attention has been paid to a unique feature of this region, namely, that it is the only southern hemisphere coastline where the West
Wind Drift (WWD) reaches a solid continental barrier.
There is a clear
need for improvement of our knowledge of the region.
If an inventory of oceanographic expeditions in the area Is performed, a striking reality becomes apparent:
even though several cruises
have sampled the region there is a great variety in depth of maximum
sampling, variables measured and degree of synopticity.
Only two major
oceanographic cruises have produced a good coverage of the area in the
sense of a dense sampling grid.
These are the PIQUERO Leg III 1969
(S.I.O., 197k) and the R/V Professor DERYUGIN 1968 (Mamayev, 1973).
Neither of these cover the total area off southern Chile, and only
PIQUERO Leg III has stations closer than 200 nautical miles to the
coast and complete water column sampling.
A composite data base of
these PIQUERO data plus the 1967 SCORPIO Expedition (s.i.O., 1969)
covering from 430
to 63° S and from the coast to 91° W, hereafter called
the study area, form the basis for the analysis and results presented in
this study.
The present study examines the water mass structure present in the
water column off southern Chile.
It also analyzes the horizontal dis-
tribution of physical and chemical characteristics of the Subantarctic
Water, Antarctic Water, Equatorial Subsurface Water, Western Pacific
Subsurface Water, and Antarctic Intermediate Water.
The Western Pacific
Subsurface Water is presented as a water mass carried eastward into the
area of study by advective processes.
This study also treats the geostrophic velocity patterns of this
extreme Southeastern Pacific area on vertical and horizontal sections.
The southernmost extension of the Peru-Chile Undercurrent is established
from a chart of acceleration potential on the 26.84 isopycnal, combined
with the analysis of the 8-S structure distribution and vertical sections of geostrophic velocity relative to 3000 db,
3
I I,
2.1
BACKGROUND
Water masses and water mass structure
The water masses present in the study area have been described in
general by authors such as Sverdrup etaL (1942) and Defant (1961).
A
more detailed study of the distribution, of the water mass characteristics
in. the Southern Hemisphere has been done by Taft (1963).
He des-
cribed the distribution of properties using isentropic analysis on the
isanosteres of 125, 100, 80 and 60 cl/ton.
Detailed studies of water
mass distributions and their characteristics off the Chilean Coast have
been prepared by Gunther (1936), Wyrtki (1966; 1967), Brandhorst (1971),
Robles etal. (1974), Sievers and Silva (1975), and Silva and Konow
(1975).
Nevertheless, none of these papers covered the area south of
43° S.
Reid (1965) performed an analysis of the Antarctic Intermediate
Water distribution in the Pacific Ocean.
He also made use of isentropic
analysis showing the intermediate water mass distribution and its geostrophic flow on the 80 cl/ton isanostere.
The distribution of properties of the Pacific Deep Water (Common
Water)
(1972).
in the Southern Hemisphere south of 40° S was studied by Callahan
He also used isentropic analysis, selecting for this purpose
the isanosteres of 50 and 30 cl/ton.
A detailed analysis of the vertical water mass structure in the
Eastern South Pacific was done by Mamayev (1973).
He discussed the dif-
ferent i-S structures present in the area and elaborated a rather complex
classification.
Mamayev (1973) described two i-S structure types, one
subtype and two transient types for the area between 30°
53° S and
4
800 W - 109° W.
Based on his classification system, he presented a 1-S
structure distribution chart.
2.2
Geostrophic circulation
The geostrophic circulation of the southern part of the Eastern
South Pacific Ocean (south of 350 s) has not been described in as much
detail as has been done off Peru (Wooster and Gilmartin, 1961; Wyrtkl,
1963; and Zuta and Guillen, 1970), or off northern Chile (Brandhorst,
1971; Robles etal., 1974; and Sievers and Silva, 1975).
Nevertheless,
this area has been included in several general circulation studies, for
instance, by Reid (1961, 1965, 1967b), Wooster and Reid (1963), Warren
(1970), Reid arid Arthur (1975), and Wyrtki (1975).
West Wind Drift - Division at the Chilean Coast
2.2.1
One of the most outstanding features in the circulation pattern in
this area, is the bifurcation of the WWD off southern Chile.
The fact
that the WWD impinges the South American Continent and divides into a
northward and southward branch has been known for a long time,
Perez-
Ros.ales is cited by Gunther (1936) to have reported in 1857 some thermal
observations relative to the characteristics of the Cape Horn Current
and the Peru Coastal Current at their commencement off the coast of
Chile,
Gunther (1936) also cites the textbooks of Hoffmann in 1884 and
Somerville In 1923 as describing the WWD division at
4Q0
branches in "the Cape Horn Current flowing to the south
S into two
the Peru or
Humboldt Current to the north."
Charts of Pacific Ocean Currents based on ship drift including the
area where the WWD reaches the South American Continent have been pre-
5
sented by authors such as Sverdrup etal. (19142) and Defant (1961).
These charts present the WWD approaching the coast from the t4-SW and impinging at about 15° S.
The geostrophic circulation of the sea surface
relative to 1000 db prepared by Reid (1961), shows in a broad way the
WWD impingement at
430
of 9Q0 W) from the W-NW.
S but with a final approach to the coast (east
Wyrtki's (1975) sea surface mean annual dyna-
mic topography relative to 1000 db showed a final W-NW flow direction in
a fashion similar to that of Reid's (1961).
Sandoval (1971) has pre-
pared a chart of the sea surface dynamic topography relative to 500 db
for the Eastern Pacific Ocean (00
be observed also.
470 s) where this W-NW direction can
Nevertheless, some geostrophic velocities inferred
from his chart are doubtful (for example 168 cm/sec at 1470 S and 78° w).
Silva and Neshyba (1977) prepared a sea surface dynamic chart relative
to 500 db using available austral summer data in the region 35° - 56° S,
east of 91° W.
They found that the W1IID has a final approach from the
W-NW reaching the coast off Chiloe Island N 1420 s).
They also describe
an apparent correlation between the dynamic flow and the bathymetry of
the Chile Ridge.
2,2,2
Peru-Chile Undercurrent
The presence of a subsurface poleward flow close to the Chilean
Coast-was first described by Gunther (1936).
He based his description
on the temperature, salinity and oxygen distributions.
Wooster and
Gilmartin (1961) further describe the poleward subsurface flow between
6° S and 2140 S using both vertica.l sections of geostrophic flow relative
to 1000 db and a set of direct measurements by drogues.
They further
proposed that this flow, which they called the Peru-Chile Undercurrent,
should extend at least as far as 41° S, using as a basis a set of north
south (along coast) vertical sections of salinity and oxygen.
A dynamic topography chart on 200/1000 db was prepared by Wooster
and Reid (1963) to analyze the poleward currents in the Eastern Pacific
This chart shows the extension of a poleward subsurface flow
Ocean.
along the Chilean Coast as far south as 40° S.
Brandhorst (1971), using
isentropic analysis on the 150 and 140 cl/ton isanosteres, described the
extension of the Peru-Chile Undercurrent as far as 42° S.
He based his
results on the distribution of properties of the Equatorial Subsurface
Water, rather than geostrophic computations.
Vertical sections of geostrophic velocity relative to 1000 db at
24° S and 28° S off the coast of Chile prepared by Sievers and Silva
(1975), also show the Peru-Chile Undercurrent.
and
5
Flow magnitudes were 10
cm/sec, respectively, which are compatible with those of Wooster
and Gilmartin (1961) off Peru.
2.2.3
Flow of intermediate, deep, and bottom levels
Dynamic topography charts at intermediate levels have also been
prepared by Reid (1965) and by Johnson (1972).
The first author des-
cribed the Antarctic Intermediate Water flow along the 80 cl/ton
isanostere (sigma-t
27.29), while the second showed this. flow along
the 27.1 isopycnal.
Johnson (1972) showed a general northward flow of
the Antarctic Intermediate Water along the Chilean Coast.
In Reid's
(1965) chart, the situation is not very well defined due to the scarcity of data.
Relative to the deep water circulation, Callahan (1972) proposed a
deep southward flow along the coast of Chile which carries oxygen poor
7
waters to the Antarctic.
This low oxygen water joints the Antarctic
Circumpolar Current west of the Drake Passage, flowing through it to
the South Atlantic.
The dynamic topography maps at 2000/3000 db and
2500/3500 db prepared by Reid and Arthur (1975) are in agreement with
Callahan's (1972) proposed flow.
The bottom circulation in the Bellinghausen Basin was described by
Gordon (1966) based on potential temperature, salinity and oxygen distributions,
He pointed out that the flow velocity in the basin is
probably moderate in the western (east of East Pacific Ridge) and
northern (south of Chile Ridge) sections and weak in its eastern branch
(along Southern Chi1e)
I I.
3.1
DATA AND METHODS
Cruises and data
The Leg III of PIQUERO Expedition took place off the southern coast
of Chile during January 13 to February 16, 1969.
The cruise occupied
51 oceanographic stations including the development of seven current
meter arrays in the Drake Passage.
Only 43 of these stations were con-
sidered in the present study; 8 coastal stations from 40 to
omitted because. they lacked nutrient data.
550
S were
The 43 PIQUERO stations
selected covered the area between 1+5 to 63° S, 91° W to the coast of
Chile and they are shown in Figure 1.
The current meter data have been
analyzed by Reid and NowlIn (1971), and part of their results have been
included in the interpretation of the geostrophic velocity profiles.
Eleven of the easternmost stations of the section along /+30 S
(deeper than 2000 m) of the SCORPIO Expedition (April 30 to May 8, 1967),
were added to the PIQUERO Leg III statIon grid to obtain a better
coverage to the north (Figure 1).
This composite permits construction.
of smooth connections between the hydrosections of the two cruises when
contours of the different variables are traced, even though the cruises
are separated in time by two years and were taken in different seasons
(austral summer for PIQUERO and austral autumn for SCORPIO).
Additional
SCORPIO stations toward the west along 13° S at 93°, lO14, 116°, 127°, 139°,
150°, 162° and 169° W, were used in the analysis of the Western Pacific
Subsurface Water Mass.
The basic data from both cruises consist in observations of temperature, salinity, dissolved oxygen, and nutrients.
Descriptions of
the methods and data have been presented in dat.a reports (S.I.O., 1969
900
850
80°
70°
75°
65°W
4 0°S
A
A
SCORPIO
f31
4
-S-66
S-68
S-70
j 0 PIQUERO
5-73 S-76
450
000
p-SI P-49
S
0
000
0
0
P
P-L5
P-41
500
0'
C
:°
0
550
0 P35
cc'
p37
0 °P-30
P-29
p39
D
0
P-28
oP-18
P-27
P-26
p-g
E
P-25
op-j
°P-20
OP-14
op-21
0
60°
oP16
F
op-22
op-J3
P-U
op-23
P-lb
P-Jo
.
P-9°
1
Figure 1.
_____
iii.
The study area, showing locations of SCORPIO (s) 1967
and PIQUERO (P) 1969 oceanographic stations.
Letters
A - F define vertical sections.
10
for SCORPIO and S.I.0., 1971f for PIQUERO),
To analyze relationships between horizontal surface mixing and
nutrient distribution in coastal. water off Chile, additional data were
incorporated from the HUDSON 70 Expedition.
These data consist mainly
in observaflons of tel nperature, salinity, and dissolved oxygen in the
Chilean Archipelago.
Some HUDSON 70 oceanographic stations did have
additional phosphate, nitrate, and silicate observations (CSS HUDSON,
1970).
3.2
Methods of data analysis
Water masses
3.2.1
Water masses were identified using diagrams of potential temperature (0) versus salinity (s)
(Helland-Hansen, 1916).
The movement or
distribution of water masses was studied using Wust's (1935) core
method.
An additional technique employed to dlsriminate between water
masses used oxygen (02) versus S
values of 0
values at which they occurred.
with the S
2
plots to correlate the specific
It will be
max
shown later this technique provides the basis for identification of the
Western Pacific Subsurface Water (wpssw)
in the study area.
Linear re-
gressions of phosphate and nitrate versus salinity were used to examine
relationshIps between horizontal surface mixing and nutrient distribut i on.
3.2.2
Distribution of properties on sigma8 surfaces
The distribution of water mass properties along surfaces of con-
stant potential density is an established method of analyzing horizon-
tal advection and mixing (Montaomery, 1937; and Sverdrupetal., 1942),
The selection of specific isopycnal surfaces for this purpose involved
calculating the averages of the density values associated with salinity
maxima or minima, for the PIQUERO and SCORPIO stations within the study
A mean sigma-O value of 26.8
area,
was obtained for Equatorial Sub-
surface Water (ESSW), whfle 27.12 defined Antarctic Intermediate Water
(AAIW).
Geostrophic analysis
3.2.3
Geostrophic flow patterns at intermediate levels were inferred
from distribution of acceleration potential along the Isentropic surfaces described above.
On the sea surface, where a specific isopycnal
cannot be defined, the horizontal flow pattern was inferred from sea
surface dynamic topography computed relative to 3000 db.
Vertical sec-
tions of geostrophic velocity relative to 3000 db were prepared to complement the information given by horizontal charts of dynamic topography
and acceleration potential.
Choice of level of no motion
3.2.4
The choice of the level of 3000 db level as a no motion for the
computation of the geostrophic velocities is a key decision in this
method,
In strongly stratified waters, the level of no motion is found
at a shallower depth than in more nearly stratified waters (Neumann,
1968),
In high latitudes, such as found around the Antarctic Continent,
the weakly stratified waters will show a very deep level of no motion.
Gordon (1966), for example, indicated that a level of no motion does not
exist for zonal flow in the northern and western Drake Passage.
He also
points out that the level of no motion in the Drake Passage slopes
12
steeply down toward the north, reaching the bottom about half way across
the passage.
Cabrera-Muro (1977), using the method of Defant (1961),
did not find a level of no motion at high latitudes in the South Pacific
due to the lack of stratification.
Johnson (1972) selected a depth of 2500 m as a level of no motion
based on:
"the Pacific Deep Water (PDW) considered to be moving gener-
ally south, and the AAIW moving north, would have a point of flow re-
versal (or level of no motion) between them"
Reid (1961, 1965, 1973b),
and Reid and Arthur (1975), have used 1000, 2000, 3000, 3500, or 4000
db, as a reference level depending upon the actual value of the feature
which they are discussing.
Computations of the level of no motion at different latitudes
using PIQUERO-SCORPIG data, also leadto similar conclusions to those
of Gordon's (1966) and Cabrera-Muro's (1977).
South of 550 S, Defant's
(1961) method did not indicate the presence of a level of no motion in
the water column.
Since the study area includes stations where a level
of no motion does not exist, or at least can not be defined with Defant's
method, an arbitrary depth of 3000 m was selected as a reference level
for dynamic computations.
The choice of 3000 db was based on:
one reference level satisfies the total area Included;
(1)
No
(2) Flow at great-
er depths is usually considered to be weaker than at shallower depths;
(3) 3000 db is the deepest common standard depth for most of the stations
used here; and (4)
It is desirable to compare these results with those
of Reid and Nowlin (1961) from the PIflUERO data across the Drake Passage
Stat ions P-9, 16, 31, 50, 51 and S-69 were not used in this analysis
since depths were less than 3000 m.
13
DESCRIPTION OF WATER MASSES IN THE STUDY AREA
Iv,
,+.1
General
An analysis of all the B-S diagrams for the study area reveals the
presence of seven distinct water masses at depths below the seasonal
thermocline.
These water masses are shown in Figure 2.
They are the
Subantarctic Water (SAAW) , Antarctic Water (AAw) , Equatorial Subsurface
Water (ESSW), Western Pacific Subsurface Water (WPSSW), Antarctic Intermediate Water (AAIW), Pacific Deep Water (PDW), and Antarctic Bottom
Water (AABW).
Descriptions of the temperature, salinity, dissolved oxygen and
nutrient contents of these surface, subsurface, and intermediate waters
The deep and bottom water masses, PDW and AABW,
are given below.
rexpectively, are not described in detail here because the characteris-
tics of these deeper waters do not change appreciably over the study
area and they already have been described by Gordon
Lynn. (1971), and Callahan. (1972).
(1966), Reid and
The description of the Subantarctic
Water includes a discussion of the influence of estuarine waters of the
Chilean Coast.
The Western Pacific Subsurface Water i.s discussed in
detail because this is a new water mass. designation for this region
which has resulted from the water mass structure analysis presented in
this study.
1,2
Subantarctic Water
The SAAW occupies the surface layer in the whole area of study,
north of the Polar Front Zone.
tribution (Figure
As a surface water, its temperature dis-
is dominated by the influence of seasonal varia-
14
15°
S-76
S-66
P-24
SAAW
lro
26.0
ESSW
2
o
o
.
cD
SAAWL
1
5°
8
*
.2
w.Pssw
AAIW
sPDW
AA\'
8
J
PAAB.W
I
I.
34.5 %0
34.00/oo
coI
Ioo
Figure 2.
8-s diagrams of reprseritative stations showing the different water masses present in the study area.
Depth of
the base of seasonal thermocline is indicated by stars.
15
850
90°
800
750
65°W
700
40°
A \A A
A
- 13
4 5C
-
13
12
A AA
\..
8
S 5
0
T
I
0
0
0
PFZ(3-2
6 Q(
T"C
Oj
1-I
0
0
Figure 3.
Temperature distribution on the sea surface, from the
SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions;
PFZ marks the Polar Front Zone.
16
tions of solar heating.
Since the data was taken during summer and
early fall, a seasonal therrnocline was present in the thermal structure.
Close to the Chilean coast, relatively low surface salinities are
present (Figure 1+).
than 31L0
Reid (1969) showed surface waters of salinity less
extended as far as 1000 W at i0° S.
Figure 4 shows that t.he
lowest values, less than 33.69, are located along the coast; these apparently result from coastal runoff.
Pickard (1971, 1973), using data
from the HUDSON 70 Expedition, demonstrated increasing salinity trends
from fresher inside waters of the Chilean fjord region to the oceanic
waters offshore.
Since the SAAW is in the surface layer, its dissolved oxygen is
high (Figure 5).
The dissolved oxygen increases towards the Antarctic
as a result of the increase in oxygen solubility with decreasing tem-
perature, hence the direct correlation between the patterns of isotherm
and iso-oxygen contours.
Surface phosphate and nitrate values decrease toward the coast,
with the lowest values off the Chonos Archipelago ('
and 7).
1450 s)
(Figures 6
These low values might be explanable as. depletion of phoshate
and nitrate from surface waters by phytoplankton and/or mixing of
oceanic waters with comparatively lower nutrient estuarine waters..
The
first explanation implies that the lowering of phosphate and nitrate
would be independent of salinity.
The second, on the other hand, would
require a covariation between salinity and nutrient content consistent
with a two point mixing process.
Phosphate and nitrate observations in
some of the Chilean fjords and adjacent coastal waters were also made
by the HUDSON 70 E.xpediton (CSS HUDSON, 1970).
Even though the surface
17
750
800
85°
65°W
700
400 S
:.
-
t
t
t
i
b
/A33.6
450
33.9
g
33.06
V..
33.7
500
311
550
0
>34.1
\
0
0
600
33.9-___
,9%o
0
-
>33.9g.
0
L I
Figure
i
.
II
.1
I
I III It
1 I#i(
Salinity distribution on the sea surface, from the
SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
18
900
800
85°
7
7S
65°W
40 OC
6.0
6.0
6.0
/Jl
:
45° :46.t
000.
p
000
0
0
0
_---6.S'
5Q0
0 0
-
0
0
0
0
550
00
0
:
:
60°
8.0.
0
I
I
Figure 5.
i
t
.
I
t i
i
t
I
t
i
i
i
I
i...i
t
I
I
t
t
c
Dissolved oxygen distribution on the sea surface, from
the SCORPIO (A) 1967 and HQUERO (o) 1969 Expeditions.
19
no
qio
f0
B5°W
4005
(
-
)
/1
L5° -
-
,/o10;01
500
ff
0
((.°
\''
0
550
0
-
-
-
0
-
0
0
0
0
[!ii
POpmoI/!
1.8
1
Figure 6.
I
I
I
I
LI Ii i
I
I
I
t ILc
Phosphate distribution on the sea surface, from the
SCORPIO (a). 1967 and PIQUERO (o) 1969 ExpeditionS.
20
750
800
85°
700
65°W
4 0°S
I
A
A
6423
A
450
8 4\
;;'
@1
550
0
N
- - - -
-
60°
NO3pmo///'
222L
ititi
Figure 7.
Nitrate distribution on the sea surface, from the
SCORP0 () 1967 and PIQUERO (°) 1969 Expeditions.
21
data is highly variable (
pmol/l
0-1
1.3 iimol/I
in phosphate,
0 - 14
in nitrate), comparatively Tower phosphate and nitrate contents
are the general trend observed in the estuarine region relative to the
adjacent coastal waters.
Plots of surface phosphate and nitrate vs.
salinity show a significant direct linear correlation at the 99
confi-
dence level (Figure 8), for the study area where the salinity ranges
between 33
3'+°/
(Figure 4)
.
This dependency supports the second
explanation, that is, the low phosphate and nitrate values close to the
coast seem to be caused principally by a mixing process between oceanic
and estuarine waters.
Because of the r2 values (Figure 8), only 7l
of
the phosphate and 65? of the nitrate variability can be explained by
means of the respective linear regressions.
Therefore, the biological
and/or chemical processes could also account for an important fraction
of the observed nutrient variability.
The surface silicate content of the SAM in the study area is zero
with some exception where the silicate reaches 1-2 pmol/1 (Figure 9).
Surface silicate measured by the HUDSON 70 Expedition in the fjord area
shows highly variable silicate content (z
1
113 pmolIl).
Neverthe-
less, it generally shows relatively high values at the heads of the
fjords with decreasing values toward the ocean.
The above discussion of coastal surface nutrients is general be-
cause the data are Hmited.
More work should be done on these coastal
waters to answer some questions such as:
How important are the phos-
phate and nitrate depletions by primary producers in this area?
How is
the high silicate estuarine water, generally present at the head of the
estuary, depleted before it reaches the ocean?
Biologically?
Geo-
15
NO-4 57.3+13.8 S o/
PO =-25.6+O.78S°/.o
r2 0.65
J r2O.71
.7
.
.
1.0
iii:
1
-a
0
E
Icy,
III
0
z
0
..
5
0.5
1
1
33.0
33.5
eol
.? 100
Figure 8.
34.0
33.0
33.5
34.0
eo/
.
/00
Regression of surface phosphate or nitrate with salinity for surface waters
of salinity range 33 -. 34°/oc,, from t.he SCORPIO 1967 and NQUERO 1969 ExRegression equation and correlation coefficient r2 are given.
peditions.
NJ
N.J
23
00
85°
700
75°
80°
65°W
400 S
>0>O
>0
A
A
A
A\yA A13
0
450
000
0
0 0
0
: .
50°
5",,
0
00
550
0
-4'
0
0
0
0
0
0
0
0
0
00
600
0
5 .._.io
0
I
I..'
2525
0
25
<25 0
25
F SiQyrno///
I
Figure 9.
I
I
t
t
Silicate distribution on the sea surface, from the
SCORPIO (A) 196] and PQUERO (o) 1969 Expeditions.
24
chemically?
Both?
L3 Antarctic Water
Because of the Polar Front Zone location and the station distribu-
tion, only five stations are situated within the Antarctic zone (P-9
to P-13).
The vertical temperature structure of the MW is character-
ized by a temperature minimum localized at 100 - 150 m, which reaches
its lowest value of -1.13° C at station P-JO.
The Antarctic surface
salinity is lower than that of the Subantarctic zone (Figure If), which
is probably the result of ice melting during the summer (Gordon etal.,
1977).
Due to the low temperatures (Figure 3), the oxygen solubility
in this area is higher than that in the Subantarctic.
This results in
an increase in dissolved oxygen in the MW (Figure 5).
The phosphate,
nitrate and silicate content in the AAW are relatively high.
values above 1.4 prnolfl
Surface
in phosphate, 18 pmol/1 in nitrate and 15
mo1/l
in silicate are observed (Figures 6 to 8).
4I4
Equatorial Subsurface Water
Within the study area, the core of the ESSW is characterized by a
salinity maximum, an oxygen minimum, a nutrient maximum and a sigma-e of
26.84.
The sigma-e surface is deeper along the coast and in the north-
ern portion of the study area (Figure 10).
It slopes gently upward to-
ward the South, intersecting the sea surface at about 58° S.
The potential temperature on the 26.84 isopycnaI varies from 60 C
to 8°C, with its maximum values off Chiloe Island
(
43°
s)
(Figure 11).
Temperature inversions of the order of 0.1° C above the ESSW salinity
maximum are present at stations P-48 and P-I9; similar inversions have
25
850
750
80°
700
65°W
1. O°S
A
A
A
A A A\A 44
'-% 250
450
0
0
500
N
200
00
0
o
o
't
I
150
I
'
I
)
'.
0
0
100 \
0
0
0
0
55°
$50
00
600
0
Figure 10.
Bathyrnetry in meters of the 26.8 sigma-9 surface,
from SCORPIO (A) 1967 and PIQUERO (o) 1969 Expeditions.
26
900
800
85°
7Q0
75
4 QOC
>6.5
450
\000
-
6.5
0
0
0
0
500
8.OR
a
A
7.0
o
0
0
.1
-..
1.v
o
'
<6.5
0
':
'
ef-
0
.._o0
0
550
0
,o
\
>6.5
'0'
:
f
0
v0
..
0
0
0
00
1
0
eec
Figure H.
Potential temperature distribution on the 26.8k
sigma-9 surface, from SCORPIO (A) '1967 and PIQUERO
(o) 1969 Expeditions.
2.7
been described for the ESSW north of 34° S by Silva and Sievers (1974)
an.d Sievers and Silva (1975).
The description of "warm water" as a
characteristic of the ESSW was first given by Gunther (1936).
The salinity distribution on the 26.84 isopycnal shows a maximum
in the NE portion of the region (Figure 12).
The salinity decreases to-
wards the south due to mixing with the less saline SAAW and AAIW.
of 50° S the ESS
South
salinity maximum can not be identified.
The oxygen content shows a distribution similar to that of the
salinity and temperature but exhibits a minimum instead of a maximum
(Figure 13)
.
The lowest values appear near the coast and in the north-
ern portion of the study area.
Values as low as 1.35 ml/1 were present
at 223 m (sigma-e = 26.78) at station S-76.
The low oxygen content
(<5.0 mill) persist to 52° S; this is a little farther south than the
saHnity maximum.
The phosphate, nitrate and silicate Content of the ESSW show relatively high values close to the coast and towards the north (Figures 14
to 16).
Silva and Konow (1975) have shown that the ESSW nutrient con-
tent off the coast of Chile is characterized by a maximum only south of
25° S.
This maximum content in the vertical structure originates be-
cause of the comparatively lower nutrient values of the 5MW above and
the AAIW underneath the ESSW.
25 pmol/l
Values of 1,8 pmol/l in phosphate and
in nitrate in the maximum persist as far as 52° S.
The low
oxygen content, high salinity and nutrients are remnant features of the
highly oxygen depleted and nutrient enriched waters brought to the south
by the poleward Peru-Chile Undercurrent from the eastern tropical areas.
28
00
850
80°
75°
700
65°W
LU °S
A
A
A
A
U
& t A\A A
450
36.2
00
o
0
000
500
362°
550
<34.1
0
60°
Figure 12.
Salinity distribution on the 26.81-i.sigma-O surface,
from SCORPIO (A) 1967 and PIQUERO (°) 1969
Expeditions.
29
850
7Q0
750
800
65°W
£0°S
A A!%A A6
-
.-... 5.5_'.
450
lJt
r
I' \
5Q0
-
0%:::\
0
0
550
0
\
0
>6$
0
0
[eJJ
02m1/l
Figure 13.
Dissolved oxygen distribution on the 26.84 sigmaO
surface, from SCORPIO (A) 1967 and PIQ.UERO (0) 1969
Expeditions
-
30
85°
800
75°
700
65°W
4 0°S
450
5Q0
1.6
°
0
55°
0\
1'
600
PO4pmo//I
Hgure 1-i.
Phosphate distribution on the 26.8k sigrna-E3 surface,
from SCORHO (A)
Expeditions.
967 and PQUERO () 1969
31
10°
85°
80°
75°
700
65°W
4 0°S
I
450
50°
°
:To:
55°
600
NOJJmoI/I
Figure 15.
Nitrate distribution on the 26.84 sigma-8 surface,
from SCORPIO (A) 1967 and PIQUERO () 1969
Expedluons.
32
800
85°
75°
70°
65° W.
L0°S
203
L5°
010
\
"
I
15
10
0
E.]
500
5'
0
550
0.,
0
0
0
0
0
0
0
0
0
60°
Sf02 pmol//
Figure 16.
Silicate distribution on the 26.81k signi-A surface,
from SCORPIO () 1967 and PIQUERO (°) 1969
Expeditions.
33
14.5
Western Pacific Subsurface Water
From the B-S diagrams of stations S-66 and S-76 (Figure 2), it can
be seen that the small subsurface salinity maximum in station s-66 is a
remnant of the major one present in S-76.
A superposition of 0-S dia-
grams from S-76 towards S-66 (not shown) indicates that this subsurface
maximum decreases in intensity westward.
ward extension of the ESSW.
This would indicate a west-
However, some characteristics of the ESSW
such as the low oxygen content do not seem to support this interpreta.tion.
A plot of the oxygen content. vs. subsurface salinity maximum,
for th.ose stations north of 550 S, shows that the 02-S pairs tend to
form two groups (Figu.re 17).
One group is associated with a high oxygen
content (>1k ml/i) and the other with lower oxygen content (<1 mill),
The existence of these distinct of clUsters in the 0
2
vs. S
max
plot in-
dicates the presence of two different water masses.
A superposition of 0-S diagrams taken from the SCORPIO Expedition
(section at 13° s)
(S.I.O.,, 1969) from S-30 (169° w) to S-75 (76° w)
shows that a subsurface salinity maximum can be traced as far as S70
at 82° 14.
It is proposed that this water mass, which is characterized
by a subsurface salinity maximum not associated with an oxygen minimum,
originates in the southern portion of the Western South Pacific,
As the waters located around 170° W move towards the east, surface
salinity decreases, probably due to AAW transport across the Polar Front
Zone (i.e. ring formation; Joyce and Patterson, 1977) and/or runoff from
Southern Chile (Pickard, 1971).
This decrease in the surface salinity
combined with the presence of low saflnity AMW at intermediate depths
causes the formation of a subsurface maximum (Figure 18).
U
This maximum
34
7.0-
P-3
PPL7 WPSSW
P-2
6.0-
S6
S-7O
S-72'
S71
5.0-
S
S-72--
40-
E
-P-51
.-P-8
3.0-
ESSW
--S-7S
2.0-
-S-76
1.034.1
Figure 17.
.2.
.3
..4
34.5
Dissolved oxygen vs. subsurface salinity maximum showing
the separate groups of 0
pairs corresponding to two
550
different water masses,
rom stations north of
S of
the SCORPIO (s) 1967 and HQUERO (P) 1969 Expeditions.
35
0
C,,
0
0
i.t)
U)
a
Lfl
th
0
(1)
0
Cl)
15°
U'
0
0)
-
Cl)'?
!
t1
.-
0
.
o.8
26.0
.
0
5°
O27.O
33.5%0
34.O%o
34.5°/oo
c.01
/00
Figure 18.
Superposition of 0-S diagrams along 13° S from
169° W to the Chilean Coast from selected stations
Stars indicate the depth of the
of SCORPIO 1967.
base of the seasonal thermocline.
36
decreases in intensity eastward due to mixing with the less saline
waters located above and below it.
Even though the salinity maximum de-
creases eastward, the core of the WPSSW increases in density due to temperature decrease.
The name of Western. Pacific Subsurface Water (WPSSW)
is given to this water mass which in the study area has a mean potential
density of 27.06.
One notes in Figure 18 that the e-s diagrams for station S-75
(dashed line) departs markedly from the trend of the salinity maximum
between station S-30 and S-.70, showing an increase in salinity and a
decrease in density.
This change is attributed to the presence of the
ESSW.
A more exhaustive analysis on the properties of the WPSSW is beyond
the objectives of this study because of the area involved.
Therefore,
the WPSSW nutrient characterization as well as its spatial extension is
work to be accomplished in the future.
1i.6
Antarctic Intermediate Water
North of 550 S the bathymetry of the 27.12 sigma-8, where the AAIW
is located, presents rather constant depths of around 600 m (Figure 19).
South of 550 S it slopes upwards retching the sea surface south of the
Polar Front Zone.
The potential temperature has a relatively homogeneous distribution
along the 27.12 isopycnal..
Close to the Polar Front Zone, as this water
mass shoals, the potential temperature drops rapidly at the rate of
about 30 C in 100 km (Figure 20).
The salinity of the AAIW in the study area is also fairly uniform,
with values between 3.20 and 3'+.25°/
north of 55° S (Ficiure 21).
The
37
90°
85°
80°
75°
700
65°W.
4005
450
>600
(OO( .i4°
50°
\600
0o
/
0
0
°
ep.
0
\
550
°-';' \J
Th200/
600
'o100
rn
FFgure 19.
50
0
0
Bathymetry in meters of the 27.12 sima-O surface,
from the SCORPIO () 1967 and PIQUERO () 1969
Exped it ions.
38
)00
750
800
85°
700
65°W.
4Q0 S
£ 86.MLJ
450
000
>6.5
0
0
0
0
o
0
o
500
a.
-
0
0\
0
0
0
I.
0
0
550
0
0
/
-0
0..
0
0
4.0
0
0
2.0
0
600
e°c
Figure 20.
0j
Potential temperature distribution on the 27-12
sigma-9 surface, from the SCORPIO (A) 1967 and
PIQUERO (o) 1969 Expeditions.
900
85°
750
80°
7Q0
50W
/.0° 5
34.25
AlA
A
A
A
450
000
0
o
0
0
00
0
.
9
<34.25
500
0
0
0
550
iso
600
34123
0
0
Figure 21.
Salinity distribution on the 27.12 sigrria-G surface,
from the SCORPIO (t) 1967 and PQUERO () 1969
Expeditions.
40
salinity vertical distributions In the three northernmost oceanographic
sections A, B and C (not shown), comprise a layer of relatively homogeneous salinity with values between 314.20 and
34.30°/ee.
This nearly
homogeneous layer, located between 200 and 900 rn, contains the charac-
teristic AAIW salinity minimum and becomes shallower and less saline as
it approaches the Polar Front Zone.
As in the case of potential temperature and salinity, the dissolved
oxygen is also fairly uniform (Figure 22).
Oxyge.n increases poleward
mainly because the water on the 27.12 sigma-O surface is cooler and
fresher, increasing oxygen solubiuity.
Close to the Chilean coast and
between the oxygen poor ESSW above and POW below, the AAIW shows a maximum oxygen content.
Beyond the area where the ESSW oxygen minimum is
present, the AAIW does not show a relative maximum in dissolved oxygen.
Phosphate and nitrate in the AAIW along the 27.12 isopycnal, exhibit nearly homogeneous distributions (Figure 23 and 24).
The silicate
distribution on the 27.12 sigma-8 is rather different from that of phosphate and nitrate, even though north of 550 S it is also nearly homogeneous (Figure 25).
Closer to the Polar Front Zone, silicate dimin-
ishes to a minimum (<5 pmol/I) at the Polar Front Zone, increasing again
southward.
This situation is the result of the use of a fixed mean
isentropic surface to describe the AAItI distribution.
The 27.12 sigmaO
surface represents the core of AAIW in most of the area, but close to
the Polar Front Zone it reaches depths lower than 100 m and this is where
the SAAW or AAW is located.
Therefore, the minimum in silicate
on
sigma-e 27.12 near the Polar Front Zone can be explained by the presence
of low silicate SAAW, whle the increase to the south of the Polar Front
41
9Qo
7Qo
800
650w.
I
_1
I.
44
A
4
A
So
87
/
°°ocf)
5L
r
I-
5f) 0 0
/
0
5001
'o5,'
/
fo
sso/o,/
I
-
I
0
0
0
0
°
7.0
I
1.
0
0
0
22.
DISSOlVe d OXyg
Surf
' fr0
1969 XPed i.
d'lsIrjb
,,
e
SCo10
O the 27 /2
967
100
1,00 S
\,tItt,%ptlI't23
3eçoc?\0
otons
çSà'
U
tO
43
900
850
80°
75°
700
65°W.
40
A
Z
&
45 0
0 0 0
00 O.
0
0
4
0
>25
5
0
0
°
0
0
00
5!
..
0
0
0
0
BC
NO3 pm oil!
Figure 2L.
Nitrate distribution on the 27.12 sigma-B surface,
from the SCORPIO (Li) 1967 and PIQUERO (o) 1969 Exped it ions.
I4
900
85°
80°
750
700
65°W.
/,00s
L
4
A
A
A
L5°
<15
F-
15151
A
(.3
Ioo
:'
I
15
500
N:/' °°°:R
'><J
550
fl\0oo
S
Czo15
H
60°
$102 simol/I ',
Figure 25.
Silicate distribution on the 27.12 sigma-S surface,
from the SCORPIO (L\) .1967 and PIQtJERO (°)
peditions..
1969 Ex'
k5
Zone is explained by the presence of high silicate AAI4.
was not evident in
t'-'
This situation
phosphate and nitrate distributions because of
the relatively high values of these properties in SAAW in contrast to
the silicate.
In a similar fashion to oxygen but in an opposite sense, nutrients
in the AAIW show a minimum concentration in the vertical structure
where the ESSW is present.
Reid (l973a) points out that although the
nutrients exhibit minima in the vertical, the actual concentrations in
the AM'iI are high, as. might be expected from waters of a high latitude
origin.
These high concentrations define minima only because of the
relative higher concentrations of the overlying ESSW carried poleward
from the eastern tropical areas (Reid, 1973a) and the high concentrations of the PDW beneath.
46
V.
5.1
WATER MASS STRUCTURE TYPES
General considerations
When for a given area the temperatures and corresponding salinities
of the wate.r column are plotted against each other, the points fall on
a characteristic curve showing the structure of the salinity-temperature
relationship.
This well defined curve is the end result of the vertical
mixing processes that different original water masses have experienced
during their movement toward the given area.
The grouping of similar
8-S curves and the division of the study area into regions with similar
types of curves will give important information bearing upon features
observed in the circulation patterns.
5.2
Water structures present in the region
The PIQUERO-SCORPIO 8-s diagrams of the water column sampled north
of the Polar Front Zone, from the seasonal thermocline to the bottom,
can be grouped into four different types, each with a more or less
characteristic structure.
8-S diagrams representative of each of these
structures are shown in Figures 26 to 29.
A geographic distribution of
stations classified according to these structures is shown, in Figure 3O
The water masses present in these structures are given in Table I.
TABLE I.
Water mass constituents of 8-S structures present
in the area north of the Polar Front Zone.
Water Masses
Structure
I
II
III
IV
SAAW
SAAW
SAAW
SAAW
-
ESSW - AAIW - PDW
AAIW - PDt4 - AABW*
WPSSW
AAIW - PDW - AABW*
AAW - PDW - AABW
*May not be present in some stations showing this characteristic structure.
15°-
-S-76
---'a--
49
P-48
\J-5OTh
10
100
26.O
10o
-
.2
P
-
CD
..
___100
-
5°
FIgure 26.
from the
Representative stations showing Structure Type
Stars
1969
Expeditions.
1967
and
PQIJERO
(P)
SCORPIO (s)
indicate depths of the base of the seasonal thermocline.
I
15°
-
,SOm
S-67
- - - * S 66
.6
100
26.0
1oo
.8
cT$= 27.0
2
/
7
3000.
/iI*Loo 0
.8
34.5 %0
34.O'Y00
r0I
,o0
Figure 27.
Representative stations showing Structure Type II from
Stars indicate depth
the SCORPIO (s) 1967 Expedition.
of the base of the seasona' thercnocline.
19
150
P-47
S
S - a a
- 3 7
P-18
1O°-26.o
w
1OO
50
L.V
5Orn-N
.6
.8
300
-'-600
cT927.O
1500
.2
-
1500
3OOO
6
-J:
-
3O0O
000
-
34.O%o
34.5 %o
J(Oo
Figure 28.
Representative stations showing Structure Type III from
Stars indicate depth
the PIQUERO (P) 1969 Expedition.
of the base of the seasonal therniocflne.
50
15°. - - .
2 4
P 23
P-21
100_
o
0
0
50m--
\5Orn
ioo7
26.8
jI30
8
(-6o0
.uu
=27.O
1500
,1U0+
300 600
2
no
U -
6
I
J
34.00/00
Figure 29.
*3000
F
P
1.000
.8
I
I
-:
3.5%o
Representative stations showing Structure Type IV from
Stars indicate depth
the PIQUERO () 1969. Expedition.
of the base of the seasonal thermocUne.
51
850
90°
'
400
'-I
I
I
I--i
750
800
I
-I
I
I
I
I
I
1
700
I
I
I
I
J
15-
65°W.
I
I
I
I
I
L
-
15-
;r27O
':::
4:0
,28.0
/28.0
:
0
TelIuIIlIIIiI
3&Oso/35.O
0- ., .........
0 ° V
-
340S%o35°0
0
-
27.O
550
:°i
:
S
60°
V STRUCTURE I
STRUCTURE/I
P-14()
CD
_
,cro27.0
:
28.0
° STRI.ICTLIRE Iii
STRUCTURE/V
Figure 30.
(ri
340
s/oe
35.0
Distribut ion of 8-S structure types north of the Polar
Front Zone for the SCORP0 1967 and PIQUERO 1969
Stations in the study area.
--
-
52
Some coastal stations are under the influence of the adjacent estuarine system, showing low surface salinity values (Figure 4).
This
low surface salinity produces a shallow halocline, which has not been
considered in the water mass structure analysis.
Mamayev (1973) has performed an analysis of 1-S structures of the
upper 2500 m in the Eastern South Pacific based on data taken by the
R/V Professor DERVUGIN in 1968.
His work included the area between 300
and 530 S, 200 nautical miles off Chile and
1100
W,
Therefore, there is
a region where Mamayev's (1973) study overlaps this study.
For this
coimion region, Mamayev (1973) described a 1-S structure named Subantarc-
tic type I, which corresponds to structure III of this work.
He also
described a transient type V, 100- 150 km farther to the NW of the
study area.
This transient type V, structure II
in this work, is still
present in the study area even though its WPSSW component is very weak.
Mamayev (1973) showed that the transient type V extendedwestward as
far as 102°W.
Farther to the west. he. described a transient type IV
which does not show the maximum and minimum salinity content. of the
WPSSW and AAIW, showing instead a nearly homogeneous salinity layer. between 300 and 1000 m.
The superposition of 0-S diagrams from SCORPIO
(Figure 18) shows that structure II
[equivalent to Mamayevs (1973)
transient type vi extends much farther to the west than 102° W.
How-
ever, these diagrams do not show a nearly homogeneous salinty layer at
subsurface and intermediate levels as shpwn by Mamayev's (1973) transient type IV west of 102° W.
Structure II should be considered as ex-
tending westward to the point where the surface salinity minimum can not
be considered as such.
53
Another study which includes the study region and which considers
the water mass structure of the water column is that of Johnson (1972).
He described two structures with three water masses in each.
Those
structures are the basis of his three point vertical mixing analysis.
Neither of Johnson's (1972) structures includes the ESSW, even though
he recognized the presence of the Peru-Chile Undercurrent and two of his
working stations in this area are from SCORPIO Expedition (S-72 and S-76).
Basically, Johnson's (1972) two structures correspond to structure III
of this study, where the AABW was not included due to the maximum depth
he considered (2500 m).
Since the ESSW has an important role in the upwelling off the
central Chilean Coast (Braridhorst, 1971; and Silva, 1973) and its
southernmost extension has not been clearly established, the geographic
distribution of structure
I
will be discussed in detail.
As can be ob-
served from the water mass structure distribution chart (Figure 30),
structure
limit.
I
extends as far south as 48° S with 79° W as its westernmost
Its distribution resembles a wedge, with its apex pointing to-
ward the south,
South of 50° S the ESSW salinity maximum can not be
traced, because it has been completely dissipated due to vertical mixing
with the less saline SAAW and AAR4.
Tiard the west a similar dissipa-
tion occurs, nevertheless a slight salinity maximum still persists in
This maximum is not
some of the remaining stations of sections A and B.
a remnant of the ESSW from structure I, but rather Ft belongs to the
WPSSW from structure II as has been discussed before.
located at the western boundary of structure
I
,
Station S-72,
is a good example of such
a situation, because of its slight double salinity maximum at 251
and
54
399 m (Figure 31).
In addition to the differences In depth and density,
the upper salinity maximum in. S-72 is associated with a minimum in oxy-
gen, while the lower one is not (See Figure 15, S-72 and 5-72').
This
situation can be interpreted as the result of the interleaving of
structures
I
and II
in the same oceanographic station.
From the above discussion it may be seen that the ESSW, carried
southward by the Peru-Chile Undercurrent, spreads as far south as 48° S
This result complements that of Wooster and Gilmartin (1961), extending
their proposed southernmost limit for the ESSW spreading by about
1000 km.
The distribution of structure II has been limited to the west of
section A.
Nevertheless, some of the stations in section B classified
in structure III, could have been classified in structure II.
This is
because one structure transforms into another by slow mixing of some of
the water mass components; therefore, transient or intermediate structures are often present.
Structure III
is the prevailing structure found in t.he study area.
It covers about 75, of the region studied.
Structure IV, which includes
MW, is found in the neighborhood of the Polar Front.
15°-
S-72
2i
--'-72.-
4
.6
10°- 26O96
-
o
.2
177201
4
o
i5j:
6
325
-4-399
50
5007"
.8
665-
7'698
O27O
864
-
2
/
-
./
1260
w2
,"
2151
ht'3oj5
'
j,B54
361.7-..
.6
no
31
.8
I.
I
I
I
34.0%
1
34.5 %0
eoi
100
Figure 31.
e-s diagram of the SCORP!O station S-72 showing the
presence of two weak subsurface salinity maxima at
251 and 399 m. The star marks the base of the seasonal
thermocl inc.
56
VI.
6.1
GEOSTROPHIC CIRCULATION
General considerations
Since most ocean currents satisfy geostrophic conditions and until
recently direct current measurements have been scarce and difficult to
obtain, most classical open ocean current studies are based on geo
strophy.
As it stated in the Introduction, the flow pattern in the
study area is known only in its gross features.
Therefore, the need
exists for a more detailed study.
To improve knowledge of the flow pattern of the study area, this
chapter will discuss the geostrophic flow on the sea surface and on the
26.81+ and 27.12 isopycnals, all relative to 3000 db.
strophic velocity sections relative to 3000 db will
ment the isentropic analysis.
Vertical geo-
e used to comple-
A short discussion on the 8-S structure
relative to the geostrophic circulation will be also used to explain
some of the flow features.
6,2
Flow on the, sea surface
The sea surface can not be represented by a single isopycnal;
therefore, It is not possible to apply isentropic analysis to it.
Geo-
strophic flow at the sea surface must be interpreted from a dynamic
height topography map.
The sea surface dynamic topography relative to 3000 db (Figure 32)
shows a general flow toward the south in the southern half of the study
area.
This flow corresponds to the southern branch of the .!WD, after
its division off South America north of the area of study (Silva and
Neshyba, 1977).
The coastal part of this poleward flow corresponds to
57
900
750
800
85°
700
65°W.
400
A AU/
5O
%"1:o
0
o$o
!
/
550
-1.50
1./.0..
-130
°icn I
60°
dynm
1.20 ..-
0
1.00-'--
0
Figure 32.
Dynamic topography of the sea surface relative to
3000 db, from SCORHO (A) 1967 and PiQUERO (o) 1969
Exped it ions.
58
the Cape Horn Current, which also carries southward the less saline and
comparatively lower nutrient estuarine water along the coast of Chile.
South of
550 S the general surface flow pattern is toward the East
with much larger velocities than those of the northern part.
eastward flow is
This
in agreement with generally recognized Antarctic Cir-
cumpolar Circulation in this part of the ocean (Sverdrup etal., 1942;
Gordon, 1967; and Gordon and Bye, 1972).
The main geostrophic flow across the Drake Passage is concentrated
north of 58° S, with maximum velocities greater than 30 cm/sec.
South
of 58° S the geostrophic velocity decreases toward the pole which is in
agreement with previous reports of Ostapoff (1961), Gordon (1967), and
Reid and Nowlin (1971).
6.3
Flow on the 26.84 sigma-e surface (ESSW)
The thermosteric anomaly at the core of the ESSW has been shown by
Wooster and Gilmartin (1961) to decrease from 160 cl/ton (sioma-t
26.44) off Peru, to less than 140 cl/ton (sigma-t = 26.65) off central
Chile.
They suggested that this density increase arises from mixing
with water of lower temperature,
If one analyzes the meridional sections of salinity off the Peru
and Chile coast, prepared by Gunther (1936) or Wooster and Gilmartin
(1961) or Silva and Konow (1975), it can be observed that the ESSW core
diminishes in salinity and deepens as it moves southward.
This could
be explained in terms of a more active erosion of the ESSW core due to
a stronger vertical mixing at its upper boundary (with SAAW) , than at
its lower boundary (with AAIW).
The reason for this difference in
upper and lower boundary mixing rate can be the relatively hiqher shear
in the former compared to that in the latter (see for example, Figure
35).
Because of the change in density of the ESSW core as it moves
southward along the Chflean Coast, this core can not be represented by
a single isopycnal, but by a family of them.
The isopycnal of 26.814
represents the mean potential density where the ESSW core is located
off southern Chile.
Geostrophic. flow on this isopycnal (Figure 33) can be divided
into two parts as was done with the sea surface flow.
The. southern
part, south of 500 S, has a relatively fast geostrophic flow pointing
toward the east.
This is the continuation at subsurface levels of the
Antarctic Circumpolar Current.
In the northern part, mainly north of
5Q0 S, the geostrophic flow is significantly slower.
The eastern por-
tion of the northern area shows a southward flow, while the western
portion the general flow is to the north.
The poleward flow close to the coast of Chile can be interpreted
as a remnant of the Peru-Chile Undercurrent.
Nevertheless, a more
rigorous explanation of the origin of this poleward flow can be based
on an acceleration potential chart which includes a much larger extension to the north of the study area,
Reid's (1965) acceleration poten-
tial relative to 1000 db on the 125 cl/ton isanostere, which is close to
26,84 sigma-B, shows a poleward flow off Peru and Chile which is not defined south of 36° S.
A farther southward extension of this poleward
flow may be reasonable, since the available data at that time in this
region was very scarce,
It is reasonable to connect the poleward flow
from Reid's (1965) chart with this study area, thus inferring that the
Q0
85°
80°
700
75°
65°W.
L 0°S
2.20
-
/
6
65°
I ..
/ 2.175#
_I
H
-
2.75
I
1
,
5'
2.175
tr
ri
fo
ol/o\O
50°
2'15
0
010
i
5 5°.
2.00
0
\
1 70
°
600
dynm
Figure 33.
Acce]erdtion potential on the 26.8!! siymaB surface
relative to 3000 db, from SCORPIO (A) 1967 and
PIQUERO () 1969 Expeditions.
[4
Peru-Chile Undercurrent is present at this high latItude.
Nevertheless,
further evidence is required and will be presented later.
6.4
Flow on the 27.12 sigma-A surface (AAIW)
The general circulation on the 27.12 isopycnal (Figure 31+) is very
similar to that present on the 26.84 sigma-B surface.
Again, main
characteristics are the relatively slow flow in the northern portion of
the area and a much faster flow in the southern part.
North of 550 S the flow direction is not very well defined, but a
northward movement can be accepted as the general trend.
Reid's (1965)
acceleration potential map on 80 cl/ton isanostere (sigma-B = 27.29),
which he selected as the AAIW representative, does not show any detail
in this area probably due to the scarcity of data.
The acceleration
potential map on the 27.10 sigma-t surface prepared by Johnson (1972)
shows the flow in this region, north of 550 S, with a general northward
direction.
Nevertheless, the data used by Johnson (1972) for thIs
region is also sparse (8 stations located mainly on the periphery of
the study area)
South of 55
S the flow is well defined toward the east, with
relatively high velocites.
Again, this flow corresponds to the pre-
sence of the Antarctic Circumpolar Current at this level.
Since the flow on the 27.12 isopycnal in the vicinity of the
Polar Front is directed mainly toward the east, the AAIW formed in the
PIQIJERO-SCORPIO area will be carried eastward.
Therefore, this area
will act mainly as a source region for Pacific AAI'd to the Atlantic
Ocean.
62
B°
OO
80°
75°
65°W.
70°
40°S
A
45°
A
A
\
'1
oc
(
\
50°
55°
5 QO
dynm
Fi9ure 3+.
Acceleration potential on the 27.12 si9ma-A surface
relative to 3000 db from SCORPIO (t)
967 and
PIQUERO (o)
969 Expeditions
63
6,5
Flow across vertical sections
Vertical sections of geostrophic velocity relative to 3000 db are
shown in Figures 35 to 40,
These vertical sections complement the in-
formation given by the horizontal flow on selected isentropic surfaces,
showing the geostrophic velocity
n the water column across sections
perpendicular to the coast (sections A to F in Figure 1).
6.5.1
The coastal flow
The coastal subsurface flow associated with the ESSW on the charts
of acceleration potential on the 26.84 sigma-e extends as far as 150 km
off the coast at section A (43° S) (Figure 35).
The flow shows a maxi-
mum of 5.9 cm/sec at 250 m depth, which is also the depth at which the
salinity maximum of the ESSW is found.
In section B
(48°
s)
(Figure 36), the coastal flow shows neither a
subsurface maximum nor a decrease in the velocity shear in the subsurface layer associated with the ESSW.
Nevertheless, the ESSW salinity
maximum is found at station P-48 (see Figure 26) at a depth of 252 m.
The maximum velocity of 4.0 cm/sec occurs at the surface at a distance
of 140 km offshore; the poleward flow extends to about 400 km offshore,
In section C
(51°
s)
(Figure 37), the maximum velocity of the
coastal poleward flow is again found at the surface (30 m depth) but at
a value of 6.7 cm/sec.
offshore,
In this section, the flow extends some 300 km
From sections C through F the general trend of the coastal
flow is to increase in velocity, varying in offshore extent to a niaximum of about 560 km at section F..
SECT/ON
S-67
S-66
S-68
I
2Z08
.
5-71 5-72
S-b
4L
'4'ftf j3
:\[. o.7fl12S4
/
-I-H
I
II.2I 0.iI'
I
27:l2..J-__-----
HTH1 +-U-'
.i
I
L
S-75 S-76
S-73 5-74
.
.
J;
.
I}
.
:'
(
1
:\
.
(:I
.
.
2000
cr/sec
t
1000
500
0
DISTANCE (km)
flgure 35.
Geostrophic velocity relative to 3000 db at section A,
from SCORPIO 1967 Expedition. Positive values indicate
northward flow; negative values indicate southward
flow.
Solid horizontal lines represent the potential
density at which the cores of ESSW (26.814), WPSSW (27.06)
and MIW (27.12) are found; dshcd line is the base of
the seasonal thermocline (BST).
65..
SECTION B
P-41
P-h2
IUUU
P-43
P-45
P-44
500
P-46
P-47
P-48
0
DISTANCE (kin)
Figure 36.
Geostrophic velocity relative to 3000 db at section B,
from PIQUERO 1969 Expedition.
Positive values indicate
northward flow; negative values indicate southward flow.
Solid horizontal lines represent the potential density
at which the cores .f ESSW (26.81+) and AAIW (27.12) are
found; dashed line is the base of the seasonal thermcr
cUrie (BST).
0
C',
-4.
0,
0,
0
0
F
C-
LUa
(I)
0
0)
a,
C',
0
67
SECTION D
1000
500
0
DS TA N CE I k m)
Figure 38.
Geostrophic velocity relative to 3000 db at section D,
from PtQUERO 1969 Expedition.
Positive values indicate
northward flow; negative values indicate southward flow.
Solid horizontal lines represent the potential density
at which the cores of ESSW (26.81+) and AAIW (27.12) are
found; dashed line is the base of the seasonal thermo-
dine (BST).
SECTION E
- --
.. fin
n__In
0_b
DISTANCE (km)
Figure 39.
Geostrophic velocity relative to 3000 db at section E,
from PIQIJERO 1969 Expedition.
Positive values indicate
eastward flow; negative values indicate westward flow.
Solid horizontal lines represent the potential density
at which the cores of ESSW (26.84) and AAIW (27.12) are
Found; dashed line is. the base of the seasonal thermo
dine (BST).
69
SECTION F
P-l0
P-li
P-12
P-13
P-14
P-iS
P-17
cm/sec
1000
500
0
DISTANCE (km)
Figure hO.
Geostrophic velocity relative to 3000 db at section F,
from PIQUERO 1969 Expedition.
Positive values indicate
eastward flow; negative values indicate westward flow.
Solid horizontal lines represent the potential density
at which the cores cf ESSW (26.84) and AAIW (27.12> are
found; dashed line is the base of the seasonal thermo-
dine (BST).
70
6.5.2
Identification of the subsurface coastal flow with the Peru-Chile
Undercurrent
A central theme of this study is the question of the southernmost
extension of the Peru-Chile Undercurrent.
In this sense, the coastal
flow described in the previous section may now be correlated with previously published data on the Undercurrent along the coasts of Peru and
Chile as follows:
From 6° to 24° S, STEP
Expedition (Wooster and Gflmartin,
1961); reference level 1000 db
I
From 24° to 28° S, MARCHILE VIII Expedition (Sievers and Silva,
1975) ; reference level 1000 db,
- At 35° S, geostrophic flow computed from stations 83 and 84 of
section 35° S of the SCORPIO Expedition (S.l.O., 1969); reference
level 2000 db.
From 43° S to about 57° S, geostrophic flow computed for the
study area using SCORPIO and PIQIJERO data; reference level
3000 db.
Results from these expeditions and sources are combined in Figure 41.
Arrows show the approximate position of the core of the Peru-ChIle Undercurrent from 6° S to the northern limit of the study area using data
from the first three of these sources.
Within the study area, the Undercurrent is clearly delineated
through section A (43° 5), as discussed in the previous section.
At
latitude 45° S, the PIQUERO Expedition yields a short, near coastal
section consisting of three stations, P-49 through P-51 (see Figure 1)
Geostrophic flow relative to 2500 db has been computed for this set of
stations, and profiles are shownin Figure 42.
The profile for flow be-
tween P-1+9 and P-50 shows two maxima above 300 m:
the first occurs at
the surface at a value of 5.7 cm/sec; a second, less pronounced maximum
of 2.9 cm/sec occurs at 200 m,
The latter is located wIthin the subsur-
71
110°
Figure 41.
I0O
900
80°
700
60°W
Composite of oceanographic sections showing approximate
positions and geostrophic velocities of the core of the
1960 (Wooster
Peru-Chile Undercurrent. based on STEP
and Gilmartin, 1961), tIARCHILE VIII 1972 (Sievers and
Silva, 1975), SCORPIO 1967 (this study) and PIQUERO 1969
(this study) Expeditions. Respective reference levels
are given in the text.
I
72.
-4
-6
A
U
I
V cm/sec.
0
-2
I.
I.
I
I.
It
I
I
.
+2
I.
.
¶
P5O-P51
PL9.-P5O\
It
1000
\.
E
I.
IUi
a
2000
..
Figure 42.
Geostrophic velocity profile relative to 200 db for
N50 and P-50 - P-51.
PIQUERO 1969 stations pairs P19
Positive values indicate northward flow, negative values
indicate southward flow.
73
face salinity maximum of the ESSW.
A similar subsurface maximum is
found between stations P-50 and P-51.
It is, therefore, reasonable to
conclude that the Undercurrent still exists at this latitude, 45° S.
At J8° S, section B, the subsurface salinity maximum, oxygen minimum, and nutrient max.inium which are characteristic of ESSW water are
present at about 250 m depth.
However, the geostrophic velocity regime
(Figure 36) does not show a subsurface velocity maximum.
At 510 S,
section C, remnant.s of the low oxygen and high phosphate an.d nitrate
still persist, but salinity and silicate do not show a maximum in the
vertical distribution.
Using the criteria of coincident subsurface
velocity maxima and subsurface ESSW characteristics, one must conclude
that the Peru-Chile Undercurrent loses its identity around 48° S.
How-
ever, subsurface flow does exist at section 8, as well as at sections
further south, but can not be uniquely identified with the Peru-Chile
Undercurrent characteristics.
The southernmost extension of the Peru-Chile Undercurrent is not
absolutely established in this study.
Further work must be done to
answer some questions related to the Undercurrent in this area, such as:
Does this southern extension undergo seasonal fluctuations?
Does the
latitude of the WI4D impingement affect its southern and/or western extension?
If the WWD impingement off Chile is under the influence of
seasonal variations latitudinally, how will this affect the Undercurrent?
6.5.3
Geostrophic flow south of 48° S
The increase in the flow velocity in the surface coastal poleward
flow or Cape Horn Current, may be the result of influx of runoff water
74
from the adjacent coast, elevating the sea level height and hence the
flow velocity.
The piling up effect due to the pressure exerted by the
WWD against the coast maybe important
The relatively high velocities
present in sections E and F, is the result of the constriction effect
of the Drake Passage to the zonal flow (Gordon and Bye, 1972).
The flow across the Drake Passage (Figure 40) follows the pattern
described by Reid and Nowlin (1971), as would be expected since they
also used PIQIJERO data.
Nevertheless, there is a difference between
their geostrophic velocity values at the station pair P-15 to P-17 compared to those shown in section F (Figure 40).
Even though Reid and
Nowlin's (1971) velocity values at this station pair are referred to
3400 db, this can not account for a sea surface velocity difference of
13 cm/sec between the two studies.
This difference seems to be the re-
sult of a computational error, since their surface value of 36.1 cm/sec
could not be obtained by this writer from the corresponding station data
set.
As a result, the fastest geostrophic velocities are, in fact,.
located between the station P-l4 and P-15, and not between P-15 and
P-17 as shown by Reid and Nowlin (1961).
Direct current measurements near the bottom in the Drake Passage
were reported by Reid and Nowlin (1971) to show mean daily speeds
ranging from 0.5 to 14.7 cm/sec.
These relatively high. speeds at the
bottom of the passage are further evidence of the absence ofany given
level of no motion For geostrophic computations in this area.
6.6
Geostrophi.c circulation and water mass structure
Since the geostrophic circulation is causally linked to the density
field distribution, this type of circulation is directly related to the
75
e-s structure present in a given area.
At times, circulation features
can be more easily explained in terms of 9-S or water mass structure.
The eastwardflow south of 570 S(Figures 32 to 34), is dominated
by structure IV
(FIgure 30).
ward bend at station. P-23.
This eastward flow shows a sharp south-
A detailed analysis of the 0-S structure
present at P-23 indicates a transient structure between structures III
and IV.
This means that. the water column in this sLation still shows
remnants of structure III, even though it can be classified as structure IV.
Therefore, remnants of characteristics of a northern station
are present in the south, giving as a result the observed bend in the
flow lines.
The clockwise eddy at station P-19 (Figures 32 and 34) seems also
to be produced by a transient structure between III and IV (Figure 43).
In this station, the e-s structure between 71k and 1535 m resembles
more closely structure IV than III.
This could be the result of the
presence of water which contains a major fraction of cold AAW in its
composition.
Joyce and Patterson (1977) ,
In describing a clockwise
ring formation at the Polar Front Zone, stated that:
withi.n the Polar
Front.Zone'the minrnur&' (temperature) "deepens abruptly and erodes
splitti.ng into multiple extrema of interleaving Antarctic/Subantarctic
waters.
Since t.he waters south of the Polar Front Zone are associated
withä comparatively lower dynamic height than that of those to the
north, t.h.e presence of AAW north of the Polar Front Zone will produce a
lesser dynamic height in comparison to that of surrounding waters.
Dynamic topography charts of 0/500, 500/1000, 1000/2000 and 2000/3000 db
(not shown), indicate the presence of this eddy at all these levels.
76
15°p-.19
-
10°-
o
Om
0
-'-76
.6
126
8
* Y15O3,/'
O27.O
-
255
51OIL+.h
817
1737
/
919
-
3157
.6
.8
00
--3871
', roi
, .t0,
OL,.0 100
O't.) (00
c,0I
qJ,O0
Figure 143.
0-S diagram of station P-19.
The segment between 71I and
1535 in resembles a structure type IV, while the whole
structure corresponds to structure type HI.
Star marks
the base of the seasonal therinocline.
77
The highest dynamic difference between adjacent stations
(
10 dyn cm)
is found in the 50.0/100 and 1000/2000 db zones which indicates that the
segment 500 to 2000 m of the water column is the main contribution to
the observed lower relative dynamic height of N19.
On the other hand, counterclockwise flow east of the Drake Passage
has been described by Gordon etal. (1977).
They associated this flow
with the presence of SAAW within the Polar Front Zone,
The poleward flow on the 26.8'i isopycnal (Figure 33)
for the presence of structure
off southern Chile.
I
is responsible
Nevertheless, this
structure is also present westward of the core of this southward flow.
This situation might be the result of horizontal diffusion rather than
advect ion.
There is no evidence inside the study area of any relatively important flow carrying the WPSSW component of structure II eastward.
Thus, the presence of structure II
in this area may be interpreted as
mainly due to a diffusive phenomenon.
Nevertheless, Reid's (1965) ac-
celeration potential maps on the 125 and 80 cl/ton isanosteres, show a
general eastward flow between
o°
and
500 S.
This flow, which weakens
eastward and turns northward around 90° S off Chile, may be responsible
for the presence of structure II
in this area.
78
VII.
CONCLUDING REMARKS
To summarize this detailed analysis of the water mass structure
and geostrophic circulation of the study area between 13° and 63° S,
91° W to the Chilean Coast, some concluding remarks can bemade:
1.
Previous studies of the high salinity, low oxygen poleward
flow (Gunther, 1936; Wooster and Gilmartiri, 1961) have placed its
southernmost extension at 41° S.
This study demonstrates clearly that
this limit can be extended to at least as far as 48D S.
South of this
latitude the subsurface flow can. not be definitely associated with the
ESSW characteristics of the Peru-Chile Undercurrent.
Further study must be performed to establish whether or not the
Peru-Chile Undercurrent's southernmost boundary is subject to seasonal
variations.
(1)
Questions to be answered by future work in the area are:
How does the latitude at which the WWD impinges the Chilean Coast
affect the southern and/or western extension of the Peru-Chile Undercurrent? and (2)
How does the probable seasonal fluctuation in the
latitude of impingement of the WWD affect these samefactors?
2.
The presence of the WPSSW is proposed in this work as water
carried from the extreme Southwestern Pacific Ocean into the study area
below the surfacelow salinity water off southernChile.
This water
mass is characterized by a subsurface salinity maximum, not associated
with an oxygen minimum as is the ESSW.
Further work has to be performed
to establish its nutrient characteristics as well as its spatial dis
tribut ion.
3.
The low phosphate and nitrate contents of the surface coastal
waters off southern Chile, seem to have their origins in the mixture of
79
ocean-waters with estuarine waters. of comparatively less rich nutrient
Two point mixing explains-around 60
content.
70
of the nutrient
variability of the waters in the study area where the- salinity ranges
between 33
34°/,
Biological and/or chemical processes may also be
important causes of the relatively low nutrient concentrations of thse
coastal waters.
More interdisciplinary work must be carried -out in
order to explain this feature.
1f.
The presence of seven water masses
in the region of study were
established from the 8-S diagrams below the seasonal thermocl me:
Subantarctic Water, Antarctic Water, Equato rial Subsurface Water,
Western Pacific Subsurface Water, Antarctic intermediateWater, Pacific
Deep Water and Bottom Water.
5.
Geostrophic flow in the study area can be divided in general
into two regions.
North of
550
S, where the circulation is relatively
slow, with its eastern boundary flow toward the south along the Chilean
Coast.-
South of 55° S, the circulation is much faster than in the
northern portion.
The flow has an eastward direction with high
velocities characteristic of jhe Antarctic Circumpolar Current in this
region.
80
BIBLIOGRAPHY
Brandhorst, t4..
1971.
Cond.iciones oceanograficas estivales frente a la
costa de Chile. Rev. Blol. Mar., Valpara.iso, 14(3).:
484.
Cabrera-Muro, H.
1977.
Precise North-South oceanographic transect In
the Pacific Ocean.
Corvallis., Oregon State
Master's Thesis.
University.
74 pp.
Callahan, J.E.
The structure and circulation of the Deep Water
1972.
in the Antarctic.. Deep Sea Res., 19:
563-575.
CCS HUDSON.
1970.
Data. report of HUDSON-70 Expedition. (MEDS Ref:
1810-69050).
Marine Environment Data Service.
Department of the
Environment.
Ottawa, Canada.
Defant, A.
1961.
Physical Oceanography Vol.
London.
285 pp.
I.
Pergamon Press,
Gordon, A.L.
1966.
Potential temperature, oxygen and circulation of
bottom water in the Southern Ocean. Deep Sea Res.,.j1:
1125-11.38.
Gordon, A.L.
1967.
Science,. 156:
Geostrophic transport through the Drake Passage..
1732-1734.
Gordon, A.L. and J.A.T. Bye.
Surface dynamic topography of
1972..
Antarctic Waters.
J. Geophys. Res., 77(30): 5993-5999.
Gordon, A.L., D.T. Georgi and H.W. Taylor.
Antarctic. Polar
1977.
Front Zone in the Western Scotia Sea - Sunier 1975. J. Phys.
Oceanogr., 7(3):
309-328.
Gunther, E.R.
1936.
A report on. the oceanographical investigations in
the Peru coastal current..
Discovery Rep., 13:
107-276.
Hel land-Hansen, B. ,l9l6.
Nogen hydrografiske metoder.
Naturforsker mote, 16:
357-359.
Skand.
Johnson, R.E.
Antarctic Intermediate Water in the South Pacific
1972.
Ocean.
Ph.D. Thesis.
Corvallis, Oregon State University.
170 pp.
Joyce, T.M. and S.L. Patterson.
1977.
Polar Front in the Drake Passage.
Cyclonic ring formation at the
131-133.
Nature, 265:
Mamayev, 0.1.
1973.
Water masses of the Southeast Pacific Ocean.
Oceanography of the South Pacific 1972, comp. R. Fraser.
New
Zealand National Commission for UNESCO, Wellington:
1973.,
pp. 71-79.
In:
Montgomery, R.B.
1937.
A suggested method for representing gradient
flow in isentropic surfaces. Bull. Amer. Meteor. Soc., 18:
210212.
Neumann, G.
1968.
Ocean currents.
Amsterdam.
352 pp.
Elsevier Scientific Publishing Co.
Ostapoff, F.
1961.
On the mass transport through the Drake Passage.
J. Geophys. Res., 65(9):
2861-2868.
Pickard, G.L.
1971.
Some physical oceanographic features of inlets of
Chile.
J. Fish. Res. Bd. Canada, 28: 1077-1106.
Pickard, G.L.
1973.
Water structure in Chilean fords.
In:
Oceanography of the South Pacific 1972, comp. R. Fraser.
New Zealand
National Commission for UNESCO, Wellington:
pp. 95-1014.
1973.
Reid, J.L., Jr.
1961.
On the geostrophic flow at the surface of the
Pacific Ocean with respect to the 1,000 decibar surface. Tellus,
13(4) :
489-502.
Reid, J.L., Jr.
1965.
Intermediate waters of the Pacific Ocean.
Johns IIopkins Oceanogr. Stud., No. 2,. 85 pp.
Reid, J.L.
1969.
Sea-surface temperature, salinity and density of the
Pacific Ocean in summer and winter. Deep Sea Res., Supp. to Vol.
16:
215-224.
Reid, J.L.
1973a.
Transpacific hydrographic sections at Lats. 143° S
and 28° S:
the SCORPIO Expedition - III.
Upper water and a note
on southward flow at mid-depth. Deep Sea Res., 20:
39-149.
Reid, J.L.
1973b.
The shallow salinity minima of the Pacific Ocean.
Deep Sea Res., 20: 51-68.
Reid, J.L. and W,D. Nowlin, Jr.
Transport of water through the
1971.
Drake Passage.
Deep Sea Res.., 18:
51-64.
Reid, J.L. and R.J. Lynn.
On the influence of the Norwegian1971.
Greenland and Weddell Seas upon the bottom waters of the Indian
and Pacific Oceans. Deep Sea Res,, 18:
1063-1088.
Reid, J.L. and R.S. Arthur.
Interpretation of maps of geo1975.
potent ial anomaly for the deep Pacific Ocean. J. Mar. Res. Supp.
Vol. 33:
37-52.
Robles, F., E. Alarcon and A. Ulloa.
Water masses at the
1974.
Northern Chilean Zone and their variations in a cold period (196.7)
and warm periods (1969, 1971-73). Presented at Reunion de Trabajo
sobre el Fenomeno El Niño. Auspiciado por la Comision Oceanografica Intergubernamental. Guayaquil-Ecuador, 9-12 Diclembre
1974.
59 numbered pages.
Sandoval, E.T.
1971.
The sumer distribution of Tuna in relation to
the general oceanographic condition off Chile and Peru.
Far Seas
Fisheries Res. Lab. Bull., 5: 23-88.
Scripps Institution of Oceanography.
Physical and chemical data
1969.
from SCORPIO Expedition in the South Pacific Ocean.
USNS Eltanin
Cruises 28 and 29.
SlO Reference 69-15.
12 March - 21 July 1967.
WHOI Reference 69-56.
La Jolla, California. 89 pp.
Scripps Institution of Oceanography.
Physical and
Data Report.
1974.
chemical data.
PIQUERO ExpeditIon, 16 December 1968 - 16 April
1969.
SlO Reference 74-27.
59 pp.
Sievers C., HA. and N. Silva S.
Masas de agua y circulacion en
1975.
330 S.
el Oceano Pacifico Sudoriental.
Latitudes 18° S
(Operacion Oceanografica "MARCHILE VIII").
Cienc. y Tec. del Mar.
Contrib. CONA No. I: 7-67.
Silva S., N.
Variaclones estacionales de temperatura, salinidad
1973.
y contenido de oxigeno en la zona costera de Valparaiso
(Septiembre de 1969 - Agosto de 1970).
mv. Mar., 13(3): 89-112.
Silva S., N. and H.A. Sievers S.
Masas de agua, velocidad
19713.
geostrofica y transporte de volumen entre Valparaiso e Isla
Robinson Crusoe (Oceano Pacifico Sud Oriental).
Pacifico Sur.,
No. 2:
102-120.
Silva S., N. and D, Konow H.
Contrlbuclon al conocimiento de
1975.
las aguas en el Pacifico Sudoriental.
Expedicion Krill. Crucero
3-1i.
Julio-Agosto 1974.
63-75.
Rev. Corn. Perrn. Pacifico Sur. 3:
Silva S., N. and S. Neshyba.
1977.
Corrientes superficiales frente a
la Costa austral de Chile.
Cienc. y Tec. del Mar. Contrib. CONA
No. 3:
(in press).
19132.
Sverdrup, H.U., M.W. Johnson and R.J. Fleming.
The oceans, their
physics, chemistry and general biology. Prentice-Hall, New York,
1087 pp.
Taft, B.A.
1963.
Distributions of salinity and dissolved oxygen on
surfaces of uniform potential specific volume in the South
J. Mar. Res. 21(2):
Atlantic, South Pacific, and Indian Oceans.
129-1136.
Warren, B.A.
1970.
In:
General circulation of the South Pacific.
Scientific Exploration of the South Pacific. Warren S. Wooster
(Ed.).
pp. 33-49.
Wooster, W.S. and M. Gilmartin.
1961.
J. Mar. Res.. 19(3): 97-112.
The Peru-Chile Undercurrent.
83
Wooster, W.S. cnd J.L, Reid, Jr.
Eastern Boundary Currents.
1963.
hi:
The Sea, deas and Observations on Progress in the Study of
the Seas.
Vol. 2.
tl.iI.I. Hill (Ed.).
Interscience Publishers.
New York, pp. 253-280.
tlUst, G,
1935.
Die Stratosphare des Atlantischen Ozeans, Inn: Wiss:
Ergebn. Deutsche Atlant. Exped,
"meteor," 1925-1927. Vol. 6,
Pt. 1, Sect. 2:
109-288.
Wyrtki, K.
1963.
The horizontal and vertical field of motion in the
Peru Current.
Bull. Scripps Inst. of Ocean., 8(4): 44-72.
Wyrtki, K.
1966.
Oceanography of the Eastern Equatorial Pacific Ocean.
Oceanogr. Mar, Biol, Ann. Rev., 4: 3368.
Wyrtk, K. 1967. Circulation and water masses in the Eastern Equatorial
Pacfic Ocean.
hit. J. Oceanol. and Limnol., 1(2):
117-147.
Wyrtki, K.
1975,
Fluctuations of the dynamic topography in the Pacific
Ocean. J. Phys. Oceanogr., 5:
450-459.
Zuta, S. and 0, Guillen.
Oceanografia de las aguas costeras
1970.
del Peru, Bol. Inst. Mar. Peru-Callao, Vol. 2, No. 5, pp. 157-324,