Drq-Sea
Pergamon
0967_0637(95)EOOO2&8
Research I, Vol. 42, No. 4. pp 42S436. 199.5
Copyright 0 1995 Elsrwer Sc~cnccLtd
Prmted in Great Bmain. All rights reserved
09h706371Y4 $9 50 + 0.w
Currents in the deep ocean off Chile (30”s)
GARY SHAFFER,* SERCIO
SALINAS,~
OSCARPIZARRO,?ANDRESVEGA? and
SAMUELHORMAZABALt
(Received 23 February 1994; in revised form 20 September 1994; uccepted 17 November 1994)
Abstract-Results
are reported from the first recording current meter observations
in the eastern
boundary current system off Chile. Currents at 100 m and 3400 m were observed for a 4-6 month
period during the austral spring-summer-fall
period of 1991-1992 at a deep ocean site 150 km off
the Chilean coast and 70 km seaward of the axis of the Peru-Chile
Trench. Results indicate
energetic inertial oscillations, low eddy kinetic energy at 3400 m, southward flow, decreasing from
austral spring to summer, at 100 m and 3400 m, and quite strong eastward flow, increasing from
spring to summer, at 100 m. The latter result may help toexplain satellite images, showing the zone
of high biological production near the coast in the study region to be very narrow during the austral
summer despite strong coastal upwelling in that season. The authors current observations,
local
geostrophic
current profiles and abyssal potential temperature
distributions
indicate a five-layer
structure for the alongshore flow in the eastern boundary current system off Chile: northward flow
above 100 m, between about 400 m and 1700 m (maximum around 600-700 m. northward transport
of Antarctic Intermediate
Water) and below 3400 m; southward flow from above 100 m to about
400 m (maximum at 180 m, Peru-Chile
Undercurrent)
and from about 1700 m to below 3400. This
proposed vertical structure is consistent with temperature,
salinity, dissolved oxygen and nutrient
distributions
of the study area and bears resemblance
to eastern boundary
current simulations
(MCCREAR’I
etal.. 1987).
INTRODUCTION
THE eastern South Pacific remains one of the least explored, major units of the world
ocean. Our concept of the circulation in this vast region rests almost entirely on tracer
distributions and geostrophic calculations based on sparse hydrographic data collected
over many years (LONSDALE,1976; MANTYLAand REID, 1983; WUNSCHet al., 1983; REID,
1986). A cornerstone for such analyses remains the data from the Scorpio cruises along 28
and 43”s over a quarter of a century ago (STOMMELet al., 1973). In terms of synoptic,
hydrographic data, this situation is improving due to recent World Ocean Circulation
Experiment hydrographic sections across the region. When this new data has been
analyzed an improved picture will emerge. Still, long records of directly observed currents
have been lacking in the eastern South Pacific, records needed to establish the nature and
amplitude of low frequency variability in the region as well as to help pin down absolute
“Niels Bohr Institute of Astronomy,
Physics and Geophysics,
Department
of Geophysics.
University of
Copenhagen,
Haraldsgade
6,220O Copenhagen
N, Denmark and Born6 Institute for Ocean and Climate Studies,
Holma 4235,454OO Brastad, Sweden.
tSchoo1 of Marine Sciences, Catholic University of Valparaiso,
Casilla 1020, Valparaiso,
Chile.
425
426
G.
SHAFER
PI al.
current velocities
and to test the assumption
of stationarity
upon which geostrophic
calculations
and their interpretation
rest.
The eastern boundary current system of the South Pacific-the
Peru-Chile
Currentremains one of the least known of its kind in the world ocean. This is particularly
true off
Chile, where direct current observations
have been lacking even over the shelf and slope
(except for a few current profiles, JOHNSON et al., 1980). Without such observations,
our
present view of the structure of the Peru-Chile
current system also derives from tracer
distributions
and geostrophic
calculations
(WOOSTER and GILMARTIN, 1961), methods
which have been used to address the Peru-Chile
Undercurrent,
for instance (SILVA and
NESHYBA, 1979). In analogy to the currents observed off California or Peru (WINANI rt ~1..
1987; HUYER etnl., 1991), we would expect considerable
low frequency, quasi-geostrophic
variability off Chile, as coastal-trapped
waves (BRINK, 1982), for instance. Such variability
might severely limit the usefulness of geostrophic
calculations,
based on hydrographic
sections across the Peru-Chile
Current, for defining mean current speeds and structures in
this system.
Here the observations
from moored current meters at a station in the deep ocean
seaward of the Peru-Chile
Trench at 30”s (Fig. I) are reported. To the authors knowledge,
these are the first recording
current meter records from the Chile Basin. In fact, the
authors are unaware of any other such records from the eastern half of the South Pacific
ocean between 15% and 55”s. These results are also compared with geostrophic
calculations based on CTD cast data from the same area and time period.
OBSERVATIONS
AND METHODS
A deep sea mooring was deployed at 29”59.3’S and 73”ll. 1 ‘W (Sta. C2, Fig. 1) in a water
depth of 4200 m on 3 November 1991 and recovered on 18 April 1992. This site is located in
76”
72”
74”
70’‘W
28”
.32”S
Fig.
I
The study arca and locations
of moored current
(C2. C4).
meter observations
(C2) and CTD casts
Currentsin the oceanoff Chile
427
a region of low abyssal hills (100-200 m height) at the eastern edge of the Chile Basin about
150 km off the Chilean coast. It lies about 70 km seaward of the axis of the Peru-Chile
Trench which reaches depths of over 6200 m in the study region. As one component of an
ongoing international study of the Peru-Chile Current system in association with the Joint
Global Ocean Flux Study, the mooring was equipped with five Aanderaa RCM 7 current
meters (refitted, former model RCM 4; the RCM 7 features straight-paddle rotor, vectoraveraged currents and a solid state memory) at nominal depths of 100,400,900, 1900 and
3400 m and three sequentially-sampling sediment traps at 400,900 and 3400 m. All RCMs
had speed, direction and temperature sensors, and the one at 100 m had a pressure sensor.
The RCMs at 400 and 900 m failed and inspection of the results from 1900 m indicated
unreliable direction data. Good data over half-hour sampling periods were recovered until
23 February 1992 at 100 m and throughout the mooring period at 3400 m. Data from these
two depths are considered here.
Short cruises within the Peru-Chile Current study wcrc made at the beginning and end
of the mooring period (cruises 1 and 3, respectively) as well as in its middle (cruise 2).
These cruises were made with R.V. Ahare Moha, a new research vessel of Instituto
Foment0 Pesquero, the Chilean fisheries research institute. Here CTD data from deep
casts (to 3000 m, the length of available cable) at Stas C2 and C4 (Fig. 1) on each cruise is
considered. The CTD used was an EG&G Mark III purchased in 1991 and recalibrated by
the manufacturer in March 1992 just before cruise 3. To the original salinity data of cruise 1
and 2,0.02 was added, a decision based on an intercomparison of CTD between 2000 and
3000 m (a depth range of weak currents, see below) for the three cruises at Sta. C2 and on
the assumption that the cruise 3 data with the newly-calibrated CTD were more accurate.
After this correction, we believe that the salinity data of the three cruises are accurate to
within 0.005. The deep temperature data agreed well, to within O.Ol”C, on all three
cruises. The gcostrophic calculations presented below are independent of the abovementioned salinity correction.
At 3400 m, current speeds below the manufacturer’s maximum threshold speed of 2 cm
s _’ were present in 69.5% of the observations; zero rotor rotation (nominal speed of 1.1
cm s-‘) occurred in 59.8% of the observations. Still, regular rotation of the current
direction through tidal cycles always occurred at stall-speeds. In the following, we have
replaced nominal speeds of 1.1 cm s-’ by 0.55 cm ss’ while retaining measured direction.
Spectra were calculated on hourly data by the fast Fourier transform. Before the spectra
were computed, the mean and trend were removed and a Hanning taper was applied.
Rotary energy spectra were calculated according to the method of MOOERS (1973). The
original hourly data were also low passed filtered with a 121-point cosine-Lanczos filter
with half amplitude point at 0.016 c hh’ (60 h).
MOORED
CURRENT
RESULTS
Frequency spectra
The autospectra of the clockwise and anticlockwise kinetic energy density for the meters
at 100 and 3400 m depths are shown in Fig. 2. Both spectra exhibit a spectral gap near
0.016-0.02 c h-’ which separates increasing energy at low frequencies from large energy in
the inertial and tidal bands. The prominent diurnal-inertial peak (0.042 c h-l) is quite
sharp with less spreading toward lower frequencies at the deep meter. Well-defined peaks
also exist in the semidiurnal tidal band and, in particular for the shallow meter, at 0.0125
428
G. SHAFFERet al.
Fig. 2. Clockwise and anticlockwise
spectra of kinetic energy density for current meter records
from 100 m and 3400 m depths at Sta. C2 in Fig. 1. Spectral calculations were made with lo,20 and
40 degrees of freedom for the frequency ranges 10-‘-10~2 cph, lo-‘-lo-’
cph and IO-‘-O..5 cph,
respectively,
and are based on the entire record length of each record.
c h-’ (8 h). The latter peak may represent a harmonic of the energetic, 24 h period motion
as might some of the energy in the semidiurnal
band at 100 m. The spectra of the currents at
3400 m is rather flat toward low frequencies
and the ratio of kinetic energy between the
shallow and deep meters increases by about an order of magnitude from semidiurnal
to the
lowest frequencies.
Anticlockwise
components
are dominant
at frequencies
near and above the diurnalinertial frequency
with the exception
of the semidiurnal
band at 3400 m for which
clockwise and anticlockwise
components
are comparable.
Clockwise currents are dominant at 100 m for low frequencies
but at 3400 m only for frequencies
less than about 0.003
c h- ’(12 d). The evidence points toward inertial oscillations as the dominant motion in the
diurnal-inertial
band. Examination
of the raw records (not shown) for the shallow meter
shows a considerable
background
of diurnal oscillations punctuated
by various, several
day events of strong diurnal oscillations. The strongest of these events can also be traced to
the deep meter record as bursts of diurnal motion. At 30” latitude,
there exists the
possibility of resonant response of inertial oscillations
to diurnal forcing (cf SHAFFER,
1972).
Current statistics
Calculations
of means, variances,
covariances
and eddy kinetic energy for the
passed records of the two meters are shown in Table 1. The values are computed for
complete record length of each meter. Calculations
for the deep current meter using
alternative
interpolation
of currents below the rotor threshold (values below 2 cm
low
the
an
s-’
429
Currents in the ocean off Chile
Table 1.
Depth
Cm)
100
3400
Statistics of current meter records
&!
ii
(cm s-‘)
I;
(cm s-l)
(cm s-‘)
9.4
0.2
-5.3
-0.6
38.3
10.1
2””
I %I,,
U’V’
K,
(cm2sK2) (cm* s-*) (cm* s-*) (cm* s-*)
63.3
1.4
44.3
0.5
28.8
0.3
53.8
1.0
Means, variances and eddy kinetic energies (Ke = f(u’z + ~‘1)) calculated from
the entire length of low-passed current meter records from 100 m and 3400 m depths
at Sta. C2 in Fig. 1. The half amplitude point of the low pass filter used was 60 h.
Positive u and v are toward 90” and O”, respectively. V,,,,, is the maximum speed
observed from the original, half-hourly records.
replaced by 1 cm s-‘, observed direction) yielded values within 10% of those reported in
Table 1 except for u and v which became 0.2 cm ss’ and -0.7 cm s-‘, respectively.
Mean southward flow was observed at 100 m and 3400 m depths. 5.3 cm s-’ and 0.6 cm
respectively, during the study period. The shallow meter apparently sampled
S -I,
poleward flow within the Peru-Chile Undercurrent (WOOSTER and GILMARTIN, 1961). The
deep meter registered weak, but rather stable, southward flow: the mean and standard
deviation of the low-passed data were about equal for this component (Table 1).
Furthermore, the mean southward flow at 3400 m is not an artifact of the subthreshold
interpolation since a recalculation of the v component based on replacement of all
observed current speeds values below 2 cm s-l by zero yields -0.4 cm s-r.
The eastern South Pacific is isolated from the rest of the Pacific below about 3000-3500
m by the East Pacific Rise. An analysis of potential temperature at 3800 m by LONSDALE
(1976) indicates that the Chile and Peru Basins at this depth and below are ventilated from
the south via sills of about 3800 m depth across the Chile Ridge in the southeast corner of
the Chile Basin. The western half of this basin is relatively shallow, 3800 m or less, and the
abyssal, northward flow implied by the potential temperature distribution is likely to be
concentrated at the western flank of the Peru-Chile Trench. Thus, a transition to mean
northward flow might be expected at some depth between 3400 m and the bottom.
The eddy kinetic energy observed at 3400 m, 1.Ocm2 sd2, is among the lowest observed
in the ocean, comparable with the lowest levels found in the deep central and eastern
basins of the Atlantic and North Pacific oceans (TAFT, 1981; SCHMITZet al., 1988). Also,
our value for K, is about three times less than that derived from deep observations near the
eastern boundary of the North Pacific Ocean off California but located about twice as far
offshore as our mooring site (NOBLE et al., 1987). This low level of low-frequency energy is
consistent with the view that the mooring site is too far seaward to be greatly influenced by
coastal-trapped waves-the mooring is 150 km offshore while the local slope width is about
60 km and the local internal deformation radius is about 30 km-or by trench waves for
which motions are strongly trapped over the trench itself (BRINK, 1983). Some of the high
eddy kinetic energy at 100 m reflects the strong trend in the data at that depth, a trend
which we have interpreted as a seasonal change.
Low-frequency
motions
The low passed time series of velocity components for the current meters at 100 m and
3400 m are shown in Fig. 3 (note the different velocity scales in the figure). Both records
430
G.
SHAFEER
et al.
Time
0
20
Ill
I
40
I
I
60
III
60
(days)
I
100
I
I
120
I
,
140
I
/
160
I,
30-
100m
20 -
-30 1
47
3_
h
3400
m
2-
I,
‘:
1991
1992
Fig. 3. Low-passed
time series of the velocity components
from the two current meter records. The half
amplitude point of the low pass filter used was 60 h. Positive u and v are toward 90” and 0”. respectively.
exhibit slow, anticlockwise
rotation over the first two months of the record, with rotation
from southward to eastward flow at 100 m and from westsouthwestward
to eastward flow
at 3400 m. Rather regular fluctuations of about one month period can be seen in the nearly
four month record of u at 100 m; a weak expression of these 30-day oscillations can also be
discerned at 3400 m. The curves in Fig. 3 illustrate the result in Table 1 that low frequency
zonal variability exceeds low frequency meridional variability at the study site.
Eastward flow at 100 m grows steadily throughout
the record resulting in quite strong
mean flow, 9.4 cm s-‘; the mean easterly flow during the last month of the record is 19.2
cm s-r. The flow evolution at 100 m (and at 3400 m) may reflect changes in circulation
Currents in the ocean off Chile
431
patterns on seasonal time scales and large space scales. The record of alongshore flow (Fig.
3) is also consistent with this interpretation: Relatively strong southward flow at 100 m
during November and December in the late austral spring and early austral summer is
followed by a cessation of southward flow for the rest of the summer. Observations off
central Chile (FONSECA,1985) indicate a summer maximum of the salinity within the
salinity maximum of Equatorial Subsurface Water (WOOSTERand GILMARTIN,1961)
advected southward in the Peru-Chile Undercurrent. This summer maximum is consistent
with stronger, southward flow in the months preceding, just as revealed in these
observations within the Undercurrent. Other interpretations are possible for the observed
low frequency, time evolution based on this limited data set. Such might be mesoscale
eddies, El Nine-related anomalies (a weak El Niiio event occurred during 1991-1992) or
secular circulation changes.
It is not unlikely from these results and the analyses of the geostrophic flow presented
below that this eastward flow extends into the ocean surface layer just above. Eastward
surface currents of large horizontal extent would carry oligotrophic, open ocean surface
water toward the coast. Such a flow pattern would be expected to greatly influence primary
productivity in the study area by restraining nutrient-rich, recently-upwelled water, or
waters with high phytoplankton biomass deriving from such nutrient injections, from
spreading seaward. Indeed, as would be expected from the interpretation of observed
eastward currents in terms of seasonal variation, ocean color satellite images of the study
region during the austral summer show blue, oligotrophic water impinging upon the
Chilean coast in the study region (THOMASet al., 1994) despite strong upwelling during this
season.
COMPARISON WITH GEOSTROPHIC CALCULATIONS
Figure 4 shows calculations of geostrophic current from the Sta. pairs C2 and C4 for
cruises 1-3 (curves a-c). Each curve represents current speed in the direction perpendicular to the line between C2 and C4 (positive speed toward 15”) with the absolute velocity
adjusted to the component in this direction of currents observed from the meter at 3400 m
(dots at the bottom of each profile in Fig. 4). The dots represent a five day average at the
beginning and end of the records for cruise 1 and 3, respectively, and a centered 10 day
average for cruise 2. Since geostrophic calculations were only possible to about 3000 m, the
CTD cast length, geostrophic currents were extrapolated to 3400 m using the current shear
calculated for the depth range 2600-3000 m. Since these shears were quite weak, the error
made by this extrapolation is probably small.
Also shown in Fig. 4 is the mean speed and its (low-passed) standard deviation for the
component toward 15” of currents observed at 100 m for cruises 1 and 2 (dots with bars
adjacent to curves a and b, respectively) calculated for the same time window as for the
3400 m currents above. Agreement is quite good between these shallow observed currents
and the estimate at the 100 m level based on calculated geostrophic shear adjusted to tit the
observed deep currents. This agreement is remarkable in the face of the approximations
made (comparison of observed currents from the position of one of the two stations used in
the geostrophic calculations, the point in the center of the line formed by them being about
40 km shoreward of the mooring station, extrapolation of deep CTD data, comparison of
cruise 1 geostrophic calculations with observed current mean values centered several days
afterward).
432
G.
SHAFFER et
al.
Each of the three geostrophic profiles exhibits similar vertical structure: a relative
minimum centered at 150-200 m depth and relative maxima at the surface and in the depth
range 600-1200 m. As for the moored current meter results above, these profiles indicate
southward flow during the austral spring giving way to northward flow in the austral
summer whereby the amplitude of this tendency increases from the bottom toward the
ocean surface. A choice of zero current at 1000 m for the above geostrophic calculations, a
reference level commonly chosen in earlier studies in the region (i.e. SILVAand NESHYBA,
1979) would have resulted in significant changes in the absolute currents of Fig. 4.
The above results encourage an estimate of a mean current profile in the deep ocean
seaward of the Peru-Chile Trench off Chile for the spring-summer-autumn
period of
1991-1992 by averaging the three, adjusted geostrophic profiles of Fig. 4. This mean
profile is shown in Fig. 5 together with mean currents (toward 15’). calculated over the
entire record length of the recording current meter observations at 100 m and 3400 m
(heavy dots in Fig. 5). There is reasonable agreement at the depths of the moored current
meters between means estimated from the three adjusted, geostrophic profiles and means
based on the continuous, recording current meter records.
The mean current profile of Fig. 5 indicates southward (toward 195”) flow in the PeruSpeed
-20
0
-15
“‘t”““““““‘i
-10
-5
( ms
-I
5
1
10
!C
--
Fig. 4. Profiles of alongshore
geostrophic
current (positive toward 15”) calculated from CTD cast data from
Stas C2 and C4 in Fig. 1 and adjusted to observed currents at 3400 m at Sta. C2. For cruise 1 (curve a), casts were
made on 02/11/91. lo:15 (C2) and 03/11/91, 16:30 (C4), for cruise 2 (curve b), casts were made on 06/02/92,20:00
(C2) and 07/02/92.17:30
(C4) and for cruise 3 (curve c), casts were made on 24/04/92,03:20
(C2) and 25/04/92,
2O:lO (C4).
Currents
in the ocean off Chile
-3
Speed
-1
-2
(cm
0
s-‘)
1
433
2
3
4
Fig. 5. Mean protiles of alongshore
current (positive toward 15”) calculated
as the average of the three
adjusted, geostrophicprofilesprescnted
in Fig. 4. The heavy dots represent mean currents (positive toward WC)
at 100 and 3400 m depths calculated over the entire record length of the recording current meter observations
from these depths.
Chile Undercurrent with a maximum of about 2 cm s--l centered at 180 m depth as well as a
deeper equatorward undercurrent with a maximum exceeding 1 cm s-’ in the depth range
500-1100 m. Northward flow is indicated near the surface, approaching 2 cm s-’ at the
surface. The mean current profile exhibits a deep zero crossover near 1700 m below which
southward flow prevails, near 1 cm s-’ at depth.
The mean profile of Fig. 5, although based on limited data, appears to be consistent with
the distribution of water masses in the study region implied by potential temperaturesalinity diagrams (Fig. 6) of the six CTD profiles from Stas C2 and C4 on which we based
our geostrophic calculations. Surface water of low salinity marks net equatorward flow
from higher, rainier southern latitudes. The southward flow maximum in Fig. 5 fits the
shallow salinity maximum in Fig. 6 like a glove. This salinity maximum marks Equatorial
Subsurface Water in the Peru-Chile Undercurrent. The deep salinity minimum in Fig. 6
marking the Pacific Ocean mode of Antarctic Intermediate Water, thought to be formed
off southernmost Chile (MCCARTNEY,1977), coincides with the deep, equatorward
undercurrent of Fig. 5.
Distributions of dissolved oxygen, phosphate and nitrate in the study region at 28”s from
the SCORPIO expedition show an oxygen minimum (nutrient maxima) centered at about
200 m, an oxygen maximum (nutrient minima) centered at about 600 m and an oxygen
434
G.
o-i34.0
34.2
SHAFFER
34.4
et al.
34.6
34.6
31 0
Salinity
Fig. 6. Potential temperature-salinity plots of data from the three CTD casts at Sta. C2 and the three casts at
Sta. C4 used to make the geostrophic calculations presented in this paper. Mean observed depths of the shallow
salinity
maximum
(180 m, Equatorial
Subsurface
Water) and the deep salinity minimum
Intermediate
Water) are indicated as are several sigma-t surfaces.
(620 m, Antarctic
minimum (nutrient maxima) centered at about 1200 m (WARREN, 1973; REID, 1973). The
two shallow structures are most certainly associated with the Peru-Chile Undercurrent
and the deep, equatorial undercurrent carrying Antarctic Intermediate Water. It can be
argued that the structure centered at 1200 m reflects a level of weak advection and long
residence times whereby remineralization leads to low oxygen and high nutrient levels.
Thus, the long term zero crossover inferred in this way lies somewhat above the deep
crossover at about 1700 m from our simple analysis (Fig. 5).
SUMMARY
AND DISCUSSION
The recording current meter observations
reported here from a site at 30”s with a water
depth of 4200 m seaward of the Peru-Chile
Trench are the first such observations from the
eastern boundary current system off Chile. They show the presence of energetic fluctuations of diurnal period throughout the water column, fluctuations which are probably
associated with inertial oscillations. Southward flow, decreasing from austral spring to
summer, is indicated at 100 m and 3400 m. Rather strong, eastward flow, increasing from
spring to summer, was observed at 100 m and may help explain satellite images showing an
unusually narrow zone of high production in the study region although this season is
characterized by strong southerly winds and strong coastal upwelling. However, from the
limited data, it cannot be ruled out that the possibility that the “seasonal” current trends
observed at 100 m depth may be associated with the passing of a mesoscale eddy.
Currents
in the ocean off Chile
435
These observations, when taken together with local geostrophic current profiles,
delineate the following mean alongshore current structure: northward flow above 100 m
and between about 400 m and 1700 m (maximum around 600-700 m); southward flow from
above 100 m to about 400 m and from about 1700 m to below 3400 m. Although this
structure is based on limited data from the spring-summer-autumn
of 1991-1992 only, it is
consistent with the long term mean, alongshore current structure as might be interpreted
from available tracer data from the study area. Furthermore, abyssal potential temperature observations suggest northward flow from below 3400 m to the bottom here. This fivelayer current structure is consistent with rather direct pathways to the study area of
observed water masses with well-defined temperature-salinity
signatures. For instance,
the deep equatorial undercurrent may be a significant conduit of Antarctic Intermediate
Water from its region of formation in the extreme southeastern Pacific to the rest of the
Pacific (given speeds and depth ranges for this undercurrent from Fig. 5, an assumed width
of about 1000 km would lead to a northward transport of about 10 Sv).
Although the data base is limited the authors cannot refrain from offering the following
interpretation for their results: abyssal, northward flow is due to deep internal mixing in
the Chile and Peru basin, mixing which maintains the horizontal density gradient driving
the flow across the sills of the Chile Ridge near 41%. Southward flow between about 1500
m and 3500 m may be associated with the mid-depth outflow of the Pacific deduced by
various authors (REID, 1973; WUNSCHet crl., 1983; REID, 1986). The three-layer current
structure in the upper 1500 m is probably an expression of the eastern boundary current
system off the west coast of South America. For instance, poleward undercurrents are
ubiquitous in such systems (WOOSTERand REID, 1963). Clearly there is a qualitative
resemblance between this paper’s observations and the results of eastern boundary current
models developed by MCCREARY
and co-workers (MCCREARY
and CHAO, 1985; MCCREARY
et al., 1987). In these stratified models, the boundary currents are “spread” away from the
coast beyond the local internal radius of deformation by the/3 effect and horizontal mixing.
In addition to a poleward undercurrent, they predict, for steep topography as in our study
area, the appearance of a deep, equatorial undercurrent akin to the one which emerged in
our analysis.
Acknowled~~menrs-This
work was supported by grants from the Swedish Agency for Rcscarch Cooperation
with Developing
Countries,
the Swedish Natural Science Research Council and the Danish Natural Science
Rcscarch
Council. Support from the Catholic University
of Valparaiso
to S. Salinas and 0. Pizarro is
acknowledged.
We wish to thank Arturo Nakanishi, the captain of Abate Molinu, and the Abate Molina crew,
Dierk Hebbeln, Department
of Geoscience,
University of Bremcn. and Per-lngvar Sehlstcdt and Leif Djurfeldt.
Oceanographic
Institute, University of Gothcnburg,
for their cxccllent help with the field work. Dicrk Hebbcln
was also heavily involved in the design of our joint deep sea mooring.
REFERENCES
BRINK K. H. (1982) A comparison
of long coastal trapped wave theory with observations
off Peru. .I~w& (I/
Physical Ocemngraphy. 12. 897-913.
BRINK K. H. (1983) Some effects of stratification
on long trench waves. Jo~nul of PhysicalOceanography, 13,
496-500.
FONSECAT. (1985) Fisica de las aguas costeras de la zona Central de Chile. Trdku (Chile), 4. 337-354.
HUYER A., M. KNOLL, T. PALUSZKIEWICZ and R. L. SMITH (1991) The Peru Undercurrent:
a study in variability.
Deep-Sea Research, 38(Suppl. 1). 247-279.
JOHNSON D., T. FONSECA and H. SIEVFRS (1980) Upwclling in the Humboldt Coastal Current near Valparaiso.
Journal of Marine Research, 38, 1-16.
436
G. SHAFFERet al.
LONSDALE
P. (1976) Abyssal circulation of the Southeastern Pacific and some geological implications. Journal of
Geophysical Research, 81, 1163-1176.
MANTYLA
A. W. and J. L. REID (1983) Abyssal characteristics of world ocean waters. Deep-Sea Research, 30,
805-833.
MCCARTNEY
M. S. (1977) Subantarctic mode water. Deep-Sea Research, 24(Suppl.), 103-119.
MCCREARY
J. P. and S-Y. CHAO(1985) Three-dimensional shelf circulation along an eastern ocean boundary.
Journal of Marine Research, 43, 13-36.
MCCREARY
J. P., P. K. KUNDUand S-Y. CHAO(1987) On the dynamics of the California Current system. Journal
of Marine Research, 45, l-32.
MOOERSC. N. K. (1973) A technique for cross spectrum analysis of pairs of complex-valued time series, with
emphasis on properties of polarized components and rotational invariants. Deep-Sea Research, 20, 11291141.
NOBLEM., R. C. BEARDSLEY,
J. V. GARDNERand R. L. SMITH(1987) Observations of subtidal currents over the
northern Californian continental slope and adjacent basin: some evidence for local wind forcing. Jourrza/of
Geophysical Research, 92,1709-1720.
REID J. L. (1973) Transpacific hydrographic sections at Lats. 43”s and 285: the SCORPIO Expedition-III.
Upper water and a note on southward flow at mid-depth. Deep-Sea Research, 20,39-49.
REID J. L. (1986) On the total geostrophic circulation of the South Pacific Ocean; flow patterns, tracers and
transports. Progress in Oceanography. 16, 1-61.
SCHMITZW. J., J. F. PRICEand P. L. RICHARDSON
(1988) Recent moored current meter and SOFAR float
observations in the neastern Atlantic near 32”N. Journal of Marine Research, 46,301-319.
SILVAN. and S. NESHYBA(1979) On the southernmost extension of the Peru-Chile Undercurrent. Deep-Sea
Research, 26, 1387-1393.
SHAFFER
G. (1972) A theory of time-dependent upwelling induced by a spatially-and temporally-varying
wind
with emphasis on the effects of a seabreeze-landbreeze cycle. Kieler Meeuesforschung, 28. 139-161.
STOMMELH.,D. D. STROUP,J. L. REID and B. A. WARREN(1973)Transpacific hydrographicsections at Lats. 43”s
and 28%: the SCORPIO Expedition-l.
Preface. Deep-Sea Research, 20, 1-7.
TAFT B. A., S. R. RAMP,J. G. DWORSKI
and G. HOLLOWAY
(1981) Measurements of deep currents in the central
North Pacific. Journal of Geophysical Research, 86, 1955-1968.
THOMASA. C., F. HUANG, P. T. STRIJBand C. JAMES(1994) A comparison of the seasonal and interannual
variability of phytoplankton pigment concentrations in the Peru and California Current systems. Journal of
Geophysical Research, 99,1355-1370.
WARRENB. A. (1973) Transpacific hydrographic sections at Lats. 43”s and 28%: the SCORPIO Expedition-l!.
Deep Water. Deep-Sea Research, 20,9-38.
WINANTC. D., R. C. BEARDSLEY
and R. E. DAVIS(1987) Moored wind, temperature and current observations
made during Coastal Ocean Dynamics Experiments 1 and 2 over the northern California shelf and upper
slope. Journal of Geophysical Research, 92. 1569-1604.
WOOSTER
W. S. and M. GILMARTIN
(1961) The Peru-Chile Undercurrent. Journal of Marine Research, 19, 97122.
WOOSTERW. S. and J. L. REID (1963) Eastern boundary currents. In: The sea, Vol. 2, M. N. HILL, editor,
lnterscience Publications, New York, pp. 253-280.
WUNSCH C., D. Hu and B. GRANT(1983) Mass, heat, salt and nutrient fluxes in the South Pacific Ocean. Journal
of Physical Oceanography.
134,125-753.