GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L20307, doi:10.1029/2004GL020998, 2004
Interdecadal variability of the thermocline along the west coast of
South America
Oscar Pizarro
Departamento de Fı́sica de la Atmósfera y del Océano, COPAS & PROFC, Universidad de Concepción, Concepción, Chile
Aldo Montecinos
Departamento de Oceanografı́a, COPAS & PROFC, Universidad de Concepción, Concepción, Chile
Received 13 July 2004; accepted 21 September 2004; published 20 October 2004.
[1] Using gridded MBT and XBT data for the period
1955 – 2002, we analyze the interdecadal variability of the
thermocline along the western coast of South America
between the equator and 32°S. Results show a relatively
small, but coherent, interdecadal oscillation of the depth of
different isotherms representing the thermocline along the
region. This oscillation is well correlated with interdecadal
SST anomalies. At the end of the 1960s the thermocline was
on average 10 m shallower than during the beginning of the
1980s, while SST anomalies changed from 0.3°C to 0.5°C
during the same period. The transition from shallow to deep
thermocline depths during the mid-1970s is consistent with
the change from cold to warm conditions observed at the
surface. Changes in the depth of the thermocline base may
modify the properties of the subsurface water that feeds the
coastal upwelling, with further consequences for the
I N DE X T ER MS : 4215
ecosystem of the region.
Oceanography: General: Climate and interannual variability
(3309); 4223 Oceanography: General: Descriptive and regional
oceanography; 4522 Oceanography: Physical: El Nino.
Citation: Pizarro, O., and A. Montecinos (2004), Interdecadal
variability of the thermocline along the west coast of South
America, Geophys. Res. Lett., 31, L20307, doi:10.1029/
2004GL020998.
to the surface layer impacting biological productivity.
Coastal upwelling variability is directly induced by fluctuations of the local alongshore wind stress. However, low
frequency changes of the thermocline depth may modify
the characteristics of the upwelled water, and thereby
the properties of the water in the surface layer. This
phenomenon has been recognized to be important off Peru
[Barber and Chavez, 1983; Huyer et al., 1987] and off
northern Chile [e.g., Morales et al., 1999; Blanco et al.,
2002] during El Niño episodes. Similarly, at decadal and
longer timescales, thermocline fluctuations may also modulate the variability of surface waters and can be associated
with major changes in fisheries and plankton observed
during the 1970s in this region [e.g., Yañez et al., 2001;
Alheit and Ñiquen, 2004]. However, such long-period
fluctuations of the thermocline have been poorly documented along this emblematic upwelling region. Here,
based on gridded expandable bathythermographs (XBT)
and mechanical bathythermographs (MBT) observations
we analyze the interdecadal variability of the thermocline
and its relation to SST along the western coast of South
America in the period 1955– 2002.
2. Data and Methods
1. Introduction
[2] Sea surface temperature (SST) shows significant
interannual and interdecadal variability along the western
coast of North and South America [e.g., Montecinos et al.,
2003]. While interannual SST oscillations along these
regions have been directly related to ENSO dynamics, the
origin of decadal and longer timescale variability in the
Pacific Ocean is unclear and remains an issue of debate and
controversy. One of the most plausible hypothesis to explain
SST fluctuations at interdecadal timescales, point out that
low-frequency disturbances of the thermocline in the eastern
equatorial Pacific modulate the thermocline depth along the
coast of North and South America, and these changes are, in
turn, related to changes in SST [e.g., Clarke and Lebedev,
1999]. Due to coastal upwelling, SST variability off Peru
and Chile may be very sensitive to subsurface changes of
the thermocline depth, which may also affect surface
salinity, concentration of gasses –like oxygen, nitrous oxide
and carbon dioxide – and may modulate the nutrient supply
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL020998$05.00
[3] We analyzed temperature profiles along the west
coast of South America from 1955 to 2002. The data are
monthly gridded values (2-degree latitude by 5-degree
longitude) at standard levels prepared by the Joint Environmental Data Analysis Center (JEDAC) at Scripps Institution
of Oceanography (courtesy of Dr. Warren White). These
data are based on XBT and MBT observations compiled by
the National Oceanographic Data Center. Details of the
quality control and the objective interpolation methods used
can be found at http://jedac.ucsd.edu. In addition, we use
SST, sea level and wind data from Antofagasta (23°300S).
Atmospheric pressure was added to the sea level records,
using a scale factor of 1 cm per hPa, to form adjusted sea
level (ASL) values. Time series of thermocline depth were
calculated for 16 grids off the west coast of South America
between the equator and 32°S. Typically, grids extend from
1 to 5 degrees of longitude offshore. Note that information
is not available for the grid centered at 30°S.
[4] To characterize the thermocline depth we selected two
specific isotherms representing the mid-thermocline and the
thermocline base for each grid point. Then, monthly anomalies of these isotherm depths were used to compute annual
mean anomalies. These annual time series were then
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captured by this data set, as it is shown when a comparison
with other variables (see below) is made.
3. Interdecadal Thermocline Variability
Figure 1. Monthly temperature time series for 3 selected
grids along the western coast of South America, between
the equator and 32°S, based on JEDAC XBT and MBT data
(left panels). Right panels show the long-term mean
temperature profiles. Circles, asterisks, and triangles
indicate the mean depth of the isotherms that are
representatives of the mid thermocline (Zm, maximum
gradient), the base of the upper thermocline (Zb), and the
minimum temperature gradient (Zp) within the upper 400 m,
respectively. The lines in the left panels show the depth of
these selected isotherms.
smoothed – using a 7-year running mean applied twice – to
obtain the interdecadal time series [cf. Montecinos et al.,
2003]. The same filter was applied to other variables.
Finally, interdecadal time series at 30°S were obtained by
linear interpolation of the time series at the neighbor grids.
Note that positive (negative) thermocline-depth anomalies
correspond to shallower (deeper) than normal thermocline
levels.
[5] Monthly temperature time series and their mean
profiles for 3 selected grids are shown in Figure 1. Based
on the mean profiles we selected isotherms to represent the
middle and the base of the thermocline for each grid point.
For the mid-thermocline we chose the isotherm with the
largest vertical temperature gradient (denoted here by Zm).
When the isotherm representing the mid-thermocline
reached the surface, we assumed Zm to be equal to zero.
The isotherm representative of the thermocline base was
defined as the shallower isotherm (below the surface mixed
layer) where the gradient was smaller than 0.03°C m1
(denoted by Zb). On the other hand, below the upper
thermocline near 150– 250 m depth, the vertical gradient
of temperature decreases, creating a local minimum. This is
a common feature in eastern ocean boundaries, related to a
downward bending of the subsurface isotherms near the
coast in connection with an alongshore poleward subsurface
flow. Consequently, we also considered an isotherm representative of this minimum gradient observed below the
surface layer for each grid point (denoted by Zp). Lack of
data during a number of years in some of the coastal grids is
an important limitation – time series in Figure 1 illustrate
the typical data density along the study region – , but we
consider that interdecadal variability is relatively well
[6] Most of the time series of Zm, Zb, and Zp show
positive trends, indicating that the depth of the thermocline
has increased along the western coast of South America
during the last 50 years. The mean Zb trend for the whole
study region is 0.34 m yr1 (17 m per 50 years). However,
the trends at different latitudes differ considerably, varying
from 0.60 m year1 (30 m in 50 years) at 6°S, to 0.12 m
year1 (6 m in 50 years) at 12°S. Thus, to analyze
interdecadal fluctuations we detrended the annual time
series of Zm, Zb and Zp. We use the standard EOF analysis
to obtain a common mode of variability of the thermocline
in the study region.
[7] The first principal components (PC1s) of Zm, Zb, and
Zp explain 52%, 73%, and 60% of the total interdecadal
variance, respectively (Figure 2a). The three PC1s are
relatively well correlated (r = 0.89 and r = 0.90 for PC1
Zm-PC1 Zb, and PC1 Zb-PC1 Zp respectively, and r = 0.66
for PC1 Zm-PC1 Zp). Figure 2a shows that the thermocline
was shallower than average during the period 1965 – 1974
and after 1988, while it was deeper between 1976 and
1986. The correlation between the PC1 and the original
interdecadal time series as a function of latitude is shown in
Figure 2b. The spatial pattern shows that the PC1 Zb is
relatively well correlated with the original interdecadal time
series, presenting significant values along the whole region
(correlations higher than 0.55 are significantly different
Figure 2. First principal components (PC1) of the
interdecadal time series of the depth of three selected
isotherms (Zm, Zb, and Zp see Figure 1) from coastal grids
along the western coast of South America (a). Alongshore
structure of the first EOF mode (b), presented as correlation
between the PC1 and the respective time series. Standard
deviation of the observed time series for each grid point (c).
In all figures the circles, asterisks, and triangles are
associated with Zm, Zb, and Zp time series, respectively.
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from zero at 95% confidence, using an estimated integral
timescale of 2.75 years). PC1 Zp presents larger correlations
near the equator, decreasing southward, while PC1 Zm is
well correlated at subtropical latitudes, decreasing equatorward. On the other hand, the interdecadal variability of Zb
remains rather constant along the coast with a standard
deviation of about 5 m (Figure 2c), while the amplitude of
Zm is small near the equator increasing to about one-half the
amplitude of Zb off the equatorial region. In contrast, the Zp
variability tends to decrease from equatorial to subtropical
latitudes, except in the last grid, where it shows the largest
standard deviation. Therefore, the spatial pattern of the first
EOF mode and the relatively constant interdecadal standard
deviation of Zb along the coast indicate that the PC1 Zb
is the best index representing the common interdecadal
variability of the thermocline along the tropical and subtropical western coast of South America.
[8] Long records of data along the Chilean coast are
scarce. However, near Antofagasta a time series of coastal
temperature and sea level, as well as meteorological data
from the commercial airport, are long enough to address
low frequency variability. Additionally, given the importance of pelagic fisheries during the last decades, the region
off northern Chile is one of the best-sampled regions of the
eastern South Pacific in terms of hydrography; thereby we
have there a relatively independent data set to compare our
results. Figure 3 shows the interdecadal time series of
meridional (near alongshore) wind, ASL and coastal SST
anomalies observed at Antofagasta during part of the second
half of the last century. The time series of Zb in the grid
centered at 24°S is also shown, together with the PC1 Zb. To
us, the most striking feature of the figure is that interdecadal
SST and ASL anomalies apparently do not change in
response to the upwelling favorable wind. In fact, the
maximum equatorward wind observed at the beginning of
the 1980s coincides with a maximum in SST and ASL. On
the other hand, SST and ASL anomalies are (inversely) well
correlated with the depth of the thermocline base anomalies
(r = 0.86 and r = 0.78 for SST and Zb, and for ASL and
Zb, respectively). At this location, the interdecadal Zm and
Zb time series are well represented by the PC1s, as derived
from the correlation shown in Figure 2b. This is also the
case for SST changes along the west coast of South
America, as shown by Montecinos et al. [2003]. These
authors studied the interdecadal variability of SST by mean
of EOF analysis applied to data from 9 coastal stations
between 5°S and 37°S. A linear regression between SST at
Antofagasta and Zb at 24°S showed that – for interdecadal
timescales– an increment of the depth of the thermocline
base in 1.1 m involves an increment of the SST in 0.1°C (r =
0.90). Since both SST [Montecinos et al., 2003] and Zb
interdecadal time series show relatively constant variance
and EOF structure along the coast, a similar relation
between these variables should be valid at other latitudes
in our study region.
[9] We also analyzed the zonal wind stress along the
equatorial Pacific as a possible forcing of the interdecadal
variability observed along the west coast of South America.
Wind stress was estimated using the difference of the sea
level pressure (SLP) between the eastern and the central
equatorial Pacific, according to the methodology proposed
by Clarke and Lebedev [1997]. The SLP at each region was
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Figure 3. Interdecadal time series of meridional wind (a),
sea level and SST (b) anomalies near 23.5°S, and
interdecadal time series of Zb for the coastal grid centered
at 24°S (c). Thin line in (c) is the reconstructed Zb for this
location using the first EOF mode. Bottom panel (d) shows
a simulated thermocline depth in the eastern equatorial
Pacific based on zonal wind stress anomalies estimated from
the zonal sea level pressure gradient between the eastern
and the central equatorial Pacific (see text for details).
computed by averaging the Smith and Reynolds [2004]reconstructed SLP data inside boxes delimited by 2°N and
2°S, and 170°E and the date line (central box), and 100°W
and 90°W (eastern box). Based on the estimated wind
stress, we computed changes of the thermocline depth in
the eastern tropical Pacific (Zeq) using the relationship
proposed by Clarke and Lebedev [1999]. According to this
relation, long timescale fluctuations of Zeq are proportional
to the integral of the zonal wind stress along the equatorial
Pacific. Effectively, Zeq presents interdecadal changes –
with a minimum depth during the mid-1970s and a maximum at the beginning of the 1980s – that are notably similar
to those observed in coastal SST (with opposite sign) and Zb
(Figure 3d).
[10] Hydrographic data from off northern Chile (19°–
24°S, 72° –73°W) were also used to estimate interdecadal
changes of the base of the thermocline (Zbh). Large gaps in
the data prevented the possibility of having a continuous
interdecadal time series. Thus, based on our previous
results, we calculated average of Zbh on these data for
different periods (Figure 4). These results are consistent
with those based on the JEDAC data shown in Figure 2a.
4. Discussion and Conclusions
[11] During the second half of the last century, the
thermocline along the west coast of South America presented relatively small – but significant, and spatially
coherent – interdecadal variability. At this timescale, the
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depth of the thermocline base changed about 10 m, thus,
during the early 1980s Zb was about 5 m deeper than
average, coinciding with a period of positive SST anomalies
of about 0.6°C. In contrast, during the end of the 1960s, the
thermocline was shallower and SST presented anomalies of
about 0.3°C. The transition from positive (negative) to
negative (positive) anomalies of the thermocline depth
(SST) occurred during mid-1970s, while an inverse transition seems to have occurred during the second half of the
1980s. The mid-1970s change has been widely documented,
and is frequently identified as a climate regime shift, while a
most recent change to cold conditions during the 1990s is a
matter of present investigation [e.g., Chavez et al., 2003].
Such a transition in the eastern South Pacific is also
suggested by our results.
[12] At interdecadal timescales, an increasing of the
coastal SST in 0.1°C corresponds to a deepening of the
thermocline of about 1.1 m – within a coastal region
extending a few hundred kilometers offshore. Such a
change is in agreement with the variability of the eastern
equatorial Pacific thermocline depth, estimated from the
zonal wind anomalies in the equatorial Pacific. In contrast,
changes of the local wind at Antofagasta (Figure 3a) and the
integral of the alongshore wind stress from the equator to
24°S (based on COADS data, not shown) are not able to
reproduce the observed interdecadal thermocline fluctuations. These results support the idea [e.g., Clarke and
Lebedev, 1999] that long time oscillations of the thermocline depth, as well as SST, along the Pacific coast of the
Americas are controlled by changes in the equatorial zonal
wind.
[13] Montecinos et al. [2003] showed that the relative
importance of the interdecadal SST signal increases with
latitude in the subtropical region, reaching similar values as
those for the interannual SST signal. Here, the variability
(standard deviation) of Zb and Zp between the equator and
10°S at interannual timescales is about 4 –5 times larger
than that observed at interdecadal timescales. Farther south,
this variability is reduced to about 2 –3 times the interdecadal one. It is worth noting that the correlation between the
PC1 SST and PC1 Zb at interannual timescales is also
highly significant (r = 0.85).
[14] If the observed interdecadal fluctuations of the
thermocline depth were responsible for a coherent and
persistent change of the SST in the region, then these
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Figure 5. Annual landing of anchovy (solid line) and
sardine (dashed line) from northern Chile between about
18.5°S and 24°S [from Yañez et al., 2001]. Low-pass
filtered time series for the same species are shown by the
smooth lines.
changes may also have an impact on the nutrient upwelling
and on other variables that regulate biological productivity
and plankton distribution, and so modulate the abundance
and distribution of commercial fisheries. Figure 5 shows the
annual landings of anchovy and sardine in northern Chile.
These fisheries have experienced long time changes during
the last 50 years. A comparison of the interdecadal time
series of the anchovy and sardine landings with Zb shows
that a shallower thermocline depth is related to an increase
of anchovy catches, while the sardine catches seem to
fluctuate with an opposite phase. These results are consistent with the basin-wide ecosystem response presented by
Chavez et al. [2003].
[15] Here we have quantified the magnitude of the
interdecadal variability of the thermocline in the eastern
South Pacific, and showed that these changes are of the
order of a few meters. They are rather small compared with
the interannual oscillations observed commonly in this
region –which are of the order of several tens of meters – ,
but still they have a significant imprint on the SST. It will
be exciting to search for the influence of these small, but
long-lasting, changes of the thermocline depth on the
regional fishery fluctuations.
[16] Acknowledgments. Gridded data were obtained from the Joint
Environmental Data Analysis Center. The Hydrographic and Oceanographic
Service of the Chilean Navy (SHOA) and the Chilean Meteorological
Service (DMC) provided coastal data. This work was supported by grants
from the Chilean National Research Council (FONDECYT 1020294, FIP
2003-33 and FONDAP COPAS) and Fundación Andes, Chile. AM was
supported by a MECESUP/UCO-0002 graduate scholarship.
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Figure 4. Annual mean values of the thermocline base off
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for different periods. The thick curve shows the PC1 Zb
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A. Montecinos, Departamento de Oceanografı́a, COPAS & PROFC,
Universidad de Concepción, Concepción, Chile.
O. Pizarro, Departamento de Fı́sica de la Atmósfera y del Océano,
COPAS & PROFC, Universidad de Concepción, Concepción, Chile.
([email protected])
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