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A new synthesis of coral δ18O records to evaluate spatiotemporal characteristics of tropical
decadal variability
T.R. Aulta, J.E. Colea, M.N. Evansa,b, H. Barnetta
a.
b.
University of Arizona, Department of Geosciences
Laboratory of Tree Ring Research, University o Arizona
ABSTRACT:
This study investigates the spatial and temporal scales at which decadal variability is
coherent, significant, and meaningful. 30 annualized coral d18O records are divided into subsets
with roughly uniform temporal coverage. Principal component analysis is then performed on
each subset. SSA and MTM are applied to the leading PCs from each subset as well as the
individual, annualized records.
Results from PCA isolate at least two patterns of variability. One is a strong, roughly
linear trend. The other is a spatial pattern identical to ENSO with variability on interannual to
interdecadal timescales. Evidence of volcanic forcing is present in both patterns.
In the ENSO-like pattern there is interannual variance, PDO-related variability during the
20th century and a 10-12 year cycle during the late 19th century. Prior to the 20th century, we do
not find clear evidence of the PDO. This work does not provide evidence for a coherent ~16yr
cycle in the ENSO system. Nonetheless, the results suggests the full range of tropical variability
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directly linked to the ENSO system is greater than can be observed from 20th century
instrumental records alone. More proxy-based studies of tropical variability during past centuries
to further examine other timescales of variability are needed.
KEYWORDS: SEA-SURFACE TEMPERATURE; PACIFIC; INTERDECADAL
VARIABILITY; DECADAL VARIABILITY; MULTIDECADAL; CLIMATE VARIABILITY;
OSCILLATION; ENSO; DYNAMICS; CORAL; ISOTOPE
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Introduction
During the 20th century, significant interdecadal shifts have been observed in Pacific
instrumental sea surface temperature (SST), sea level pressure (SLP) and wind stress records
[Zhang et al., 1996; Mantua et al, 1997; Garreaud and Battisti, 1999; Deser, 2004]. This low
frequency variability, termed the Pacific Decadal Oscillation (PDO) or Interdecadal Pacific
Oscillation (IPO), has spatial characteristics similar to the El Niño/Southern Oscillation (ENSO)
and is roughly symmetric about the equator [Latif and Barnett, 1994; Gu and Philander, 1997;
Chang et al., 1997; Zhang et al., 1997; Deser, 2004]. Both ENSO and the PDO phenomena
impact agriculture, fisheries, and hydrologic cycles throughout North America and much of the
world [Mantua, 1997; McCabe et al, 2004]. Understanding both the full range of temporal
variability and the radiative forcing sensitivities of these phenomena is essential for making
accurate future projections of ENSO-related behavior.
Recent studies using instrumental records and climate models have proposed several
mechanisms by which Pacific decadal variability may operate [see review by Mestas-Nuñez and
Miller, 2006]. Certain mechanisms call on the advection of midlatitude SST anomalies into the
tropics that directly affect SST gradients [Deser et al., 1996; Gu and Philander, 1997; Zhang et
al., 1998] or thermocline depths [Lysne et al., 1997]. Karspeck and Cane (2002) showed that
tropical equatorial wind gradients alone could reproduce the 1976-1977 PDO shift using a linear,
wind-driven model. Similarly, Mann et al., (2005) and Cane (2005) used a Pacific oceanatmosphere model to show how changes in the radiative budget of the tropics could alter the
equatorial east-west temperature gradient, shifting the system into more El Niño or La Niña-like
conditions.
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Both observational and theoretical studies of Pacific decadal variability, however, are
limited by increasingly poor data coverage further back in time [Kaplan, 1998; Mestas-Nuñez,
2006]. From 1951-1991 the reliability of the instrumental record is high [Kaplan, 1998], but
prior to then, SST records from the tropical Pacific become increasingly rare, incomplete, and
spatially inhomogeneous [Kaplan, 1998], making for large uncertainties in any interpretation of
that data. To study SST variability prior to the instrumental record, we must use proxy data.
Tree-ring based studies extend the PDO index as far back as 1600 [Gedalof and Smith, 2001;
Evans et al., 2001]. Though these indices reliably track Pacific decadal variability during the 20th
century, they do not agree well with each other before 1900 [Evans, 2001]. Moreover, tree ring
reconstructions of the PDO are inherently biased towards terrestrial, high-latitude influences.
Corals, on the other hand, reliably track variability in SST and sea surface salinity (SSS) [see
summaries in Gagan, 2000; Cole, 2003; Lough, 2004]. In warmer or wetter conditions corals
incorporate less of the heavy isotope of oxygen (18O) into their skeletons than during cooler/drier
conditions. This inverse relationship between δ18O and SST has allowed researchers to
reconstruct local SST from individual records spanning several centuries (see references in Table
1) as well as basin-wide Pacific and Indian Ocean SST from the network of records [Evans et
al., 1999; Evans et al., 2001; Wilson et al., 2006]. Because they grow throughout the tropical
Pacific and Indian Oceans, corals are optimally located to record tropical Indo-Pacific variability,
including ENSO [Evans et al., 1998; Evans et al., 2002; Wilson et al., 2006] and tropical decadal
variability.
Our study builds upon previous work by focusing on decadal patterns of variability
resolvable from coral δ18O time series. We are primarily concerned with identifying spatial and
temporal patterns of decadal variability during the last 400 years. In doing so, we also hope to
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show which theoretical mechanisms for producing such variability are most physically consistent
with our results.
2.
Methods
2.1 Data
We examined 30 coral δ18O records from the Indian and Pacific Oceans (figure 1). Two
sites (Palmyra and Mafia) contained data from both the 20th and 17th centuries; we treated these
as a single discontinuous time series. Our Galapagos record is a composite of the Urvina Bay
record (Dunbar et al., 1994) from 1607 to 1953 and Punta Pitt (Cole et al., 1993) from 1953 to
1982. Before combining the two records, we converted both records into anomaly time series by
subtracting the means.
To accurately compare records of monthly, bi-monthly, seasonal, and annual resolution,
we converted each time series into a yearly average. Annualizing on the calendar year tended to
split anomalously high or low δ18O values corresponding to single El Nino/La Nina events into
two moderately anomalous years. Therefore, we annualized records with sub-annual resolution
on the July-June year: the δ18O value for any given year was computed by averaging the
observations from July-Dec of the first year and January to June of the second; we used the
second year as the identifier. Annualization on the July-June year emphasizes the seasonal (i.e.
northern hemisphere winter) expression of individual El Niño/La Niña events. We took no
measures to adjust records that had already been published with annual resolution.
We obtained SST values for the Pacific and Indian Oceans from the Kaplan reanalysis
MOHSSTv6 dataset [Kaplan et al., 1998] available at the Columbia IRI/LDEO climate data
library (http://iridl.ldeo.columbia.edu/). For comparison with our coral records, we annualized
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the SST dataset on the same July-June year as the δ18O records. We also used the extended
NINO3.4 index [Kaplan et al., 1998] from the Columbia IRI/LDEO website and annualized it on
the July-June year.
2.2 Principal Component Analysis
What are the timescales and spatial patterns of the most stable structures of variance in
our δ18O dataset? To answer that question, we applied principal component analysis (PCA)
(Preisendorfer, 1988). Though our longest records cover nearly 400 years, we cannot simply
apply PCA to all records over that entire time domain because individual records have variable
lengths. Instead, we selected five time intervals spanning the last 350 years and divided the data
into corresponding subsets. Records were included in the individual subsets if they contained at
least 90% of their time observations for that interval. This approach ensures that time-dependent
changes we observe within an interval do not result from changes in data coverage. The time
periods chosen were 1890-1990 (T1), 1850-1990 (T2), 1790-1990 (T3), 1703-1990 (T4), and
finally a discontinuous interval from 1650-1703 and 1887-1990 (T5) (see Table 2). We then
performed PCA on the correlation matrix for each subset.
In PCA, singular value decomposition (SVD) is performed on the covariance or
correlation matrix of a suite of time-dependent observations. The empirical orthogonal functions
(EOFs) derived by SVD describe covarying spatial structures and may be projected onto the
original data to determine the corresponding amplitudes (called principal components or PCs) of
those structures over time [Preisendorfer, 1988]. The eigenvalues from SVD may be normalized
to measure the total fraction of variance explained by each EOF/PC pair.
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Performing PCA on the covariance matrix, which preserves the variance differences
among records, did not significantly alter the results. It did, however, tend to heavily weight
central Pacific sites, which have greater interannual variance. We also performed a Monte Carlo
(rule N) significance test [Overland and Preisendorfer, 1982] to establish confidence limits for
the leading eigenvalues.
2.3 Multi-Taper Method
To identify decadal cycles of variability resolvable by our coral δ18O dataset, we
calculated the spectra of the principal components from each subset. To do so, we employed the
multi-taper method (MTM) [Mann and Lees, 1996]. By using multiple orthogonal data tapers to
obtain mutually independent spectral estimates, MTM yields a highly stable spectrum with
relatively high resolution (we used p =2, K =3) [Mann and Lees, 1996]. We performed MTM
using the SSA-MTM toolkit provided by the Department of Atmospheric Sciences at UCLA
[Ghil et al., 2002] (available online at: http://www.atmost.ucla.edu/tcd/ssa/).
2.4 Singular Spectrum Analysis
Although spectral analysis of principal components can yield useful information about the
dominant frequencies within a dataset, it cannot provide information about the spatial expression
of different frequency components [Mann and Park, 1999]. Moreover, spectral analysis of the
principal components cannot resolve coherent but out-of-phase signals that may be present in
several records [Mann and Park, 1999]. To avoid this uncertainty, we performed singular
spectrum analysis (SSA) [Vautard and Ghil, 1989] on the individual annualized δ18O time series
to identify patterns of low-frequency variability. SSA makes use of a maximum lag window (M)
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to identify oscillatory patterns in a single time series and decompose that time series into M
reconstructed components (RCs) [Vautard and Ghil, 1989]. We used an M of 20 years but found
our results to be stable with a range of values for M from 10 to 35 years [Ghil et al., 2002].
3.
RESULTS
Each PC for each subset tracks the temporal amplitude of a spatial pattern over time. All
intervals show that the first PC is significant at the 95% confidence level, and for all but the T4
and T5 intervals the second PC is also significant. The results of PCA on each subset for each
time domain are discussed below (Table 3).
3.1 Trend
Figure 2 shows the first PCs from the T1, T2, and T3 intervals, which appears as a trend
that is strongest since the mid-19th century. The trend appears roughly linear, with substantial
interannual variance, and two brief cool periods following major volcanic eruptions in 1816
(Tambora) and 1883 (Krakataua). By examining the difference between mean δ18O values for the
first and last 10 years of each PC, we estimate the magnitude of the trend to between 0.57 and
0.77‰. If we convert this δ18O value into SST using the equation derived by Grossman and Ku
(1986) (-0.22‰δ18O/1˚CSST), and recently verified for corals by Lough et al. (2004), we would
infer a change of 2.6˚ to 3.6˚C. This SST estimate is greater than instrumental observations
would support. Since coral skeletal δ18O varies with both SST and SSS, freshening of tropical
waters may be responsible for at least part of the trend as well. Using the SSS-δ18O relationship
from Lough et al. (2004) (0.63‰δ18O /1‰SSS), we would estimate a change in SSS between 0.9
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and 1.3‰. The first PC of the T4 and T5 intervals contain similar trends, but they include
interannual variability and are discussed below.
3.2 Interannual to Decadal Variability
Discussed below are the PCs for each subset with the strongest, significant correlation (p
<0.1, N=51) with our annualized NINO3.4 index between 1940 and 1990. In each case, the PC is
significant at the 95% confidence level.
For the 1890-1990 interval, the correlation between NINO3.4 and the second PC (PC2) is
-0.86 (p <0.001). Spatial loadings of the T1-PC2 are consistent with the spatial expression of the
warm phase of ENSO; strong positive loadings (i.e. warming/freshening) occur in the central
Pacific and western Indian Ocean, with moderate negative loadings (cooling/drying) in the south
Pacific. The PC2 time-series is negatively correlated with Pacific and Indian Ocean SST
throughout the tropics and well into the North Pacific (figure 3). Spectral analysis of PC2
indicates significant power at periods of 3-5 years in length with little or no power at decadal
timescales (figure 4). Two shifts in the mean occur coherently with changes in the PDO. The first
takes place during 1941-1942 and the second during 1976-1977. The 1900 and 1924 shifts
described by others (Mantua et al., 1997; Deser et al., 2004) do not appear to be significant
components of our second PC.
As above, PC2 from the 1850-1990 interval is strongly correlated with NINO3.4 (r= 0.89, p <.001). The T2-PC2 loadings and correlation field both strongly resemble ENSO with
evidence for warming/freshening in the central Pacific and western Indian Ocean and
cooling/drying in the south Pacific (figure 3 and figure 5). Spectral power is significant in the 3-5
year band and marginally significan in the 10-12 year band (figure 4). To assess whether the
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decadal power results from the different dataset used or the different time interval analyzed, we
performed PCA on the T2 subset of records over the shorter T1 interval (not shown) and found
no power in the decadal band. This result indicates that 10-12 year variance is not a bias of the
T2 sites, but rather a frequency component with more importance prior to 1890.
Despite the lack of any record from the central Pacific in the T3 subset, the 1790-1990
second PC is significantly correlated with NINO3.4 (r = 0.53, p <0.001). Strong negative
loadings occur throughout the south Pacific, with positive loadings occurring in the eastern
Pacific and western Indian Ocean (figure 3). The spatial correlation field with SST is quite
similar to those from the T1 and T2 PCA results, but there are several key differences. For one,
the correlation field is less strong and less spatially coherent. Also, the second PC from the T3
subset has significant spectral power at different interannual periods (2.6-3 year as opposed to 35 years) and no significant power in the 10-12 year band. It does, however, posses power in the
~25 year band (figure 4). Finally, δ18O values persist at anomalously low levels from 1890-1941,
alternate to anomalously high values from 1942-1977, and return to anomalously low values
until 1990. Although this low frequency oscillation appears in PC2 from the T2 interval, it is
more visible in this 1790-1990 interval.
The first PC from the 1703-1990 interval contains a strong trend and is weakly correlated
with NINO3.4 (r=-0.24, p=0.08) (figure 3 and figure 5). Negative loadings occur on Madagascar,
the eastern coast of Australia, and at Chiriqui, Panama; the only positive loading occurs in the
Galapagos. Because El Niño events produce cooler and drier conditions in the south Pacific,
where strong negative T4-PC1 loadings occur, individual events can be viewed as positive δ18O
pulses (cooling/drying) superimposed on a longer-term negative (warming) trend. The SST
correlation field is weak throughout the Pacific (-0.5 < r < 0.5) and lacks the obvious tropical
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expression of ENSO (figure 3). Instead, strongest negative correlations (i.e. warming) occur in a
latitudinal band between 18°-38° east of the international dateline and on both sites of the
equator. The strongest positive correlations (i.e. cooling) occur in the north Pacific. Multiple
spectral peaks occur between two and five years as well as ~9 years. A marginally significant
peak occurs at ~16 years. PC2 (not shown) describes an out-of-phase relationship between The
Galapagos and Chiriqui, but is not significant at the 95% confidence level.
The first PC from the discontinuous T5, 1636-1702 and 1887-1990, interval is significant
at the 95% confidence level and strongly correlated with NINO3.4 (r =-0.7, p <0.001). Strong
positive loadings occur in the central Pacific and Indian Ocean (Palmyra and Mafia,
respectively), weak positive loadings occur at The Galapagos, and negative loadings appear in
the south Pacific. The correlation field between the leading PC and Indo-Pacific SST closely
resemble the PC and correlation field from the T1 interval (which contained 30 sites). Spectral
analysis of the PC from 1636-1703 and 1887-1990 shows power at ENSO (3-5 year) frequencies
for both the 1636-1703 and 1887-1990 intervals (figure 4). There is also a ~2.7‰ trend during
the 20th century portion of this PC.
3.3 Spectral Analysis
We can examine the importance of decade-scale components in each individual record
more directly by applying SSA & MTM to the individual records. Appendix I shows the
individual MTM spectra of each site. Figure 6 shows the RCs (from SSA) of nine sites with data
going back as far as 1850 and with power at the 10-12 year timescale. These RCs explain
between 5% and 28% of the variance in each series (table 4). These records show decadal
variability that seems to be strongest and most coherent during the mid-late 19th century. Records
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from the central Pacific (Maiana) and western Indian Ocean (Tutia and Seychelles) tend to vary
coherently and in phase with each other but roughly out-of-phase with records from the south
Pacific. This synchronous basin-wide decadal variability can be seen in the amplitude of PC2
from the T2 subset (figure 6) and in the extended NINO3.4 index during the late 19th century.
Essentially, it reflects persistent warm phase/cold phase conditions of the ENSO system.
While most of the individual spectra tend to have power in the ENSO band (2-7 years)
and many of the 1850-1990 sites have power in the 10-12 year band, significant ~16 year
variability is present in five out of the 30 records (Appendix I). Non-significant spectral peaks
also occur at Moorea, the Houtman Abrolhos and Chiriqui. Of these eight, six are included in the
12 site 1790-1990 interval and all are included in the four-site T4 interval. Despite the
importance of this ~16 year variability to individual records, there is only marginal significance
of ~16 year variability in the T4 PC and it is not significant for any PC from any other subset.
4.
DISCUSSION
4.1 Trend
We have resolved two important structures of variance in a network of coral δ18O
records. The first is a roughly linear trend towards more negative values (i.e.
warming/freshening). The second has an ENSO-like spatial expression and variability on
multiple timescales and is discussed below. Figure 2 shows the trend and its spatial loadings for
the T1, T2, and T3 intervals. Nearly all sites except for The Galapagos have positive loadings
with the strongest positive loadings occurring in the southwestern Pacific and Indian Ocean.
These regions also have a strong trend in SST and are hence correlated with the δ18O trend. Since
the trend is greater in magnitude than the instrumental SST trends, we attribute at least some of
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the trend to increases in precipitation (decreased salinity). The negative loading at The
Galapagos would indicate cooling, but this change is not consistent with local SST, which is
relatively constant throughout the last century [Cane et al., 1997]. The weak negative loadings
must therefore indicate changes at the Galapagos associated with interannual variability.
4.2 Interannual to Interdecadal Variability
The second PCs from the T1, T2, and T3 subsets and the first PCs from the T4 and T5
subsets correlate significantly with NINO3.4 and are spatially similar to ENSO. We assume that
these PCs are all recording components of the same ENSO-like phenomenon. Given that the
spatial loadings of this pattern are the same for both interannual and interdecadal timescales, we
examined the PC-SST spatial correlation fields during the 20th century to better understand the
uncertainties associated with any interpretation of decadal variability in these PCs.
For example, the second PCs from 1890-1990 and 1850-1990 correlate strongly with
ENSO centers of action in the tropics and with PDO-sensitive areas in the extra-tropics. These
spatial correlations give us confidence in those PCs’ ability to faithfully track ENSO-like
variability throughout their respective time domains. Similarly, The network of corals used for
PCA from 1790-1850, has a slightly diminished correlation field during the modern period.
Despite this limitation, the density of sites in the south Pacific and Indian Ocean ensures
reasonably high correlation with ENSO and PDO-related SST variability.
On the other hand, the correlation field between the 1703-1990 first PC and SST is weak.
There are relatively few sites included in this subset, and only one (Galapagos) located in and
ENSO center of action. Also, the T4 PC1 seems biased towards recording lower frequency
variability during the 20th century (figure 6). Since we have a poor correlation field with tropical
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Pacific SST, we cannot directly attribute this decadal variability to either the ENSO-system or
the PDO. In light of this uncertainty, we do not attempt to explain decadal variability during the
period from 1703-1790.
In contrast to the T4 subset, the first PC from the T5 interval is strongly correlated with
Pacific SST. Higher correlations occur primarily due to the contribution of records from Mafia
[Damassa et al., 2006] and Palmyra [Cobb et al., 2001]. In fact, the T5-PC1 correlation with
NINO3.4 is even higher than the T3-PC2 correlation with NINO3.4 despite the T3 subset having
many more records. This improvement suggests that just one or two tropical Pacific records may
be sufficient to significantly improve our understanding of the ENSO system prior to 1840.
We now turn our attention to the decadal timescales of variability in each interval. During
the 20th century, we see evidence of PDO shifts in the second PCs from our T1, T2, and T3
subsets. During the late 19th century, we see variance in a 10-12 year band in the T2 interval with
the same spatial characteristics as ENSO (figures 3 & 6). This 10-12 year spectral peak has been
widely discussed in other studies of coral records (Cobb, 2001; Charles et al., 2003; Urban et al.,
2000) and is recorded best in the central tropical Pacific record of Maiana (Urban et al., 2000).
However, we cannot conclusively determine if our 10-12 year cycle is more accurately described
as low frequency ENSO behavior, or high frequency PDO behavior. One reason for this
ambiguity is that the key physical differences between the PDO and ENSO are atmospheric (SLP
and convergent winds) [Garreaud and Battisti, 1999]. Since corals are limited to tracking
changes in SST (and SSS), we can only expect to identify the PDO from low frequency,
persistent changes in the long-term δ18O mean as shown in figure 5.
By comparing our PCs with the Gedalof and Smith (2001) PDO reconstruction, we find
indirect evidence that the late 19th century Pacific decadal variability is tied to ENSO and not the
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PDO. In figure 7, we see the correlations between the second PCs for both the T2 and T3 subsets
are weak during the 19th century, but become considerably more coupled during the 20th. Such
discordant behavior between the tropics and high latitudes suggests the two regions were
behaving relatively independently and hence not analogous to the 20th century PDO; we do not
find evidence of a pre 20th century PDO. Instead, we interpret the 10-12 year cycle as low
frequency behavior of the ENSO system. These results are analogous to, and consistent with, the
findings of Evans et al., (2001). In this context, the extreme El Niño of 1877 can be seen as just
one oscillation of a 10-12 year signal with variance comparable to 20th the century ENSO.
The roughly 16-year variability present in eight records does not seem related to the 1012 year ENSO-like variability for several reasons. For one, it expressed differently, with the
majority of the sites occurring in the south Pacific (New Caledonia, Houtman Abrolhos, Moorea,
New Caladonia, and Abraham’s Reef) as well as the two most distant sites, Ifaty and The
Galapagos. Secondly, ~16 year variability is not significant in any of the spectra of any subsets
except for the 1703-1990 interval. This marginally significant peak looses power if either the
Galapagos or Ifaty are excluded from PCA on the T4 subset, suggesting those two records are
responsible for the bulk of the ~16-year variability in the T4-PC1. Also, the absence of a ~16year peak in the other subsets indicates that the phase of this cycle may be different from site to
site. In this case, the signals would effectively cancel each other out in PCA [Mann and Park,
1999]. Finally, since ~16-year variability is really only a significant component of the T4 subset,
which has the worst correlation field with SST, we cannot conclusively relate this cycle to
variability of the ENSO system.
4.3 Forcing Mechanisms
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We have shown that tropical Pacific SST is synchronized on decadal timescales for a
relatively brief interval from 1860-1910 in a pattern analogous to ENSO. Synchronization
between records before and after that time falls apart (figure 6). Any decadal forcing mechanism
must therefore influence, either directly or indirectly, the ENSO system and cease to be an
important driver of SST during the last 50-75 years.
To explain ENSO-like dynamics of the 10-12 year component, we test the plausibility of
the Bjerknes feedback mechanism [Clement et al., 1996; Cane et al., 1997; Mann et al., 2005;
Cane, 2005] on decadal timescales. The Bjerknes feedback proposes that increased radiative
forcing should produce La Niña-like conditions while decreased radiative forcing should produce
El Niño-like conditions. This feedback occurs because increased radiative forcing will warm the
western Pacific while upwelling prevents SST in the east from increasing, thus enhancing the
east to west SST gradient. Walker circulation will likewise intensify and establish more La Niñalike conditions. Conversely, if radiative forcing was to decrease, the east-west thermal gradient
would weaken, thus allowing the ocean to shift to a more El Nino-like state.
The two most likely mechanisms to affect the radiative budget of the tropics, and hence
the Bjerknes feedback, are solar and volcanic forcing [Cane et al., 2005; Mann et al., 2005].
Mann et al. (2005) produced an ensemble of Zebiak-Cane model simulations forced by the
Crowley solar-volcanic forcing curve [Crowley, 2000] over the last millennia. Their results show
a significant relationship between decreased (increased) radiative forcing and more El Niño-like
(La Niña-like) conditions [Mann et al., 2005; Cane et al., 2005 in both model simulations and
proxy observations [Mann et al., 2005; Cane, 2005]. Similarly, empirical analyses of the
relationship between ENSO and volcanic forcing, shows that an El Niño event is more probable
during the winter following a major, tropical volcanic eruption [Adams et al., 2003]. The
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likelihood of persistent El Niño conditions during subsequent years is also greater [Adams et al.,
2003].
We first examine the role of volcanic forcing (Mann et al., 2005; Adams et al., 2003) in
our records. PC1 from the 1850-1990 and 1790-1990 δ18O subsets (figure 2) both show an abrupt
increase in δ18O values (basin-wide cooling) following the eruption of Krakataua in 1883. At the
same time, the second PCs from 1850-1990 and 1790-1990 show shifts towards El Niño
conditions (figure 8). Another rise in δ18O (i.e. cooling) occurs in the 1790-1990 second PC just
after 1816 and the eruption of Mt. Tambora (figure 8). From these two eruptions, we infer that
tropical volcanic forcing produces a response in the network of corals that is physically
consistent with Adams et al. (2003) and the Bjerknes feedback mechanism. Volcanic events
alone, however, are not enough to produce decadal variability, especially when considering that
the 10-12 year cycle begins well before the 1883 eruption of Krakataua. Similar changes are
present in both PCs of the T2 and T3 intervals for other eruptions.
We now consider a potential relationship between our 10-12 year cycle during the late
19th century and total annual sunspot number (SSN) [Hoyt and Schatten, 1993] (figure 6). During
SSN minima (i.e. low solar irradiance), the 10-12 year δ18O cycles from Maiana (central Pacific)
and the Indian Ocean also reach their minima (indicating warming/freshening). Meanwhile, sites
in the south Pacific experience cooler/drier conditions. PC2 and the NINO3.4 also both suggest
that El Niño conditions occurred during those intervals (figure 6). These oscillations are roughly
synchronous, given the chronological uncertainties in the data, though the eastern Pacific may
lead the rest of the basin by a few years.
The late 20th century should provide an ideal time period to test Bjerknes feedback with
respect to solar forcing because sunspot numbers reached greater amplitudes in the last 50 years
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than any other period during the previous 200 years. However, there is also a strong
warming/freshening trend during that time. It is possible that this masks the 10-12 year variance
in the individual δ18O records. Prior to 1840, on the other hand, we lose many of the records that
actually record the 10-12 year cycle. This lack of spatial coverage makes 10-12 year variability
during the early 19th century intrinsically unresolvable until longer central Pacific records
become available.
The total change in the radiative budget at the tropics due to changes in solar irradiance is
quite small [White, 2006], and therefore additional feedbacks are necessary to account for the
variance explained by the 10-12 year cycles. Furthermore, we cannot rule out the possibility that
Pacific decadal variability is unforced (Newman et al., 2003). Nonetheless, the presence of a
basin-wide, ENSO-like signal, roughly 180° out of phase with SSN, indicates that solar forcing
should not be ruled out as a candidate for pacing tropical decadal variability in the past. Further
evidence is needed to verify the robustness of this phenomenon either through improved tropical
SST proxy resolution or climate model simulations. Any proposed explanation must account for
both the ENSO-like spatial structure of the pattern and multiple timescales on which it operates.
5.
CONCLUSIONS
We observe a strong trend of increasingly negative δ18O values everywhere except the
eastern Pacific. This signal overestimates the trend in instrumental SST, and hence must be at
least in part influenced by freshening of western equatorial waters where the trend is strongest.
Volcanic eruptions in the tropics interrupt this trend, and also produce a response in the ENSO
system that is consistent with the Bjerknes feedback mechanism.
Ault et al.
19
Our results reveal a broader spectrum of ENSO-like variability than can be observed during the
20th century. In the tropical coral δ18O records, ENSO and the PDO are spatially identical and
can only be distinguished by examining their timescales. Prior to the 20th century, however, we
do not find evidence of a PDO. Instead, the tropical Indo-Pacific is dominated by decadal, 10-12
year variability during the late 19th century. As during the 20th century, this variability is spatially
analogous to ENSO. Given impacts of interannual variance on climate, environmental, and
human systems worldwide, the identification of a decadal signal with comparable energy has
significant societal implications; those same impacts prolonged over decadal time scales could be
far more devastating.
Acknowledgements
Julia Cole, Mike Evans, Andrew Comrie, Jonathan Overpeck, Joellen Russel, Dave Meko, Scott
St. George, Heidi Barnet, Kevin Anchukaitis, Tim Shanahan, NOAA, NSF & UA Geoscience
Department
Ault et al.
20
REFERENCES
Adams, J. B., M. E. Mann, and C. M. Ammann, 2003: Proxy evidence for an El Nino-like
response to volcanic forcing. Nature, 426, 274-278.
Allen, M. R. and A. W. Robertson, 1996: Distinguishing modulated oscillations from coloured
noise in multivariate datasets. Climate Dynamics, 12, 775-784.
Allen, M. R. and L. A. Smith, 1996: Monte Carlo SSA: Detecting irregular oscillations in the
presence of colored noise. Journal of Climate, 9, 3373-3404.
Asami, R., T. Yamada, Y. Iryu, C. P. Meyer, T. M. Quinn, and G. Paulay, 2004: Carbon and
oxygen isotopic composition of a Guam coral and their relationships to environmental
variables in the western Pacific. Palaeogeography Palaeoclimatology Palaeoecology, 212,
1-22.
Bjerknes, J., 1969: Atmospheric teleconnections from the equatorial Pacific. Monthly Weather
Review, 97, 163-172.
Boiseau, M., M. Ghil, and A. Juillet-Leclerc, 1999: Climatic trends and interdecadal variability
from South-Central Pacific coral records. Geophysical Research Letters, 26, 2881-2884.
Cane, M. a., 2005: The evolution of El Nino, past and future. Earth and Planetary Science
Letters, 230, 227-240.
Ault et al.
21
Cane, M. A., A. C. Clement, A. Kaplan, Y. Kushnir, R. Murtugudde, and S. Zebiak, 1997:
Twentieth-century sea surface temperature trends. Science, 275, 957-960.
Chang, P., L. Ji, and H. Li, 1997: A decadal climate variation in the tropical Atlantic Ocean from
thermodynamic air-sea interactions. Nature, 385, 516-518.
Charles, C. D., D. E. Hunter, and R. G. Fairbanks, 1997: ENSO and the Monsoon in a coral
record of Indian Ocean surface temperature. Science, 277, 925-928.
Charles, C. D., K. Cobb, M. D. Moore, and R. G. Fairbanks, 2003: Monsoon-tropical ocean
interaction in a network of coral records spanning the 20th century. Marine Geology, 201,
207-222.
Clement, a. C., R. Seager, M. A. Cane, and S. E. Zebiak, 1996: An ocean dynamical thermostat.
Journal of Climate, 9, 2190-2196.
Cobb, K. M., C. D. Charles, and D. E. Hunter, 2001: A central tropical Pacific coral
demonstrates Pacific, Indian, and Atlantic decadal climate connections. Geophysical
Research Letters, 28, 2209-2212.
Cobb, K. M., C. D. Charles, H. Cheng, and R. L. Edwards, 2003: El Nino/Southern Oscillation
and tropical Pacific climate during the last millennium. Nature, 424, 271-276.
Ault et al.
22
Cole, J. E., 2003: Holocene coral records: Windows on tropical climate variability. Global
Change in the Holocene, A. McKay, R. Battarbee, J. Birks, and F. Oldfield, Eds., Arnold,
168-184.
Cole, J. E., R. G. Fairbanks, and G. T. Shen, 1993: Recent Variability in the Southern Oscillation
- Isotopic Results from a Tarawa Atoll Coral. Science, 260, 1790-1793.
Cole, J. E., R. B. Dunbar, T. R. McClanahan, and N. A. Muthiga, 2000: Tropical Pacific forcing
of decadal SST variability in the western Indian Ocean over the past two centuries.
Science, 287, 617-619.
Damassa, T. D., J. E. Cole, H. R. Barnett, T. R. Ault, and T. R. McClanahan, 2006: Enhanced
multidecadal climate variability in the seventeenth century from coral isotope records in
the western Indian Ocean. Paleoceanography, 21, -.
Deser, C., M. A. Alexander, and M. S. Timlin, 1996: Upper-ocean thermal variations in the
North Pacific during 1970-1991. Journal of Climate, 9, 1840-1855.
Deser, C., A. S. Phillips, and J. W. Hurrell, 2004: Pacific interdecadal climate variability:
Linkages between the tropics and the North Pacific during boreal winter since 1900.
Journal of Climate, 17, 3109-3124.
Ault et al.
23
Dunbar, R. B., G. M. Wellington, M. W. Colgan, and P. W. Glynn, 1994: Eastern Pacific SeaSurface Temperature since 1600-Ad - the Delta-O-18 Record of Climate Variability in
Galapagos Corals. Paleoceanography, 9, 291-315.
Evans, M. N., A. Kaplan, and M. A. Cane, 1998: Optimal sites for coral-based reconstruction of
global sea surface temperature. Paleoceanography, 13, 502-516.
Evans, M. N., R. G. Fairbanks, and J. L. Rubenstone, 1998: A proxy index of ENSO
teleconnections. Nature, 394, 732-733.
——, 1999: The thermal oceanographic signal of El Nino reconstructed from a Kiritimati
Island coral. Journal of Geophysical Research-Oceans, 104, 13409-13421.
Evans, M. N., A. Kaplan, and M. A. Cane, 2000: Intercomparison of coral oxygen isotope data
and historical sea surface temperature (SST): Potential for coral-based SST field
reconstructions. Paleoceanography, 15, 551-563.
——, 2002: Pacific sea surface temperature field reconstruction from coral delta O-18 data
using reduced space objective analysis. Paleoceanography, 17, -.
Evans, M. N., M. A. Cane, D. P. Schrag, A. Kaplan, B. K. Linsley, R. Villalba, and G. M.
Wellington, 2001: Support for tropically-driven Pacific decadal variability based on
paleoproxy evidence. Geophysical Research Letters, 28, 3689-3692.
Ault et al.
24
Felis, T., J. Patzold, Y. Loya, M. Fine, A. H. Nawar, and G. Wefer, 2000: A coral oxygen isotope
record from the northern Red Sea documenting NAO, ENSO, and North Pacific
teleconnections on Middle East climate variability since the year 1750.
Paleoceanography, 15, 679-694.
Gagan, M. K., L. K. Ayliffe, J. W. Beck, J. E. Cole, E. R. M. Druffel, R. B. Dunbar, and D. P.
Schrag, 2000: New views of tropical paleoclimates from corals. Quaternary Science
Reviews, 19, 45-64.
Garreaud, R. D. and D. S. Battisti, 1999: Interannual (ENSO) and interdecadal (ENSO-like)
variability in the Southern Hemisphere tropospheric circulation. Journal of Climate, 12,
2113-2123.
Gedalof, Z. and D. J. Smith, 2001: Interdecadal climate variability and regime-scale shifts in
Pacific North America. Geophysical Research Letters, 28, 1515-1518.
Ghil, M., M. R. Allen, M. D. Dettinger, K. Ide, D. Kondrashov, M. E. Mann, A. W. Robertson,
A. Saunders, Y. Tian, F. Varadi, and P. Yiou, 2002: Advanced spectral methods for
climatic time series. Reviews of Geophysics, 40, -.
Gu, D. and S. G. H. Philander, 1997: Interdecadal climate fluctuations that depend on exchanges
between the tropics and extratropics. Science, 275, 805-807.
Ault et al.
25
Guilderson, T. P. and D. P. Schrag, 1999: Reliability of coral isotope records from the western
Pacific warm pool: A comparison using age-optimized records. Paleoceanography, 14,
457-464.
Hoyt, D. V. and K. H. Schatten, 1998: Group Sunspot Numbers: A new solar activity
reconstruction. Solar Physics, 181, 491-512.
Kaplan, A., M. Cane, Y. Kushnir, A. Clement, M. Blumenthal, and B. Rajagopalan, 1998:
Analyses of global sea surface temperature, 1856-1991. Journal of Geophysical Research,
103, 18,567-18,589.
Karspeck, A. R. and M. a. Cane, 2002: Tropical Pacific 1976-77 climate shift in a linear, winddriven model. Journal of Physical Oceanography, 32, 2350-2360.
Kuhnert, H., J. Patzold, K. H. Wyrwoll, and G. Wefer, 2000: Monitoring climate variability over
the past 116 years in coral oxygen isotopes from Ningaloo Reef, Western Australia.
International Journal of Earth Sciences, 88, 725-732.
Latif, M. and T. P. Barnett, 1994: Causes of decadal climate variability over the North Pacific
and North America. Science, 266, 634-637.
Ault et al.
26
Linsley, B. K., G. M. Wellington, and D. P. Schrag, 2000: Decadal sea surface temperature
variability in the subtropical South Pacific from 1726 to 1997 AD. Science, 290, 11451148.
Linsley, B. K., R. B. Dunbar, G. M. Wellington, and D. a. Mucciarone, 1994: A Coral-Based
Reconstruction of Intertropical Convergence Zone Variability over Central-America
since 1707. Journal of Geophysical Research-Oceans, 99, 9977-9994.
Linsley, B. K., G. M. Wellington, D. P. Schrag, L. Ren, M. J. Salinger, and A. W. Tudhope,
2004: Geochemical evidence from corals for changes in the amplitude and spatial pattern
of South Pacific interdecadal climate variability over the last 300 years. Climate
Dynamics, 22, 1-11.
Linsley, R. K., L. Ren, R. B. Dunbar, and S. S. Howe, 2000: El Nino Southern Oscillation
(ENSO) and decadal-scale climate variability at 10 degrees N in the eastern Pacific from
1893 to 1994: A coral-based reconstruction from Clipperton Atoll. Paleoceanography, 15,
322-335.
Lough, J. M., 1997: Regional indices of climate variation: Temperature and rainfall in
Queensland, Australia. International Journal of Climatology, 17, 55-66.
——, 2004: A strategy to improve the contribution of coral data to high-resolution
paleoclimatology. Palaeogeography Palaeoclimatology Palaeoecology, 204, 115-143.
Ault et al.
27
Lough, J. M. and D. J. Barnes, 1997: Several centuries of variation in skeletal extension, density
and calcification in massive Porites colonies from the Great Barrier Reef: A proxy for
seawater temperature and a background of variability against which to identify unnatural
change. Journal of Experimental Marine Biology and Ecology, 211, 29-67.
Lysne, J., P. Chang, and B. Giese, 1997: Impact of the extratropical Pacific on equatorial
variability. Geophysical Research Letters, 24, 2589-2592.
Mann, M. E. and J. Lees, 1996: Robust estimation of background noise and signal detection in
climatic time series. Climate Change, 33, 409-445.
Mann, M. E. and J. Park, 1999: Oscillatory spatiotemporal signal detection in climate studies: A
multiple-taper spectral domain approach. Advances in Geophysics, Vol 41, 41, 1-131.
Mann, M. E., R. S. Bradley, and M. K. Hughes, 2004: Global-scale temperature patterns and
climate forcing over the past six centuries (vol 392, pg 779, 1998). Nature, 430, 105-105.
Mann, M. E., M. A. Cane, S. E. Zebiak, and A. Clement, 2005: Volcanic and solar forcing of the
tropical Pacific over the past 1000 years. Journal of Climate, 18, 447-456.
Mantua, N. J. and S. R. Hare, 2002: The Pacific decadal oscillation. Journal of Oceanography,
58, 35-44.
Ault et al.
28
Mantua, N. J., S. R. Hare, Y. Zhang, J. M. Wallace, and R. C. Francis, 1997: A Pacific
interdecadal oscillation with impacts on salmon production. Bulletin of the American
Meteorological Society, 78, 1069-10079.
Mestas-Nunez, A. M. and A. J. Miller, 2006: Interdecadal variability and climate change in the
eastern tropical Pacific: A review. Progress in Oceanography, 69, 267-284.
Müller, A., M. K. Gagan, and M. McCulloch, 2004: Effects of early marine diagenesis on coral
reconstructions of surface 13C/12C and carbonate saturation state. Global
Biogeochemical Cycles, 18.
Newman, M., G. P. Compo, and M. A. Alexander, 2003: ENSO-forced variability of the Pacific
decadal oscillation. Journal of Climate, 16, 3853-3857.
Overland, J. E. and R. W. Preisendorfer, 1982: A Significance Test for Principal Components
Applied to a Cyclone Climatology. Monthly Weather Review, 110, 1-4.
Pfeiffer, M., W. C. Dullo, and A. Eisenhauer, 2004: Variability of the Intertropical Convergence
Zone recorded in coral isotopic records from the central Indian Ocean (Chagos
Archipelago). Quaternary Research, 61, 245-255.
Ault et al.
29
Preisendorfer, R. W., C. D. Mobley, and T. P. Barnett, 1988: The Principal Discriminant Method
of Prediction - Theory and Evaluation. Journal of Geophysical Research-Atmospheres,
93, 10815-10830.
Quinn, T. M., T. J. Crowley, and F. W. Taylor, 1996: New stable isotope results from a 173-year
coral from Espiritu Santo, Vanuatu. Geophysical Research Letters, 23, 3413-3416.
Quinn, T. M., T. J. Crowley, F. W. Taylor, C. Henin, P. Joannot, and Y. Join, 1998: A
multicentury stable isotope record from a New Caledonia coral: Interannual and decadal
sea surface temperature variability in the southwest Pacific since 1657 AD.
Paleoceanography, 13, 412-426.
Terray, P. and S. Dominiak, 2005: Indian Ocean sea surface temperature and El Nino-Southern
Oscillation: A new perspective. Journal of Climate, 18, 1351-1368.
Thomson, D. J., 1982: Spectrum Estimation and Harmonic-Analysis. Proceedings of the Ieee, 70,
1055-1096.
Tourre, Y. M. and W. B. White, 2003: Patterns of coherent climate signals in the Indian Ocean
during the 20th century. Geophysical Research Letters, 30, -.
Ault et al.
30
Tourre, Y. M., Y. Kushnir, and W. B. White, 1999: Evolution of interdecadal variability in sea
level pressure, sea surface temperature, and upper ocean temperature over the Pacific
Ocean. Journal of Physical Oceanography, 29, 1528-1541.
Tourre, Y. M., B. Rajagopalan, Y. Kushnir, M. Barlow, and W. B. White, 2001: Patterns of
coherent decadal and interdecadal climate signals in the Pacific Basin during the 20th
century. Geophysical Research Letters, 28, 2069-2072.
Tudhope, a. W., G. B. Shimmield, C. P. Chilcott, M. Jebb, A. E. Fallick, and A. N. Dalgleish,
1995: Recent changes in climate in the far western equatorial Pacific and their
relationship to the Southern Oscillation; Oxygen isotope records from massive corals,
Papua New Guinea. Earth and Planetary Science Letters, 136, 575-590.
Tudhope, a. W., C. P. Chilcott, M. T. McCulloch, E. R. Cook, J. Chappell, R. M. Ellam, D. W.
Lea, J. M. Lough, and G. B. Shimmield, 2001: Variability in the El Nino - Southern
oscillation through a glacial-interglacial cycle. Science, 291, 1511-1517.
Urban, F. E., J. E. Cole, and J. T. Overpeck, 2000: Influence of mean climate change on climate
variability from a 155-year tropical Pacific coral record. Nature, 407, 989-993.
Vautard, R. and M. Ghil, 1989: Singular Spectrum Analysis in Nonlinear Dynamics, with
Applications to Paleoclimatic Time-Series. Physica D-Nonlinear Phenomena, 35, 395424.
Ault et al.
31
White, W. B., 2006: Response of tropical global ocean temperature to the Sun's quasi-decadal
UV radiative forcing of the stratosphere. Journal of Geophysical Research-Oceans, 111, .
White, W. B. and Y. M. Tourre, 2003: Global SST/SLP waves during the 20th century.
Geophysical Research Letters, 30, -.
Wilson, R., A. Tudhope, P. Brohan, K. Briffa, T. Osborn, and S. Tett, 2006: Two-hundred-fifty
years of reconstructed and modeled tropical temperatures. Journal of Geophysical
Research-Oceans, 111, -.
Zebiak, S. E. and M. A. Cane, 1987: A model El Niño/Southern Oscillation. Monthly Weather
Review, 115, 2262-2278.
Zhang, Y., J. M. Wallace, and D. S. Battisti, 1997: ENSO-like interdecadal variability: 1900-93.
Journal of Climate, 10, 1004-1020.
Zhang, Z. H., M. E. Mann, and E. R. Cook, 2004: Alternative methods of proxy-based climate
field reconstruction: application to summer drought over the conterminous United States
back to AD1700 from tree-ring data. Holocene, 14, 502-516.
Ault et al.
32
Zinke, J., W. C. Dullo, G. A. Heiss, and A. Eisenhauer, 2004: ENSO and Indian Ocean
subtropical dipole variability is recorded in a coral record off southwest Madagascar for
the period 1659 to 1995. Earth and Planetary Science Letters, 228, 177-194.
Zinke, J., M. Pfeiffer, O. Timm, W. C. Dullo, and G. R. Davies, 2005: Atmosphere-ocean
dynamics in the Western Indian Ocean recorded in corals. Philosophical Transactions of
the Royal Society of London Series a-Mathematical Physical and Engineering Sciences,
363, 121-142.
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FIGURES
’82-’83 SST
Figure 1. Map of showing location of each record (black asterisks) with the annualized (Jul-Jun) SST for the year
1983 (top) and all annualized (Jul-Jun) coral δ18O time series used in this analysis (bottom).
Ault et al.
34
Table 1. Sites, location, references and authors for all data analyzed. The time interval has been adjusted for the
annualization. Mafia (Damassa et al., 2005) and Palmyra (Cobb et al., 2001) are treated as single, discontinuous time
series. The Galapagos record is a composite of the Urvina Bay (Dunbar et al., 1994) and Punta Pitt (Cole et al.,
1993).
Site Name
Location
Reference
Length*
Pemba, Tanzania
5°S, 40°W
Barnett et al., in preparation
1877-1999
Zanzibar, Tanzania
6°S, 39°E
Barnett et al., in preparation
1852-1999
Mange Reef, Tanzania
8°S, 39°W
Barnett et al., in preparation
1889-1996
Tutia, Tanzania
8°S, 39°W
Barnett et al., in preparation
1848-1999
Mafia, Tanzania
8°S, 39°W
Damassa et al., 2006
1618-1997
Malindi, Kenya
3°S, 40°E
Cole et al., 2000
1801-1994
Ifaty, Madagascar
23°S, 43°E
Zinke et al., 2004
1660-1995
Mahe, Seychelles
4°S, 55°E
Charles et al., 1997
1848-1994
La Reunión
21°S, 55°E
Pfeifer et al., 2004
1833-1995
Houtman Abrolhos, Australia
28°S, 113°E
Kuhnert et al., 1999
1796-1993
Ningaloo Reef, Australia
21°S, 113°E
Kuhnert et al., 2001
1880-1994
Bali, Indonesia
8°S, 115°E
Charles et al., 2003
1783-1989
Cebu, Philippines
10°N, 124°E
Pätzold, 1986
1864-1980
Bunaken Island, Indonesia
1°N, 124°E
Charles et al., 2003
1861-1990
Bramble Cay, Papua New Guinea
9°S, 144°E
Cole, in preparation
1775-1993
Double Reef, Guam
13°N, 144°E
Asami et al., 2005
1791-1999
Laing Island, Papua New Guinea
4°S, 144°E
Tudhope et al., 2001
1885-1992
Madang Lagoon, Papua New Guinea
5°S, 145°E
Tudhope et al., 2001
1882-1992
Abraham Reef, Australia
22°S, 153°E
Druffel and Griffin, 1999
1638-1986
New Caledonia
22°S, 166°E
Quinn et al., 1998
1659-1992
Nauru
0°, 166°E
Guilderson and Schrag, 1999
1898-1995
Tarawa Atoll, Kiribati
1°N, 172°E
Cole et al., 1993
1895-1989
Maiana Atoll, Kiribati
3°S, 40°E
Urban et al., 2000
1841-1993
Savusavu Bay, Fiji
17°S, 178°E
Bangato et al., 2005
1776-2000
Palmyra
5°N, 162°W
Cobb et al., 2001
1636-1997
Rarotonga, French Polynesia
21°S, 159°W
Linsley et al., 2000
1768-1996
Moorea, French Polynesia
17°S, 149°W
Boiseau et al., 2003
1852-1990
Clipperton Atoll
10°N, 109°W
Linsley et al., 2000
1895-1993
Urvina Bay, Galapagos
0°, 91°W
Dunbar et al., 1994
1607-1982
Chiriqui, Panama
7°N, 82°W
Linsley et al., 1994
1709-1984
Subset
Interval
Number of sites
T1
T2
1890-1990
1850-1990
30
20
T3
1790-1990
12
T4
T5
1703-1990
1636-1702 & 1887-1990
5
4
Table 2. (Left) Name, interval, and
number of sites in each subset used for
PCA. All records in any given subset
have at least 90% of their time
observations for that interval.
Ault et al.
35
Figure 2. First principal components (left panel) and spatial loadings with (right
panel) from the 1890-1990 (Top), 1850-1990 (Middle), and 1790-1990 (Bottom)
interval subsets. PC curves are shown in units of δ18O (‰ V-PDB). Size and color
indicates relative strength of the normalized loadings; negative loadings are blue,
positive loadings are red according the legend on the bottom right (see scale on
right).
EOF Loading
SST correlation
Table 3 Interval (left column) the PCs we interpret with their characteristic timescale of variability and spatial
structure. The right column shows the portion of total correlation explained by each pattern (i.e. normalized
eigenvalues).
Subset
T1
T2
T3
T4
T5
PC
PC1-Trend
PC2-ENSO-like
PC1-Trend
PC2-ENSO-like
PC1-Trend
PC2-ENSO-like
PC1-Trend+ENSO
PC1-Trend+ENSO
Total correlation
explained
21.00%
16.70%
18.10%
12.90%
24.80%
12.00%
36.20%
37.70%
Ault et al.
Figure 3. Second (top 3 curves, left panel) and first (bottom two
curves, left panel) principal components from each subset. Scales are
inverted for all but the cyan (T4) curve so that up corresponds to El
Niño-like conditions. Spatial loadings are on the right panel
overlaying the PC-SST correlation field (from1940-1990). Subsets are
indicated by the following colors: 1890-1990 (blue), 1850-1990
(green), and 1790-1990 (red), 1703-1990 (cyan), and 1636-1702 &
1888-1990 (purple). PC curves are shown in units of δ18O (‰ VPDB). Size and color indicates relative strength of the normalized
loadings as indicated by the legend on the bottom. Correlation coefficients are
colored such that warm colors correspond to negative SST-δ18O correlations.
36
EOF Loading
SST correlation
Ault et al.
37
Figure 4. Same PCs as in figure 3 from each of the five
subsets shown for the entire interval of analysis along
with Gedalof and Smith (2001) PDO reconstruction and
the extended NINO3.4 index [Kaplan et al., 1998]. All
axes, except for the T4 interval, are oriented so that up
corresponds to more El Niño-like conditions. Thin black
line overlaid on each time series are the 21-point
weighted running means. Light, dotted lines at 1941 and
1976 show recent shifts in the PDO that also appear
present in the coral δ18O records.
Figure 5. Spectra of the PCs in figure 4. The 99%, 95%,
90%, and median confidence levels are shown with thin
dark red lines. PC1 from the T5 subset was divided into
two continuous portions for this analysis: one interval
from 1887 to 1990 (T5a) and another from 1636 to 1702
(T5b).
Ault et al.
38
Figure 6. Annualized NINO3.4 index (top panel), T2-PC2 (second panel), 10-12 year RCs from SSA of individual
records (third panel), and Hoyt Group Sunspot Number (bottom panel). All RCs have been oriented so that their
peaks represent El Niño-like conditions at each site. Grey vertical lines show periods where El Niño-like conditions
coincide with sun spot number (SSN) minima. Negative coefficients are indicated where used and shown in space
on the bottom right (figure 6b), RCs used, and the fraction of variance explained, are shown on the table below.
Site Name
RC
Variance
explained
Tutia
2-3
28.70%
Ifaty
4
5.00%
Seychelles
2-3
14.80%
Houtman Abrolhos 3-4
18.00%
Bramble Cay
5
6.10%
Espiritu Santo
2
8.60% Figure 6b. Map (above) showing the positive and negative coefficients
Maiana
2-3
14.90% used to orient the decadal RCs in figure six. Red plus signs indicate 1,
blue minus signs indicate –1, and open circles show T2 sites where this
Fiji
3-4
15.60%
pattern was not found.
Galapagos
4-5
14.40%
Table 4. Results (above) from SSA performed on 9 records. Time series were selected for SSA if they were
included in the T2 subset and contained RCs with variance at 10-12 year periods. The left column shows names, the
second column indicates with RCs account for 10-12 year variance and column three shows how much variance is
explained by those RCs.
Ault et al.
39
Figure 7. (right) Running 61-point correlation between the Gedalof
and Smith (2001) PDO reconstruction and the ENSO-like PCs from
T2 (green line), T3 (red line) and T4 (cyan line) subsets. The y-axis
is inverted so that up corresponds to negative correlations and more
El Niño-like conditions. Solid portions of the line indicate where the
correlation is significant (p<0.1) and dotted portions of the line show
where it is not.
Figure 8. First and second PCs (Trend and ENSO-like variability, respectively) from both the T2 (top panel, green
lines) and T3 (bottom panel, red lines) intervals. Vertical axes are oriented such that warming in the first PCs and El
Niño-like conditions in the second PCs is upward. Also shown is the Crowley et al (2000) curve of volcanic
radiative forcing. Grey lines indicate eruptions that produced a negative forcing of 2W/m2.or greater.
Ault et al.
Appendix I. MTM spectra of each individual record (black lines) used in this analysis with 90% confidence limits
(red lines). Grey vertical lines mark 16 year, 11 year, 6 year, and 3 year periods.
40