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A record of the Southern Oscillation Index for the past 2,000 years from precipitation proxies

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The El Niño-Southern Oscillation (ENSO) is a coupled ocean-atmosphere climate phenomenon in the tropical Pacific Ocean. The interannual climate variations have been shown to modify both the Hadley and Walker meridional and zonal atmospheric circulations, with strong impacts on global climate. Proxy-based reconstructions of the Southern Oscillation Index on a multi-decadal scale have shown that the strength and frequency of El Niño occurrences have varied over the past millennium. Here we compile reconstructions of precipitation from regions that experience substantial ENSO variability to extend the multidecadal-scale Southern Oscillation Index to include the past 2,000 years. We find that the Medieval Warm Period (~AD 800-1300) was characterized by a negative index, which indicates more El Niño-dominated conditions, whereas during the Little Ice Age (~AD1400-1850) more La Niña-dominated conditions prevailed. The Southern Oscillation Index we derive is significantly correlated with reconstructions of solar irradiance and mean Northern Hemisphere temperature fluctuations.
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LETTERS
PUBLISHED ONLINE: 14 AUGUST 2011 | DOI: 10.1038/NGEO1231
A record of the Southern Oscillation Index for the
past 2,000 years from precipitation proxies
Hong Yan1, Liguang Sun1*, Yuhong Wang1,2, Wen Huang3, Shican Qiu4and Chengyun Yang4
The El Niño-Southern Oscillation (ENSO) is a coupled
ocean–atmosphere climate phenomenon in the tropical Pacific
Ocean. The interannual climate variations have been shown
to modify both the Hadley and Walker meridional and zonal
atmospheric circulations, with strong impacts on global
climate1–3. Proxy-based reconstructions of the Southern Os-
cillation Index on a multi-decadal scale have shown that the
strength and frequency of El Niño occurrences have varied
over the past millennium4–7. Here we compile reconstructions
of precipitation8–15 from regions that experience substantial
ENSO variability to extend the multidecadal-scale South-
ern Oscillation Index to include the past 2,000 years. We
find that the Medieval Warm Period (AD 800–1300) was
characterized by a negative index, which indicates more El
Niño-dominated conditions, whereas during the Little Ice Age
(AD 1400–1850) more La Niña-dominated conditions pre-
vailed. The Southern Oscillation Index we derive is significantly
correlated with reconstructions of solar irradiance and mean
Northern Hemisphere temperature fluctuations.
The Southern Oscillation (SO) is principally a seesaw trend in
atmospheric mass involving coherent exchanges of air between
the eastern and western Pacific (Supplementary Fig. S1; ref. 2).
The traditional Southern Oscillation index (SOI) is defined as
the difference between the sea level pressure (SLP) of antiphase
oscillatory behaviour at Tahiti, in the eastern Pacific, and Darwin, in
the western Pacific16. Owing to the close link between SLP and local
convection and precipitation, the rainfalls in the western Pacific and
the eastern and mid Pacific are closely related to the SOI. For ex-
ample, Trenberth and Caron (2000; ref. 2) examined the statistical
relationship between the SOI and tropical Pacific precipitation and
showed that the SOI is persistently and positively correlated with
the precipitation over the Indo-Pacific warm pool and negatively
correlated with the precipitation over the eastern and mid-tropical
Pacific (Fig. 1, Areas in the equatorial Pacific where precipitation
was significantly (above 95% confidence level) positively correlated
with the SOI are marked in yellow and red (named positive area, or
PA) and areas with significant negative correlation with the SOI are
marked in green and blue (named negative area, or NA); ref. 2).
The influence of the SO on equatorial Pacific precipitation can
be explained by the variation of the Pacific Walker Circulation3.
During an El Niño event, when the SOI is low, the difference of
SLP between the eastern and western Pacific decreases. As a conse-
quence, the Walker Circulation is weakened, and the ascending limb
in the western Pacific switches to the mid-Pacific, leading to aridity
in the western Pacific and humidity in the mid-Pacific. At the same
time, the eastern tropical Pacific experiences a large increase, up to
an order of magnitude, in precipitation owing to the weakened and
transferred descending limb of the Walker Circulation.
1Institute of Polar Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, China, 2Institute
of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China, 3Department of Dairy Science, University of Wisconsin, Madison, Wisconsin
53706, USA, 4School of Earth and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, China. *e-mail: slg@ustc.edu.cn.
,
100 E 180 140 W 60 W
30 N
EQ
30 S
W1
W2 W3 M1 E1 E2
E3
¬0.55 0.55¬0.35 ¬0.15 0.15 0.35
Figure 1 |Locations of hydrological records. Correlations of monthly
anomalies of precipitation (NCEP reanalysis2) with the SOI from January
1979 to December 2010. Areas in equatorial Pacific with positive and
significant correlation (above 95% confidence level) are marked in yellow
and red, and areas with negative and significant correlation were marked in
green and blue. Locations of the rainfall records in the equatorial Pacific
(W1 (refs 8,11), W2 (refs 10,12), W3 (ref. 9), M1 (ref. 15), E1 (ref. 13), E2
(ref. 14) and E3 (ref. 22)) are also indicated. Locations that were
drier/wetter during the Little Ice Age than during the Medieval Warm
Period are marked in red/blue.
Based on such strong and persistent correlation between the SOI
and precipitation, in this study, we proposed a novel and SOI-like
index, SOIpr, as the difference between normalized annual rainfalls
in the PA of tropical western Pacific and the NA of the equatorial
eastern and mid-Pacific (SOIpr =wpZPwnZN; where ZPand ZN
are the normalized Z-scores of the precipitation in the PA and NA,
respectively. wpand wnare the optimal weights of ZPand ZNand
wp+wn=1. See Methods and Supplementary Fig. S4 for details). To
evaluate the feasibility of using SOIpr as a SOI proxy, we calculated
SOIpr from ad 1951 to 1997 using the instrumental precipitation
data from Galapagos of the eastern Pacific and Indonesia of
the western Pacific (Supplementary Figs S3–S5). We observed a
significant positive correlation between SOIpr and the instrumental
sea level pressure-based SOI (Supplementary Fig. S5, r=0.68,
p<0.0001, neff =40.6 (effective number of independent values; see
Methods for details)), indicating that SOIpr is indeed a good proxy
for the SOI.
Recently, several rainfall reconstructions for the PA of the
western Pacific8–12 and the NA of the eastern13,14 and mid
Pacific15 have been published (Fig. 1). The records from the PA
(refs 8–12) and NA (refs 13–15) contained substantial multi-
decadal variability, characterized by an anti-phase oscillatory
behaviour over the last two millennia (Supplementary Fig. S6),
and allowed us to reconstruct SOIpr for the past two millennia.
Considering these resolution and time span of these records
(Supplementary Table S1 and Fig. S6), in this study we chose the
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1231
precipitation records of Indonesia10 and Galapagos13 to calculate
SOIpr for the past two millennia. The Indonesian historic rainfalls
(ad 25–1955) were derived from a salinity reconstruction based on
planktonic-foraminifera δ18O and the Mg/Ca ratio (ref. 10). The
δ18O of foraminiferal calcite (δ18 Oc) reflects the combined effects of
calcification temperature (T) and seawater oxygen isotope (δ18Ow);
the latter varies as a function of salinity. The Mg/Ca ratio in the
shells is primarily temperature dependent. By measuring both δ18Oc
and the Mg/Ca ratio on the same samples, the salinity component in
the δ18O signal can be extracted10 . The salinity component indicates
the precipitation-caused freshening trend of the surface water in the
Indo-Pacific Warm Pool and is thus an indicator of precipitation.
The rainfall history of the Galapagos (7241 bcad 2004) is derived
from a lake level reconstruction which is based on the grain size
data from the Lago El Junco sediment core13. As suggested in the
hydro-climatic model simulations, El Junco lake sediment grain size
responds sensitively to the precipitation changes associated with the
Pacific Walker Circulation (PWC) and El Niño events, increasing
during wet El Niño events (weak PWC) and decreasing during the
intervening dry periods13.
The reconstructed precipitation in the Indo-Pacific has an
average resolution of 10 years, and that in Galapagos of 11 years.
We adjusted the two records to one year resolution using linear
interpolation, normalized them, and calculated SOIpr for the time
period from ad 50 to ad 1955 (Fig. 2 and Supplementary Fig. S7).
The calculated SOIpr for the past 100 years reveals a multi-
decadal variability similar to that seen in the smoothed instrumental
SOI time series (Supplementary Fig. S8). Smoothed over 11 years
(the averaged resolution of the used palaeo-precipitation record is
11 years), SOIpr and SOI from ad 1867 to ad 1955 have a significant
correlation (Supplementary Fig. S8, r=0.69, p<0.01, neff =14.6).
This result is consistent with our analysis above using instrumental
rainfall records, confirming that SOIpr is a reliable proxy for the
SOI. We also compared our SOIpr with some high resolution ENSO
reconstructions over the past 350 years4,6,7 and found that the
multi-decadal variations of SOIpr and these ENSO reconstructions
are consistent, but the long term trends are not. For example, the
boreal cold-season Niño-3 index, mainly derived from temperature
records, showed an obvious trend towards a La Niña-like state over
the past 350 years7, whereas our SOIpr, derived from hydrological
records, indicated an opposite trend toward a El Niño-like state.
Two other ENSO reconstructions4,6, based on multiple proxies
including temperature records and hydrological records, suggested
no obvious trend over the past 350 years.
The reconstructed SOIpr has five distinct centennial-scale phases
over the past 2,000 years (Fig. 2): persistent negative values during
ad 50–500, ad 1000–1400 and ad 1850–1955, interrupted by
positive values during ad 500–1000 and ad 1400–1850. For the past
millennium, the SOIpr shows three phases associated with the solar
irradiance and background climate state (Fig. 3). Negative SOIpr
values (indicating a weak PWC) are associated with higher solar
irradiance and global mean temperature during the Medieval Warm
Period (MWP) and the so-called Modern Warm Period. On the
other hand, positive SOIpr values (indicating an enhanced PWC)
are concurrent with lower solar irradiance and cooler global mean
temperature during the Little Ice Age (LIA).
The positive SOIpr and enhanced PWC during the relatively
cool LIA period (Fig. 2) suggest a more La Niña-like mean state
than that during the MWP, contradicting current mainstream
theory, mainly from sea surface temperature (SST) reconstruction,
about the tropical Pacific ENSO variability or mean state changes
over the past millennium (Supplementary Fig. S10). Coral-based
SST reconstruction from Palmary Island in the central equatorial
Pacific17 implied a more El Niño-like mean state during the
LIA than that in MWP (Supplementary Fig. S10). The SST
reconstructions in the western10,12, eastern18,19 and mid17 tropical
AD
Galapagos
Indo–Pacific
RWP DACP MWP LIA
SOIpr
Z–scores
Z–scores
Z–scores
0 200 400 600 800 1000 1200 1400 1600 1800 2000
¬3
¬2
¬1
0
1
2
3
¬2
¬1
0
1
2
¬2
¬1
0
1
2
3
Figure 2 |SOIpr reconstruction. SOI proxy SOIpr as the difference between
the reconstructed precipitation records from Indonesia, in the western
Pacific (top; ref. 10), and the Galapagos, in the eastern Pacific (middle)13.
All records were normalized to a standard Z-score before taking the
difference (see Methods for detail). High precipitation and positive SOIpr
are shown in red and low precipitation and negative SOIpr are shown in
blue. Time periods: RWP–Roman Warm Period (AD 50–400),
MWP–Medieval Warm Period (AD 1000–1300), DACP–Dark Ages Cold
Period (AD 500–900), LIA–Little Ice Age (A D 1400–1850).
Pacific suggested20,21 an LIA marked by a relatively warming period
in the eastern and central tropical Pacific, a substantial cooling in the
western equatorial ocean, and a more El Niño-like state (than that
of the MWP; Supplementary Fig. S10). In contrast with SST-based
reconstructions, most of the hydrological reconstructions from
the tropical Pacific suggested a more La Niña-like mean state
during the LIA than during the MWP (Fig. 1 and Supplementary
Fig. S10). These include records from the Indo-Pacific8–12, central
tropical Pacific15 and eastern equatorial Pacific13,14 that suggested
wetter, drier and drier conditions, respectively, during the LIA
than during the MWP (Fig. 1 and Supplementary Fig. S10). On
the other hand, a record of flood deposits near 13S, on the Peru
Margin, indicating wet conditions during the LIA (ref. 22), is also
consistent with a more La Niña-like mean state during the LIA. This
site experiences dry conditions during El Niño events (Fig. 1 and
Supplementary Fig. S2).
Theoretical models and computer simulations also gave contra-
dictory results for the mean state in the LIA and the MWP. The
main external forcing difference between the LIA and the MWP is
believed to be the minimum solar irradiance (Fig. 3) from about
ad 1400 to 1850 (ref. 23) and the near lowest surface temperature
in the Northern Hemisphere (NH) and many other places globally
over the past millennium24. Some models have been proposed for
the response of the tropical Pacific ENSO to decreased solar forcing
and mean global temperature from the MWP to the LIA. The ‘ocean
dynamical thermostat’ mechanism25 predicted a more La Niña-like
state in the MWP than in the LIA. According to this model,
a positive solar forcing and increasing mean global temperature
during the MWP would result in a large zonal temperature gradient
across the equatorial Pacific. In the western tropical Pacific, rising
atmospheric temperature will warm the sea surface, but in the east-
ern equatorial Pacific, surface warming is restrained owing to the
cooling from upwelling. The increased zonal SST gradient enhances
the equatorial trade winds, further drives cooling by upwelling,
and increases the SST gradient. This model prediction is supported
by the SST-based reconstructions17,20,21 over the past millennium.
The most recent coupled general circulation models (CGCM),
however, project a weakening of the atmospheric overturning
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NATURE GEOSCIENCE DOI: 10.1038/NGEO1231 LETTERS
AD
EASM
1000 1200 1400 1600 1800 2000
NAOms
Net radiative forcing
MWP LIA
NH–T
SOIpr
Z–scores
Z–scores
Z–scores
Z–scores
Z–scores
¬3
¬2
¬1
0
1
2
3
4
¬2
¬1
0
1
2
¬4
¬3
¬2
¬1
0
1
2
3
¬3
¬2
¬1
0
1
2
3
¬3
¬2
¬1
0
1
2
3
Figure 3 |Linkage between SOIpr, solar irradiance and Northern
Hemisphere climate. Comparison of long-term proxy records SOIpr,
Northern Hemisphere temperature (NH-T; ref. 24), NAOms (ref. 28),
EASM (ref. 29), and solar irradiance23 over the past millennium. All time
series were 30-years smoothed and normalized to a standard Z-score. The
correlations among them are presented in Supplementary Table S2.
circulation (especially for the Walker Circulation) as the climate
warms, driven by changes in the atmospheric hydrologic cycle26,27.
The water vapour in the lower troposphere increases by roughly
7% per C warming27, whereas precipitation increases more slowly,
approximately by 2% per C warming27. The increased moisture has
to be transported from the atmospheric boundary layer to the free
troposphere, weakening the boundary layer/troposphere mass ex-
change and tropical overturning circulations26,27. This mechanism
predicted a less La Niña-like state in the MWP than in the LIA, in
agreement with our SOIpr reconstruction and other hydrological
records from the tropical Pacific8–15,22.
The tropical Pacific is important for global climate change1,3,9,
and the interaction between tropical Pacific hydrology and
Northern Hemisphere climate is an important aspect of the global
climate. In this study, we examined and found a significant link
between the reconstructed SOIpr and the Northern Hemisphere
temperature for the past millennium (Fig. 3; ref. 24). Negative
SOIpr, indicative of a weak PWC, corresponded to the warm climate
in the Northern Hemisphere during the Medieval Warm Period and
the last century. Positive SOIpr occurred during the relatively cold
LIA. The significant correlation (r=0.54, p<0.05, neff =11.94,
ad 1000–1955) suggested a dynamic link between the hydrology
in the tropical Pacific and the Northern Hemisphere climate,
and this coupling may be established by the interplay between
the PWC and the Northern Atlantic Oscillation (NAO; ref. 28),
monsoonal circulations29 and other mechanisms (see Fig. 3 and
Supplementary Discussion).
Nonetheless, further research is also needed to reconcile the
contradiction between SST and hydrologic reconstructions, to
examine whether and how the centennial-scale variations of the
Pacific Walker Circulation are related to those of the ENSO during
the last millennium, and to study the relationships between the
tropical Pacific ENSO and mid-high latitude climate systems over
the past millennium and their mechanisms.
Methods
SOIpr calculation. To calculate SOIpr, we first normalized the instrumental and
reconstructed precipitation records to the standard Z-score: Z =(XV)/SD; here
X is original value; V and SD are the averaged value and standard deviation of the
time series. SOIpr =wpZPwnZN; here ZPand ZNare the normalized Z-scores
of the precipitation in the PA of the western Pacific and in the NA of the eastern
and mid Pacific, respectively. wpand wnare the optimal weights of ZPand ZN
and wp+wn=1. The values of wpand wnare chosen to optimize the correlation
between SOIpr and the instrumental SOI (Supplementary Fig. S4).
Error estimation of SOIpr.There are two kinds of uncertainties in the reconstructed
SOIpr: non-systematic and systematic. Non-systematic means those errors of local
nature, due to dating uncertainties and index measurement errors. For example,
the real age of a sample could be either earlier or later than the dated one. The
maximum dating uncertainties for the Galapagos record are ±100 years from
ad 50 to 1000 and ±60 from ad 1000 to 1890, based on a basal accelerator mass
spectrometry (AMS) AMS14C date assessment, and ±5 years in recent 110 years,
based on a basal Pb-Cs date assessment13. The index measurement errors for
the Galapagos record are not available, and we use the mean instrumental error
(±1%) instead. The dating uncertainties for the Indonesia record are ±40 years
from ad 50 to 1500 (AMS14C), ±90 years (AMS14 C) from ad 1500 to ad 1900,
and ±5 years (Pb-Cs) in the last century10. The index measurement errors for
Indonesia are available. We used Monte Carlo simulations to estimate this kind of
uncertainty. In each Monte Carlo simulation, to simulate the index measurement
noise, we generated and added Gaussian noise to the proxy records using the
available standard deviation and the Box–Muller algorithm30. The simulation of
dating uncertainties was based on the consideration that the effect of the dating
uncertainty is roughly equivalent to temporally shifting the time series by the
dating error. For each time point, we calculated the minimum and maximum
index values in the window of the dating error and assigned the index value to
an evenly distributed random number between the minimum and maximum
values. After application of these noises, both records were normalized to the
standard Z-score; the difference between the two records was recalculated and
denoted as SOIpr (see Methods above). We repeated the Monte Carlo simulation
Ntimes, obtained the distribution of SOIpr SOIpr, and calculated the standard
deviation for each time point. A Student ttest showed no significant differences
in the calculated standard deviations between N=100 and N=1,000 (p=0.69)
and greater, so we chose 1,000 for N. We estimated this uncertainty of SOIpr as
two standard deviation (95%) confidence intervals and the result is presented in
Supplementary Fig. S7.
The systematic uncertainty arises from the reservoir effect correction, and it
affects the dated ages of all subsamples of a time series globally. For example, an
over-correction of 10 years will shift all sample ages to be 10 years younger than
their actual ages. As SOIpr is the difference between two time series, this kind of
uncertainty could have a significant impact. We also used Monte Carlo simulations
to estimate this uncertainty. The reservoir age for the Indonesia record is 475±80
years10, and the estimated reservoir age for the Galapagos record is 0 ±4 years13. In
each Monte Carlo simulation, we randomly added from 80 to 80 years and from
4 to 4 years, respectively, to the dated ages for the IPWP and Galapagos records
before calculating SOIpr. The other procedures are the same as above. The result of
the error estimation from both the systematic uncertainty and the non-systematic
uncertainty (combined) is also presented in Supplementary Fig. S7.
Correlation analysis. For two time series, Xand Y, the Pearson correlation
coefficient rxy was calculated as
rxy =Pn
i=1(xix)(yiy)
(n1)sxsy
where nis the number of samples, xand yare the sample means of Xand Y, and Sx
and Syare the sample standard deviation of Xand Y.
For two time series (Xand Y) with smoothing, we have to consider and
adjust the autocorrelation in Xand Yby using the effective sample size or
effective number of independent values. Following Trenberth (1984; ref 16),
and Bretherton et al. (1999; ref. 31), we first calculated τ, the time between
independent values (or the time to obtain a new degree of freedom) according to
the following equation32:
τ=1+2
(n1)
X
l=1
rxl ryl
where rxl and ryl are the autocorrelation at lag lfor Xand Y. The effective number
of independent values was calculated as neff =n, and the student t-value for
assessing significance was calculated as
t=rxy neff 2
p(1r2
xy )
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1231
Received 17 May 2011; accepted 13 July 2011; published online
14 August 2011
References
1. Tsonis, A. A., Hunt, A. G. & Elsner, J. B. On the relation between ENSO and
global climate change. Meteorol. Atmos. Phys. 84, 229–242 (2003).
2. Trenberth, K. E. & Caron, J. M. The Southern Oscillation revisited: Sea
level pressures, surface temperatures, and precipitation. J. Clim. 13,
4358–4365 (2000).
3. Wang, C. Atmospheric circulation cells associated with the El Niño–Southern
oscillation. J. Clim. 15, 399–419 (2002).
4. McGregor, S., Timmermann, A. & Timm, O. A unified proxy for ENSO and
PDO variability since 1650. Clim. Past 6, 1–17 (2010).
5. Braganza, K., Gergis, J., Power, S., Risbey, J. & Fowler, A. A multiproxy index
of the El Nino–Southern Oscillation, AD 1525–1982. J. Geophys. Res. 114,
D05106 (2009).
6. Stahle, D. et al. Experimental dendroclimatic reconstruction of the Southern
Oscillation. Bull. Am. Meteorol. Soc. 79, 2137–2151 (1998).
7. Mann, M. et al. Global temperature patterns in past centuries: An interactive
presentation. Earth Interact. 4, 1–1 (2000).
8. Yan, H. et al. South China Sea hydrological changes and Pacific Walker
Circulation variations over the last millennium. Nature Commun. 2,
293 (2011).
9. Tierney, J., Oppo, D., Rosenthal, Y., Russell, J. & Linsley, B. Coordinated
hydrological regimes in the Indo-Pacific region during the past two millennia.
Paleoceanography 25, PA1102 (2010).
10. Oppo, D. W., Rosenthal, Y. & Linsley, B. K. 2,000-year-long temperature and
hydrology reconstructions from the Indo-Pacific warm pool. Nature 460,
1113–1116 (2009).
11. Liu, X. D. et al. A 1,100-year palaeoenvironmental record inferred from stable
isotope and trace element compositions of ostracode and plant caryopses in
sediments of Cattle Pond, Dongdao Island, South China Sea. J. Paleolimnol. 40,
987–1002 (2008).
12. Newton, A., Thunell, R. & Stott, L. Climate and hydrographic variability in the
Indo-Pacific Warm Pool during the last millennium. Geophys. Res. Lett. 33,
L19710 (2006).
13. Conroy, J. L., Overpeck, J. T., Cole, J. E., Shanahan, T. M. & Steinitz-Kannan,
M. Holocene changes in eastern tropical Pacific climate inferred from a
Galapagos lake sediment record. Quat. Sci. Rev. 27, 1166–1180 (2008).
14. Moy, C. M., Seltzer, G. O., Rodbell, D. T. & Anderson, D. M. Variability
of El Nino/Southern Oscillation activity at millennial timescales during the
Holocene epoch. Nature 420, 162–165 (2002).
15. Sachs, J. P. et al. Southward movement of the Pacific intertropical convergence
zone AD 1400–1850. Nature Geosci. 2, 519–525 (2009).
16. Trenberth, K. Signal versus noise in the Southern Oscillation. Mon. Weather Rev.
112, 326–332 (1984).
17. Cobb, K. M., Charles, C. D., Cheng, H. & Edwards, R. L. El Nino/Southern
Oscillation and tropical Pacific climate during the last millennium. Nature 424,
271–276 (2003).
18. Conroy, J. L. et al. Unprecedented recent warming of surface temperatures in
the eastern tropical Pacific Ocean. Nature Geosci. 2, 46–50 (2009).
19. Kennett, D. J. & Kennett, J. P. Competitive and cooperative responses
to climatic instability in coastal southern California. Am. Antiquity 65,
379–395 (2000).
20. Conroy, J. L., Overpeck, J. T. & Cole, J. E. El Nino/Southern Oscillation and
changes in the zonal gradient of tropical Pacific sea surface temperature over
the last 1.2 ka. PAGES News 18, 32–34 (2010).
21. Graham, N. E. et al. Tropical Pacific—mid-latitude teleconnections in medieval
times. Clim. Change 83, 241–285 (2007).
22. Rein, B. et al. El Nino variability off Peru during the last 20,000 years.
Paleoceanography 20, PA4003 (2005).
23. Bard, E., Yiou, Raisbeck & Jouzel, G. Solar irradiance during the last 1200 years
based on cosmogenic nuclides. Tellus 52, 985–992 (2000).
24. Mann, M. et al. Proxy-based reconstructions of hemispheric and global surface
temperature variations over the past two millennia. Proc. Natl Acad. Sci. 105,
13252 (2008).
25. Mann, M., Cane, M., Zebiak, S. & Clement, A. Volcanic and solar
forcing of the tropical Pacific over the past 1000 years. J. Clim. 18,
447–456 (2005).
26. Vecchi, G. A. et al. Weakening of tropical Pacific atmospheric circulation due
to anthropogenic forcing. Nature 441, 73–76 (2006).
27. Held, I. & Soden, B. Robust responses of the hydrological cycle to global
warming. J. Clim. 19, 5686–5699 (2006).
28. Trouet, V. et al. Persistent positive north atlantic oscillation mode dominated
the medieval climate anomaly. Science 324, 78–80 (2009).
29. Zhang, P. Z. et al. A test of climate, sun, and culture relationships from an
1810-Year Chinese cave record. Science 322, 940–942 (2008).
30. Box, G. E. P & Muller, M. E. A note on the generation of random normal
deviates. Ann. Math. Stat. 29, 610–611 (1958).
31. Bretherton, C. S., Widmann, M., Dymnikov, V. P., Wallace, J. M. & Blade, I.
The effective number of spatial degrees of freedom of a time-varying field.
J. Clim. 12, 1990–2009 (1999).
32. Box, G. E. P., Jenkins, G. M. & Reinsel, G. C. Time Series Analysis: Forecasting
and Control Vol. 16 (Holden-Day San Francisco, 1976).
Acknowledgements
Financial support for this research was provided by the Natural Science Foundation of
China (NSFC) (40730107) and the Major State Basic Research Development Program of
China (973 Program) (No.2010CB428902).
Author contributions
H.Y., L.S. and Y.W. designed the study and wrote the paper; Y.W., W.H., S.Q. and C.Y.
contributed to the statistical analysis and improving the English; all authors discussed the
results and implications and commented on the manuscript at all stages.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper on www.nature.com/naturegeoscience. Reprints and permissions
information is available online at http://www.nature.com/reprints. Correspondence and
requests for materials should be addressed to L.S.
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... The difference in sea level pressure (SLP) observed between Tahiti and Darwin, Australia, is the basis for the SOI. The SOI is an indicator of large-scale changes in air pressure that occur from west to east in the tropical Pacific during El Niño and La Niña events (Yan et al. 2011). ...
... However, it's crucial to recognize that droughts can also occur during both El Niño and La Niña events, highlighting the intricate interplay between drought patterns and SSTA (Marengo et al. 2016(Marengo et al. , 2021Thielen et al. 2020). On the other hand, SOI, a measure of monthly surface pressure variations (Yan et al. 2011), demonstrates a negative correlation with SPI and SPEI in most of Mato Grosso. This implies that higher pressure values are generally associated with lower drought severity, but interestingly, droughts have occurred during both low and high SOI values (Fig. 10f). ...
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Precipitation and air temperature have decreased and increased, respectively, in the three biomes of the state of Mato Grosso (Amazon - AMZ, Cerrado - CER, and Pantanal - PAN), making them susceptible to increased drought. The objective of this study was to analyze the trends of severity, intensity and duration of drought events between 1970 and 2022, and their relationships with sea surface temperature anomalies (SSTA) and the Southern Oscillation Index (SOI). The Standardized Precipitation Index (SPI) and Standardized Precipitation and Evapotranspiration Index (SPEI) were calculated using precipitation and air temperature data from the ERA5-land product. The trends of the SPI and SPEI were obtained using the Mann-Kendall (MK) test, and the number of occurrences, severity, intensity, and duration were obtained using the Run-Theory method. The results showed that the SPI and SPEI tended to decrease in the three biomes between 1970 and 2022. Moderate, severe and extreme drought events occurred in all biomes, with emphasis on the highest drought severity in the Pantanal between 2019 and 2022. The highest number of occurrences of extreme droughts by the SPI occurred in AMZ and CER between 2020 and 2022, and the highest values of severity, duration and intensity occurred in CER and PAN in the same period. However, the values of severity, duration and intensity by the SPEI occurred in AMZ and CER. The SPI and SPEI had a similar correlation pattern with the SSTA and SOI indices. The SPI had weak negative correlation with the Atlantic Multidecadal Oscillation (AMO), Southern Oscillation Index (SOI), Tropical Northern Atlantic Index (TNAI) and Tropical Southern Atlantic Index (TSAI), while the SPEI had moderate negative correlation with AMO and TSAI. In conclusion, the results of this study showed that the severity of drought in Mato Grosso increased between 1970 and 2022. The severity of drought in the Pantanal was particularly high between 2019 and 2022. SSTA anomalies and the SOI had some influence on the seasonal and interannual dynamics of drought in Mato Grosso, but not all SSTA anomalies had influence.
... Then, the correlation and significance tests were performed on the two time series at each station. Specifically, for the correlation analysis of interdecadal variation, we conducted an effective degree-of-freedom test on the p-values (Yan et al. 2011). ...
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Near‐surface wind speed (NSWS) reanalysis products have been widely used in NSWS research, yet there is a lack of evaluation for NSWS reanalysis products. Based on the 10 m wind speed observations data from stations during 1980–2017, this article evaluated the climatic characteristics of NSWS over mainland China at various spatiotemporal scales using four reanalysis datasets: ERA5, MERRA‐2, NCEP‐2 and JRA‐3Q. Compared with station observations, ERA5 and NCEP‐2 showed positive biases in the climatology of NSWS, MERRA‐2 showed negative biases and JRA‐3Q showed significantly negative biases. Regarding the spatial distribution of the climatology, ERA5 performed the best with a spatial correlation coefficient of 0.55 with stations. At the interannual scale, ERA5 showed the best performance with a correlation coefficient of 0.68 with stations, significantly better than the other three reanalysis datasets. At the interdecadal scale, MERRA‐2, NCEP‐2 and JRA‐3Q can generally reproduce the decreasing trend in NSWS from 1980 to 2011. However, none of the four reanalysis datasets can capture the increasing trend from 2011 to 2017. In terms of the annual cycle, only ERA5 can well reproduce the seasonal characteristics in different regions, including the maximum NSWS in the north and the Qinghai–Tibet Plateau in spring and the south of the Yangtze River in summer. Considering the evaluation results at different spatiotemporal scales, ERA5 exhibited good performance regarding the spatial distribution of climatology, interannual variability and annual cycle, but failed to reproduce observational features at the interdecadal scale. JRA‐3Q showed significant advantages in terms of interdecadal variability, whereas neither MERRA‐2 nor NCEP‐2 showed prominent advantages in various aspects.
... Although divergent interpretations exist depending on the region, most reconstructions agree that during the MCA, conditions dominated by El Niño (warm phase) prevailed (Conroy et al., 2009(Conroy et al., , 2010Henke et al., 2017;Ledru et al., 2013;Moy et al., 2002;Rustic et al., 2015;Yan et al., 2011). During this period, the Walker circulation weakened or reversed (Lüning et al., 2019). ...
Chapter
Paleoclimate reconstructions are essential for understanding the dynamics of the climate system and its past variations. By utilizing climate-dependent proxies, these reconstructions provide a comprehensive perspective on climatic variations that extend far beyond the limited scope of instrumental records, spanning centuries to millennia. Particularly, proxy-based reconstructions for the last two millennia provide valuable insights into natural climate variability during the preindustrial era and the anthropogenic influence on current climate change. As a result, paleoclimate studies are also critical for interpreting climate projections in the context of anthropogenic forcing. South America, with its vast and diverse climate conditions, is a region rich in high-resolution paleoclimate records, including marine, lacustrine, and fjord sediments, speleothems, ice cores, tree rings, glacial and aeolian deposits, archaeological evidence, and historical documents, among others, all of which capture past climate changes. However, despite numerous paleoclimate reconstructions conducted across the continent and significant advances in understanding its past climate, substantial research gaps remain. These gaps are particularly evident in understudied regions and poorly understood phenomena, hindering a comprehensive understanding of climate variability at both regional and continental scales. To advance paleoclimatic research in South America, future efforts should prioritize (a) the collection of high-resolution records from key locations, (b) the integration of diverse proxies and innovative methodologies, (c) enhancing our understanding of climate-proxy relationships, and (d) developing new proxy calibrations. Collaboration with local communities and indigenous peoples and adopting interdisciplinary approaches will be vital in driving the field forward.
... Conroy et al. (2009) built an index of the ratio of tychoplanktonic to epiphytic diatoms (T/E), ascribing the increase of El Junco Lake level to high values of the index because of enhanced precipitation during warmer Galapagos SST. Cold ENSO-dominated conditions prevailed during the LIA (Yan et al., 2011;Falster et al., 2023), and the harmonic response between the increase of the T/E index from El Junco Lake and the values of Sr and Ba to Ca ratios from K-Inc during LIA (r = 0.36; Fig. 8B) endorses the relationship between warm zonal SST in EEP and mild meridional wind anomalies at the 925-hPa level, with significant reductions in moisture flux into northeastern Mexico (Fig. 7B, C, and 7D). ...
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We reconstructed the hydroclimate of central-eastern Mexico over the last 700 common era (CE) based on inferences from multi-proxies from a stalagmite (K-Inc) collected at Karmidas cave, eastern México. Projections on hydroclimate variability in Mexico raise concerns about possible future occurrences of severe droughts and seasonal water balance fluctuations related to increased global temperatures caused by anthropogenic climate change (Murray-Tortarolo, 2021). The eastern region influences the production and supply of food to Mexico. Simulations of past climates, validated by paleoclimate records, yield valuable perspectives on climate change and enhance our understanding of future projections. However, the paucity of paleoclimatic records hinders understanding past hydroclimatic variations and their climatic mechanisms in eastern Mexico. Our record covers the Little Ice Age (LIA) through the Historical Interval (HI), a crucial period for understanding the climate repercussions spanning the transition from Earth's climatic history to the post-industrial era. The reduced intensity of the North Atlantic Subtropical High (NASH) during the LIA enabled a prominent negative phase of NAO-like variability from 1600 CE until the end of LIA. Consequently, preferent meridional airflow within the continent fosters the encounter of moisture-laden intrusions with the increased frequency of cold surges as the occurrence of frontal rain in eastern Mexico, impairing the amount effect on the K-Inc δ 18 O record. However, after the artificial opening of the cave in 1910 CE, the δ 18 O records of K-Inc began to exhibit a~20-year oscillatory periodicity. In this context, the trace elements of K-Inc help elucidate the climatic conditions that governed the precipitation regime during the investigated period. The visual alignment between the zonal sea surface temperature (SST) variability in the eastern equatorial Pacific (EEP) and the trace elements (Sr/Ca and Ba/Ca) of K-Inc reveals their relationship. Warm zonal SST in the EEP appears to be associated with changes in the length of the winter and summer seasons in eastern Mexico during the LIA. In contrast, over the HI, the trace elements of K-Inc show an anti-phase response to Warm zonal SST in the EEP, denoting wetter climate conditions at the vicinities of Karmidas Cave. This configuration led to questioning the influence of SST zonal variability in the EEP during the HI, which was probably masked by more relevant climate forcing. Our findings enabled us to draw climate scenarios by addressing the main climate drivers in our records.
... Our PWC index exhibits a correlation coefficient of r = 0.71 (P < 0.10) with the Southern Oscillation Index (SOI) reconstruction by Yan et al. (35) over the LM (Fig. 4C). Furthermore, there is a strong consistency (r = 0.87, P < 0.05) between our PWC index and the median of 4800 PWC reconstructions from a global-scale multiarchive compilation of proxy records for the stable isotopic composition of water from 1200 to 1849 CE (Fig. 4D) (36). ...
Article
The South American summer monsoon (SASM) profoundly influences tropical South America's climate, yet understanding its low-frequency variability has been challenging. Climate models and oxygen isotope data have been used to examine the SASM variability over the last millennium (LM) but have, at times, provided conflicting findings , especially regarding its mean-state change from the Medieval Climate Anomaly to the Little Ice Age. Here, we use a paleoclimate data assimilation (DA) method, combining model results and δ18O observations, to produce a δ18O-enabled, dynamically coherent, and spatiotemporally complete austral summer hydroclimate reconstruction over the LM for tropical South America at 5-year resolution. This reconstruction aligns with independent hydroclimate and δ18O records withheld from the DA, revealing a centennial-scale SASM intensification during the MCA-LIA transition period, associated with the southward shift of the Atlantic Intertropical Convergence Zone and the strengthening Pacific Walker circulation (PWC). This highlights the necessity of accurately representing the PWC in climate models to predict future SASM changes.
Article
Biogeochemical tracers of marine productivity and sea surface temperature (SST) are widely used to reconstruct conditions associated with upwelling systems. The connection of these tracers is complex; thus, it is necessary to use several locations or multiproxy approaches to understand upwelling system processes. In the present work, chlorin concentrations were used as tracers of chlorophyll-a to determine marine productivity and the alkenone Uk´ 37 index was used to calculate SST for the last 1350 years. This was done to understand the evolution of upwelling systems in the southern region of the California Current and the possible external factors responsible for its changes in the transitional zone of the Eastern Tropical North Pacific (ETNP). This new data set was contrasted with biogenic opal (BO) and organic carbon (OC) data quantified in the same multicore collected in the Magdalena margin. Chlorin concentrations ranged from 11.3 to 26.8 μg g􀀀 1 from 650 to 2010 CE. Its continuous increase in the last 1350 years was interrupted by two decreases in 1100 to 1200 CE and 1700 to 1800 CE. SST varied from 23.2 to 26 ◦C along the record. In the Medieval Warm Period (MWP), consistently low SSTs were interrupted by an increase to a maximum of 26.1 ◦C at ~1200 years. In comparison, SST showed a centennial variability, but no discernable trend in the Little Ice Age (LIA), rapidly decreasing towards the Current WarmPeriod (CWP). Chlorins, BO and OC suggested that primary productivity increased steadily. SST was relatively cooler in the MWP relative to the LIA, except for a 2 ◦C increase during the middle stage of the MWP and then decreased by 2 ◦C over the last 160 years. The trend of increasing chlorins, BO, and OC suggests that primary productivity was maintained under a predominant regime of La Ni˜na-like oceanographic conditions during the MWP, LIA, and CWP, except for the intervals from 1100 to 1200 CE and 1650 to 1850 CE. Both tracers suggested a reduction in primary productivity. These results demonstrated the complexity of the oceanographic processes that controlled the upwelling system to maintain high marine productivity in the California Current system during the last 1350 years.
Article
The El Niño‒Southern Oscillation (ENSO) is the most influential climatic phenomenon affecting global ecosystems, water use, and agriculture on an interannual scale. However, limited instrumental records make it difficult to fully understand the characteristics of ENSO events. In this study, we used both monthly-resolved Porites coral δ18O records, i.e., living Porites corals of δ18O‒HYDL4 (1992–2015) and subfossil Porites corals of δ18O‒HYD3 (1376–1500 CE), from Huangyan Island in the South China Sea (SCS), to reconstruct the moderate intensity ENSO and sea surface salinity (SSS) during 1376–1500 CE. The results show that the SSS was higher but the frequency of moderate-to-high intensity ENSO events was lower than the present during 1376‒1500 CE. ENSO activity was generally similar to or slightly lower than that of today, but it included several very strong ENSO events in the first substage of 1376–1450 CE, while it was relatively quiet in the second substage of 1451–1500 CE. More moderate-intensity ENSO events may have occurred in the relatively warm climate. The variation in coral δ18O was dominated by multiple factors in this region. SST, ENSO and the Pacific Decadal Oscillation (PDO) may be the dominant factors influencing the changes in coral δ18O at different timescales.
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The authors systematically investigate two easily computed measures of the effective number of spatial degrees of freedom (ESDOF), or number of independently varying spatial patterns, of a time-varying field of data. The first measure is based on matching the mean and variance of the time series of the spatially integrated squared anomaly of the field to a chi-squared distribution. The second measure, which is equivalent to the first for a long time sample of normally distributed field values, is based on the partitioning of variance between the EOFs. Although these measures were proposed almost 30 years ago, this paper aims to provide a comprehensive discussion of them that may help promote their more widespread use.The authors summarize the theoretical basis of the two measures and considerations when estimating them with a limited time sample or from nonnormally distributed data. It is shown that standard statistical significance tests for the difference or correlation between two realizations of a field (e.g., a forecast and an observation) are approximately valid if the number of degrees of freedom is chosen using an appropriate combination of the two ESDOF measures. Also described is a method involving ESDOF for deciding whether two time-varying fields are significantly correlated to each other.A discussion of the parallels between ESDOF and the effective sample size of an autocorrelated time series is given, and the authors review how an appropriate measure of effective sample size can be computed for assessing the significance of correlations between two time series.
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Here we present a high-resolution marine sediment record from the El Niño region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Niño flood events on the continent are used as paleo–El Niño–Southern Oscillation proxy data. The onset of stronger El Niño activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Niño activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Niño events is 60–80 years. El Niño events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Niño flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.
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Atmospheric circulation cells associated with the El Niño-Southern Oscillation (ENSO) are described and examined using the NCEP-NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. The warm phase of ENSO shows positive SST anomalies in the equatorial eastern Pacific and along the east coast of Asia and the west coast of North America, and negative SST anomalies in the off-equatorial western Pacific and in the central North Pacific. Associated with this SST anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the Pacific. The equatorial zonal Walker circulation cell is weakened, consistent with previous schematic diagrams. The anomalous meridional Hadley circulation cell in the eastern Pacific shows the air rising in the Tropics, flowing poleward in the upper troposphere, sinking in the subtropics, and returning back to the Tropics in the lower troposphere. The anomalous Hadley cell in the western Pacific is opposite to that in the eastern Pacific. The divergent wind and vertical velocity also show a midlatitude zonal cell (MZC) over the North Pacific. The mean MZC is characterized by the air rising in the central North Pacific, flowing westward and eastward in the upper troposphere, descending in the east coast of Asia and the west coast of North America, then returning back to the central North Pacific in the lower troposphere. The anomalous MZC during the mature phase of El Niño shows an opposite rotation to the mean MZC, indicating a weakening of the MZC.
Article
Based on a quantitative study of the common fluctuations of 14 Ca nd10Be production rates, we have derived a time series of the solar magnetic variability over the last 1200 years. This record is converted into irradiance variations by linear scaling based on previous studies of sun-like stars and of the sun’s behavior over the last few centuries. The new solar irradiance record exhibits low values during the well-known solar minima centered at about 1900, 1810 (Dalton) and 1690 ad (Maunder). Further back in time, a rather long period between 1450 and 1750 ad is characterized by low irradiance values. A shorter period is centered at about 1200 ad, with irradiance slightly higher or similar to present day values. It is tempting to correlate these periods with the so-called ‘‘little ice age’’ and ‘‘medieval warm period’’, respectively. An accurate quantification of the climatic impact of this new irradiance record requires the use of coupled atmosphere‐ocean general circulation models (GCMs). Nevertheless, our record is already compatible with a global cooling of about 0.5‐1°C during the ‘‘little ice age’’, and with a general cooling trend during the past millenium followed by global warming during the 20th century (Mann et al., 1999).
Article
Archaeological data indicates that socially and politically complex hunter-gatherer societies had become well established on the southern California coast by A.D. 1300. Major developmental changes in sociopolitical complexity are generally considered to have taken place rapidly between AD 1150 and 1300. Recently, two hypotheses have been proposed to account for this rapid cultural evolution, both invoking stressful climatic conditions as an important trigger for cultural change. One suggests that the sociopolitical development was stimulated, in part, by multiple marine and terrestrial subsistence stresses, particularly low marine productivity resulting from regional warming. The other suggests that these developments were largely driven by decreases in terrestrial productivity and water availability linked to drought. Resolution of this debate has been hampered by insufficient paleoclimatic and archaeological data. We present a well-dated, relatively high resolution (25-year intervals) oxygen isotopic marine climate record and new archaeological data from the Northern Channel Islands for the last 3,000 years. These data strongly suggest that changes in human behavior associated with increasing cultural complexity: 1) accelerated after A.D. 500 and became dominant by A.D. 1300, 2) occurred during one of the coldest and most unstable marine climatic intervals of the Holocene (A.D. 450-1300), and 3) coincided with cool, dry terrestrial conditions. Incipient cultural complexity emerged during an interval marked by inferred high marine productivity, reduced terrestrial food and water availability, and large, unpredictable variations in terrestrial resource availability. Our records suggest a strong relationship during this time between climatically induced changes in environmental conditions and social, political, and economic responses, including the emergence of more intensified fishing, and increased sedentism, violence, and trade.
Article
1] Planktonic foraminiferal Mg/Ca and d 18 O derived sea surface temperature and salinity records from the Makassar Strait, Indonesia, show a long-term cooling and freshening trend, as well as considerable centennial-scale variability during the last millennium. The warmest temperatures and highest salinities occurred during the Medieval Warm Period (MWP), while the coolest temperatures and lowest salinities occurred during the Little Ice Age (LIA). These changes in the western Pacific, along with observations from other high resolution records indicate a regionally coherent southern displacement of the Inter-tropical Convergence Zone during the LIA, with more arid conditions in the northern tropics and wetter conditions in the southern tropics. Citation: Newton, A., R. Thunell, and L. Stott (2006), Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium, Geophys. Res. Lett., 33, L19710, doi:10.1029/2006GL027234.
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The response of El Niño to natural radiative forcing changes over the past 1000 yr is investigated based on numerical experiments employing the Zebiak-Cane model of the tropical Pacific coupled ocean-atmosphere system. Previously published empirical results demonstrating a statistically significant tendency toward El Niño conditions in response to past volcanic radiative forcing are reproduced in the model experiments. A combination of responses to past changes in volcanic and solar radiative forcing closely reproduces changes in the mean state and interannual variability in El Niño in past centuries recorded from fossil corals. The dynamics of El Niño thus appear to have played an important role in the response of the global climate to past changes in radiative forcing.
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Using the climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change, this study examines some aspects of the changes in the hydrological cycle that are robust across the models. These responses include the decrease in convective mass fluxes, the increase in horizontal moisture transport, the associated enhancement of the pattern of evaporation minus precipitation and its temporal variance, and the decrease in the horizontal sensible heat transport in the extratropics. A surprising finding is that a robust decrease in extratropical sensible heat transport is found only in the equilibrium climate response, as estimated in slab ocean responses to the doubling of CO2, and not in transient climate change scenarios. All of these robust responses are consequences of the increase in lower-tropospheric water vapor.
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The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series, is the combination (Tn - Dn) where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index (Tn + Dn). It is recommended that this index of noise should also be monitored in order to determine the representativeness of the Southern Oscillation index. -from Author