Central Pacific coral evidence for increased ENSOvariance over the past 6,000 years

Helen McGregor, Steven Phipps, Matthew Fischer, Michael Gagan,

Laurent Devriendt, Andrew Wittenberg, Colin Woodroffe, Jian-Xin Zhao,

Jessica J. Gaudry, David Fink, Allan Chivas

Contributions from paleo-ENSO reconstructions

2. Extend the ENSO records to periods when the forcing is different

1. Extend the instrumental record

(e.g. Holocene; orbital forcing changes in seasonal distribution of insolation)

Reduced ENSO variance in the mid-Holocene, but…

J. Lough

<<

Paleo-ENSO variance 50-80% reduction

Model ENSO variance 10-15% reduction

Is Holocene ENSO variance really reduced?

Cobb et al. 2013 Science

Compilation of Holocene coral records suggests otherwise

Test the response of ENSO to changes in orbital forcing over the past 6,000 years

SST anomaly (°C)

• 255 years of new Poritescoral d18O data (non-continuous) combined with published records

• Single site, Kiritimati(Christmas) Island in the heart of the NINO3.4 region

• Well-preserved material

• U-Th dated

Porites coral microatolls at Kiritimati Is.

4 cm

Porites microatoll d18O and instrumental climate data

McGregor et al. Geochim. Cosmochim. Acta 2011

~4,300 yrBP microatoll – reduced ENSO

Positive anomaly years (El Niño)

Negative anomaly years (La Niña) Neutral years

Absence of moderate-strong eventsMcGregor et al. (2013) Nature Geoscience

A slight digression…

% of interannual record explained by the each year type

Inst. SST anomaly

Positive anomaly years (El Niño)Negative anomaly years (La Niña)Neutral years

4,300 yr BP d18O anomaly

What did the average El Niño event look like 4,300 yr BP?

Seasonal timing differences?

ENSO asymmetry?

What do these changes mean?

~4,300 yrBP microatoll – reduced ENSO

Positive anomaly years (El Niño)

Negative anomaly years (La Niña) Neutral years

Absence of moderate-strong eventsMcGregor et al. (2013) Nature Geoscience

New monthly resolved coral d18O data from Kiritimati Island

Pink = new coral data; red = 2-8 year band pass filter; grey = published modern coral data

Compilation of published and new records from KiritimatiIsland, central Pacific

For Kiritimati correlation of s.d. versus time is significant for• all records (pearson r 0.71 99% CI; also significant for all locations)• records < 30 years or < 50 years are excluded (pearson r 0.9, 0.93 99% CI)• the high-variance, late 20th Century coral records are excluded (pearson r 0.65,

95% CI)

0.20

0.15

0.10

0.05

2-8

ye

ar

filter

sta

nd

ard

devia

tion

(‰

)

6000 5000 4000 3000 2000 1000 0

Calendar Year BP

Comparison of coral data to Mk3L 8000 year unforced transient model simulation NINO3.4 SST

Trend coefficient (Ds/ka)

Fre

quency

010

20

30

40

50

−0.008 −0.004 0.000 0.004 0.008

Obs

Coral obs

Trend is not reproduced in the unforced model simulation when sampled with the same autocorrelation characteristics and record lengths as the coral dataset

0.50

0.45

0.40

0.35

0.30

Mk3L 2

-8 b

p filt

er

NIN

O3.4

SS

T s

.d.

(de

g C

)

8000 6000 4000 2000 0

Calendar Year BP

Mk3L 1000-year orbitally forced snapshot simulations: 2-8 bandpass filtered NINO3.4 SST

Holocene Kiritimati Island coral trend in ENSO variance is qualitatively similar to trend simulated by Holocene snapshot simulations by the Mk3L model

Why discrepancies?

Positive anomaly years (El Niño)

Negative anomaly years (La Niña)

Neutral yearsMcGregor et al. (2013) Nature Geoscience

1. Record length (e.g. Wittenberg 2009)

Short records not necessarily representative

ENSO - observations

Jan 1998

Feb 1997

0 4 8 1612 20

CMAP rainfall (mm/day)IGOSS SST (°C)

10 14 18 2622 30

SST Rainfall

Why discrepancies?

2. Location, location, location – different SST and rainfall influence on d18O

Conclusions

Analysis of coral d18O at a single site (Kiritimati Island) suggests

Increase in ENSO variance from the 6000 years ago to present

ENSO variance outside the range expected from internal, unforced variations

An ENSO response to orbital forcing

Long records emerging with the opportunity to calculate ENSO metrics for periods different from today.

0.50

0.45

0.40

0.35

0.30

Mk3L 2

-8 b

p filt

er

NIN

O3.4

SS

T s

.d.

(de

g C

)

8000 6000 4000 2000 0

Calendar Year BP

1. MODERN KIRITIMATI CORAL MICROATOLLS AND THEIR CLIMATE SIGNALS

Microatoll d18O reproducibility

Modern coral X-ray

Within-microatoll variabilityBetween-microatoll variability

McGregor et al. Geochim. Cosmochim. Acta 2011

ENSO variance from ~4,300 yrBP microatoll

79% reduction in ENSO variance ~4,300 years ago(dominent period 6-8 years)

McGregor et al. (2013) Nature Geoscience

79% reduction in ENSO variance ~4.3 kyBP: Outside the range expected for ENSO with no forcing?

Wittenberg (2009) – GFDL CM2.1 unforced model simulation

Woodroffe and Gagan 2000, Woodroffe et al. 2003; McGregor et al. 2011

El Niño year

Kiritimati Porites microatoll d18O and Sr/Ca

Kiritimati microatolls are faithful recorders of ENSO

Compilation of published and new records from the Line Islands, central Pacific

For Kiritimati correlation of s.d. versus time is significant for• all records (pearson r 0.71 99% CI; also significant for all locations)• records < 30 years or < 50 years are excluded (pearson r 0.9, 0.93 99% CI)• the high-variance, late 20th Century coral records are excluded (pearson r 0.65,

95% CI)

Ratio of annual cycle to interannual (ENSO) variance

Annual cycle explains 30% fossil coral variance, compared to 10% for modern coral stack

McGregor et al. (2013) Nature Geoscience

Changes in the annual cycle 4,300 yrBP

Green = modern (1972-2006), orange = 4,300 yrBP

• Strengthened annual cycle

• Present-day annual cycle at KiritimatiIs. is a ‘knock-on’ from eastern equatorial Pacific (EEP)

• Orbital forcing affecting the annual cycle in the EEP and affecting ENSO?

• Today strengthened annual cycle and reduced ENSO (e.g. 1920s-1950s)

• Is this a clue as to the mechanism for reduced mid-Holocene ENSO?

160˚W 140˚W 120˚W 100˚W 80˚W

EQ

10˚N

JAS

b

21 22 23 24 25 26 27 28 29 30

SST (˚C)

a

McGregor et al. (2013) Nature Geoscience

79% reduction in ENSO variance ~4.3 kyBP: Outside the range expected for ENSO with no forcing?

Wittenberg (2009), Phipps et al. (2011)

GFDL CM2.1 Mk3L

~4.3 kyBP ENSO is outside the range of internal, unforced ENSO. Another driver = external forcing (orbital changes)

Interdecadal amplitude variability ~4,300ka

McGregor et al. (In press) Nature Geo

Correlation of Kiritimati microatoll d18O with rainfall and SST

McGregor et al. 2011

Wavelet & phase plots for Kiritimati microatoll d18O, ERSST, & rainfall

Semi-annual

Annual

Inter-annual

McGregor et al. 2011

Compilation of modern d18O coral records from Kiritimati Island

Evans et. al 1998; Woodroffe & Gagan 2000; Woodroffe et al. 2003; Nurhati et. al 2009; McGregor et al. 2011; McGregor, et al. 2013

Porites coral microatolls at Kiritimati Is.

4 cm

Microatoll X-ray

McGregor et al. Geochim. Cosmochim. Acta 2011

Dome-shaped Porites coral

20 cm

Porites microatoll

Porites microatoll vs dome-shaped Porites

20 cm

Porites microatoll

Dome-shaped Porites coral

Fossil microatolls at Kiritimati Is.Kiritimati (Christmas) Island – optimal site

Hundreds of fossil microatolls

~4,300 year-old coral microatoll XM35, 175 year-long monthly resolved d18O record

McGregor et al. Quaternary Geochronology 2011

But what about the longer term picture over the Holocene?

Cobb et al. 2013 Science

3. SEASONAL CYCLE AND ENSO RESULTS FROM FOSSIL CORAL MICROATOLLS

How did El Niño and La Niña events evolve at the seasonal scale?

What can this tell us about the ENSO processes during the mid-Holocene?

ENSO: interannual variance yet seasonal changes

Jan 1998 - El Niño pattern

Feb 1997 - normal pattern

0 4 8 1612 20

CMAP rainfall (mm/day)IGOSS SST (°C)

10 14 18 2622 30

Inst. SST anomaly

1

0

-1

SS

Tan

om

(°C

)

J A S O N D J F M A M J

Month

2525

1515

55

Po

wer

S(f

)

1086420

Period (years)

ENSO power spectra

~4,300 yrBP microatoll – seasonal-scale ENSO processes

Positive anomaly years (El Niño)

Negative anomaly years (La Niña) Neutral years

McGregor et al. (2013) Nature Geoscience

East-central Pacific SST annual cycle

Apr-Jun

Jul-Sep

WOA09 SST data

Oct-Dec

Jan-Mar

4.3 kyBP insolation, annual, trade winds, changes counter ENSO development…

• Increased insolation north of equator• Strengthened ITCZ• Strengthened east Pacific

meridional winds and SST gradient • Cooler EEP SSTs• Strengthened E-W trade winds• Enhanced annual cycle

• Amping up the trade winds during the season when ENSOs develop• Reduced ENSO variability

Jul-Sep

East-central Pacific SST annual cycle

Apr-Jun

Jul-Sep

WOA09 SST data

Oct-Dec

Jan-Mar

Mid-Holocene: Stronger EP annual cycle, weaker ENSO

Jul 1996

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Jul 2012

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Nov 1996

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Nov 1997

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Nov 2012

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Jul 1997

120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W

Longitude

20˚S

20˚N

Latitu

de

Reference neutral: July & November 1996

Reference El Niño: July & November 1997

Partial ~4,300 yrBP analogue: July & November 2012

18˚C 20˚C 22˚C 24˚C 26˚C 28˚C 30˚C

Sea Surface Temperature Reynolds et al. 2002 (ERSST)

Mid-Holocene: Stronger EP annual cycle, weaker ENSO

•More northerly position of the ITCZ (Haug et al. Science 2001) increased cross-equatorial winds, stronger meridional SST gradient, stronger annual cycle•Paleo evidence

•Cooler SSTs eastern equatorial Pacific (e.g. Koutavas et al. Geology 2006)•Stronger upwelling (e.g. Carre et al. Quat. Int. 2012)•Modelling study suggests seasonality may be an important factor (Braconnot et al. Cli. Dynam. 2011) and our hypothesis is consistent with idealised model experiments (Timmermann et al. J. Clim. 2007)

• Implication: ENSO is affected by external forcing via changes in month-to-month annual cycle processes

Jul-Sep

2. MID-HOLOCENE CHANGES IN ENSO VARIANCE

Origins of interest in mid-Holocene ENSO

Holocene: 0-10,000 years ago (mid-Holocene: 4,000-6,000 years ago)Proxy records: Indirect measures of climate from biological or inorganic archives

Early work: Pollen records from lakes, lake sedimentation records, mollusc species changes, model studies, coral oxygen isotope records (d18O) Indicated ENSO had strengthened since the early to mid-Holocene

Could the mid-Holocene be a test-bed for improving our understanding of ENSO?Similar but different (orbital forcing)Longer recordsAlternate ENSO behaviours

Tudhope et al. Science 2001; Koutavas et al. Geology 2006 after Moy et al. Nature 2002

Laguna Pallcacocha (Ecuador) sediment color intensity

Papua New Guinea coral d18O ENSO standard deviation

Challenges to understanding ENSO from proxy records

1. Seasonally-resolved records capture the full range of ENSO variance but are not long enough (e.g. Wittenberg 2009)

2. Forced models do not fully capture the mid-Holocene changes

3. If long enough then do not have the seasonal resolution and/or only sample part of ENSO variance (e.g. Moy et al. 2002)

Laguna Pallcacocha (Ecuador) sediment color intensity

4. Many records are from teleconnected regions (e.g. Donders et al. 2008)

Interdecadal amplitude variability past 1ka

Fowler et al. Nature Climate Change (2012)

Insolation at the equator and 10N for 4.3 kyBP and present

El Niño-Southern

Oscillation

http://svs.gsfc.nasa.gov/vis/a000000/a000200/a000287/index.html

Southern Oscillation Index –pressure difference between Tahiti and Darwin

NINO3.4 Index – SST anomaly in the NINO3.4 region in the central/eastern equatorial Pacific (5°N-5°S, 120-170°W)

Contribution from paleo-ENSO research:

• Extend ENSO records from present back in time a few hundred centuries

• Extend the ENSO records to periods when the forcing is different enough to detect a change

– Improve models and predictability (Models do not fully capture ENSO processes)

Can ENSO respond to external forcing or is it a random oscillation of the climate system

(internal variability)?

El Niño-Southern Oscillation – driver of global interannual climate variability

-4 -3 -2 0-1 4321

IGOSS SST anomaly (°C)

El Niño

La Niña

Global impacts (e.g. Australia)

http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

ENSO predictability

Forecast skill 3-6 months in advance

“Tropical Pacific Ocean moves closer to El Niño” BOM, 18 November 2014

Australian Bureau of Meteorology

ENSO predictability

Forecast skill 3-6 months in advance

Results from IPCC-class models

IncreasedENSO variability

DecreasedENSO variabilityC

han

ges

in s

td. d

ev. o

f a

mea

n s

ea le

vel p

ress

ure

in

dex

(re

late

d t

o t

he

SOI)

✖ Models give conflicting forecasts for longer timescales

Collins et al. Nature Geoscience 2010

Barrier to ENSO predictability – intrinsic variability

CSIRO & BOM Climate change in Australia 2007

La

Niña

El

NiñoAnnu

al S

outh

ern

Oscill

ation

Ind

ex

Can ENSO respond to external forcing or is it a random oscillaton of the climate system (internal variability)?

ENSO - observations

Jan 1998 - El Niño pattern (high rainfall and warm SST central & eastern Pacific)

Feb 1997 - normal pattern (high rainfall and warm SST western Pacific)

-4 -3 -2 0-1 43210 4 8 1612 20

CMAP rainfall (mm/day) IGOSS SST anomaly (°C)IGOSS SST (°C)

10 14 18 2622 30

Jan 1999 - La Niña pattern (low rainfall and very cool SST central & eastern Pacific)

Middle Holocene ENSO

Early work: ENSO had strengthened since the early to mid-Holocene Inferred changes due to orbital

forcing

New coral records suggest ENSO displays large internal variability irrespective of forcing

Laguna Pallcacocha (Ecuador) sediment color intensity

Coral d18O ENSO variance

Koutavas et al. Geology 2006 after Moy et al. Nature 2002; Cobb et al. 2013 Science

Controversies & future directions

Teasing out the forced vsinternal variability signals

Seasonally-resolved records are still temporally and spatially sparse

Working with models to ENSO processes Understand the drivers of

past ENSO

Improve models and data-model comparison (e.g. seasonality and ENSO)

Document and understand the nature of ENSO teleconnections

Kiritimati (Christmas) Island

Kiritimati fossil microatoll d18O - details

Kiritimati fossil microatoll d18O

Woodroffe et al. (2003), McGregor et al. (in prep)

ENSO strength & relationship to annual cycle strength

a) Ratio of Annual:Interannual variance versus timeb) Interannual variance versus time c) A bivariate plot of the two indices in a, b

J

j j

jn

ns

sW

C

tjx

05.0

0

5.0 )(

)0(

dd

d

Wavelet Reconstruction:

Contributions from paleo-ENSO reconstructions

2. Extend the ENSO records to periods when the forcing is different

1. Extend the instrumental record (e.g. ICOADS Gilbert Islands, 3S-4N, 172E-177E)

(e.g. Orbital forcing changes in seasonal distribution of insolation)