Whither ENSO? Assessing El Ni ñ o/Southern Oscillation risks for the coming decades
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Whither ENSO?Assessing El Niño/Southern Oscillation risks for the coming decades
Andrew WittenbergNOAA/GFDL
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Earth's dominant year-to-year climate fluctuation:
NOAA/CPC
El Niño
Normal
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How will ENSO behave in the coming decades?
CMIP5 & AR5 – new focus on near-term (30yr) projections.
ENSO drives/confounds global SI-to-decadal variability.
If we knew the next decade would have: - mega-ENSO → insure, invest, prepare, monitor & model - no ENSO → different investments, e.g. rebuild habitats
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Both historical & paleo recordssuggest past modulation of ENSO
Vecchi & Wittenberg (WIREsCC 2010)
Historical SSTA (ERSST.v3)
Palmyra corals(Cobb et al.,Nature 2003)
Multiproxy reconstructions:e.g. Emile-Geay et al.
(2011abc, subm.)
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Projected ENSO changes (CMIP3/AR4)
correl(SST trend of 1%/yr, SST.PC1 of PICTRL)10S-10N, 120E-80W
Yamaguchi & Noda (JMSJ 2006)
std(
SLP.
PC1
of S
RES.
A2 (2
051-
2100
))/ s
td(S
LP.P
C1 o
f 20C
3M)
30S-
30N,
30E
-60W
van
Olde
nbor
gh e
t al.
(OS
2005
)
CM2.1
Weak/ambiguousnear-term
anthropogenicimpacts on ENSO
Intrinsicmodulation
Reviews:Meehl et al.
(IPCC-AR4 2007)
Guilyardi et al.(BAMS 2009)
Vecchi & Wittenberg (WIREs CC 2010)
Collins et al.(Nature Geosci. 2010)
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4000-year pre-industrial control run 1860 atmospheric composition, insolation, land cover 220yr spinup from 20th-century initial conditions big investment: 2 years on 60 processors
Delworth et al., Wittenberg et al., Merryfield et al., Joseph & Nigam (JC 2006), Wittenberg (GRL 2009)Zhang et al. (MWR 2007); van Oldenborgh et al. (OS 2005); Guilyardi (CD 2006); Reichler & Kim (BAMS 2008)
Kug et al. (JC 2010), Vecchi & Wittenberg (WIREsCC 2010), Collins et al. (Nature Geosci. 2010)
1990 control (300yr), 2xCO2 (600yr), 4xCO2 (600yr)
GFDL CM2.1 global coupled GCM atmos: 2°x2.5°xL24 finite volume ocean: 1°x1°xL50 MOM4 (1/3° near equator) 2hr coupling; ocean color; no flux adjustments ENSO & tropics rank among top AR4-class models SI forecasts; parent of GFDL AR5 models (ESM2M, ESM2G, CM3, CM2.5)
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What sort of ENSO do we simulate?
These are from asingle run with
unchanging forcings.
strong, skewed,long period,
eastward propagating(1980s & late 1990s)
weak, biennial, “Modoki”(early 1990s & 2000s)
regular &westward propagating
(1960s & 70s)
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20 centuries of simulated NINO3 SSTsannual means & 20yr low-pass
Wittenberg (GRL 2009)
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Modulation of NINO3 SST power spectrum
2000yr mean
(e.g. satellites, TAO) (e.g. reconst SST)
Wittenberg (GRL 2009)
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Centuries of weak or strong ENSOs
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CM2.1 mean state hardly differs between active/inactive ENSO centuries
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Will forecasts capture anything at all?
How predictable are decades of extreme ENSO?
Will forecast ensembles differ between epochs?
°C°C
Will forecasts capture the intensity of the epoch?Will forecasts capture events, if not their timing?Will forecasts track the control?Tiny perturbation:+0.0001C at one gridcell (equator, 180W, top 10m)NINO3 SSTA
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“Perfect” ensemble reforecasts
Some members resemble the control.(forecasts with minimum NINO3 SST RMS error over each decade)
°C°C
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“Perfect” ensemble reforecasts
Other members look nothing like the control.(forecasts with maximum NINO3 SST RMS error over each decade)
°C°C
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“Perfect” ensemble reforecasts
These are what perfect forecasts look like!(perfect model, near-perfect initial conditions, 40 members)
°C°C
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“Perfect” ensemble reforecasts
Summarize the ensemble PDF with quartiles.25th and 75th percentiles of NINO3 SSTA, from 40 members
°C°C
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“Perfect” ensemble reforecasts
Quartiles “forget” initialization after a few years.gray: 95%-bands for control quartiles, from 5000 resampled 40-ensembles
°C°C
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Ensemble spread40-member interquartile range (IQR), with 95%-band from control
°C°C
“Perfect” ensemble reforecasts
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Decadal statistics of ENSO
Can we predict the epoch-mean amplitude?Absolute value of NINO3 SSTA (degC)
°C°C
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Decadal statistics of ENSO
Smoothed measure of ENSO activityNINO3 SSTA amplitude, smoothed with 4yr running mean
°C°C
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Decadal statistics of ENSO
Ensemble forecasts of ENSO activitysmoothed NINO3 SSTA amplitude from 40 members
°C°C
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Decadal statistics of ENSO
Activity distributions also “forget” the ICs{10,50,90}-percentiles of smoothed amplitude, with 95%-bands from control
°C°C
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Long-term memory?
median6yr
10% >15yr
5-year waitmost common
recharge delay
consistentwith Poisson
But beyond 10 years?
Distribution of inter-event wait times suggests that NINO3
SSTA might have some memory beyond 5 years.
Even a purely memorylessENSO would give occasional
waits of 20 years or more,as seen in CM2.1.
Wittenberg (GRL 2009)
3822yr / 495 events= 7.7yr mean wait
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ENSO events and their nearest neighbors
strong events more
isolated
weak EN, 3yrafter strong EN
strong EN, 4yrafter weak EN
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Best hope for long-term ENSO predictability?NINO3 memory might last 5yr, following strong warm events.
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Best hope for long-term ENSO predictability?NINO3 memory might last 5yr, following strong warm events.
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Given enough years, we can say...
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CM2.1 ENSO is too strong
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CM2.1 ENSO is very sensitive to some parameters
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Pre-industrial range of 100yr spectra
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1990: ENSO strengthens, spectrum narrows
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2xCO2: slightly shorter period than 1990
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4xCO2: ENSO weaker than at 2xCO2
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Can we extrapolate ENSO projections to reality?
weak ENSOs
?
Merryfield (JC 2006)
Vecchi & Wittenberg (WIREsCC
2010)
CM2.1
CM2.0
Futu
re E
NSO
am
plifi
catio
n
Width of wind stress response
The “most realistic”
pre-industrial ENSOs show amplification
at 2xCO2
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Summary
1. CMIP3/AR4 projections were ambiguous for ENSO: a. Diverse responses to anthropogenic forcings b. ENSO modulation in models, historical/paleo records
2. 4000-year run of pre-industrial CM2.1: a. Strong intrinsic modulation of ENSO b. Extreme ENSO centuries: not due to climate shifts c. Extreme ENSO decades: - Multidecadal lulls consistent with memoryless ENSO - ENSO memory up to 5 years after strong warm events - After that, even perfect forecasts don't beat a memoryless PDF
3. With long enough ENSO records, we can still detect: a. Model biases & sensitivities to some parameters/forcings b. CO2 impacts (barely detectable with 100yr record) c. CO2 optimum for ENSO - A source of disparate model sensitivities? - How close is the optimum, and which side are we on?
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Projecting ENSO risks for the coming decades
1. What is ENSO capable of on its own? - long runs, large ensembles - historical/paleo reanalyses, pseudoproxies - impacts of extreme events
3. Understand ENSO's sensitivities - primary controls, feedbacks & nonlinearities - diverse tests: forecasts, volcanoes, paleo, idealized - model diversity + physical understanding -> extrapolation to reality
4. Decadal forecasts - does precise initialization matter for ENSO? - intrinsic modulation may dominate ENSO behavior over our lifetimes
2. Improve models, understand & convey their uncertainties - metrics: robust, grounded in theory (ICMs), community-wide - AR5: new feedbacks, better resolution/physics -> different projections?
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Reserve Slides
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Last Glacial Maximum (20ka)tropical SST cools 3°C
TC deeper & more diffuse
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Mid-Holocene (6ka)perihelion shifts from
Jan -> Oct;less SH seasonality
seasonal/ENSO confounding inpaleo proxies?
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Spectrum of NINO3 SST
Wittenberg (GRL 2009)
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less phase-locking of cold events & weak warm events
strong warm events peak in SON
warm events are stronger& rarer than cold events
CM2.1 ENSO peaksvs. calendar month
abs(NINO3) > 1 stddev
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CM2.1 mean state hardly differs between active/inactive ENSO centuries
100yr-mean SST & trades are robust diagnostics for CM2.1
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Inactive centuries have slightly warmer water in the west Pacific
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Active/inactive centuries show no systematicdifference in the scaled anomaly patterns
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Active/inactive centuries show no systematicdifference in the scaled anomaly patterns
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Warm pool intraseasonals are slightlymore variable during active-ENSO centuries
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Two extreme epochs
Activity spread40-member IQR of 4yr-smoothed amplitude, with 95%-band from control
°C°C