Transcript A review of ENSO and Climate Change

Motivation and Background
AR4 Chapter 10: In summary, all models show
continued ENSO interannual variability in the
future no matter what the change in average
background conditions, but changes in ENSO
interannual variability differ from model to model.
Based on various assessments of the current
multi-model archive, in which present-day El
Niño events are now much better simulated than
in the TAR, there is no consistent indication at
this time of discernible future changes in ENSO
amplitude or frequency.
Motivation and Background
• Largely a review of GCM
studies
• Separate changes in
mean (i.e. background)
climate and ENSO
variability
• Consider that there are
errors in models and
uncertainties in projections
Changes in
Background
Conditions
Robust Changes in the Hydrological Cycle in
GCMs
Robust Changes in the Hydrological Cycle in
GCMs
upwelling
Normal Conditions
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upwelling
Climate Change
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Climate Change
(SST Anomalies)
upwelling
• Not “El Nino-like”
• Trade winds weaken
• SSTs warm more on
the equator than off
• The equatorial
thermocline shoals
and the stratification
increase
• Upwelling weakens
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El Niño Conditions
(SST Anomalies)
upwelling
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Changes in
ENSO Variability
Changes in ENSO variability 2050-2100 in CMIP3 A1B experiments
© Crown copyright Met Office
van Oldenborgh et al 2005
El Niño Features
and Processes
Walker Circulation
atmospheric damping
external
noise
SST response to
thermocline anomalies
surface zonal current
SST response to wind
stress anomalies
zonal advective feedback
upwelling
upwelling
© Crown copyright Met Office
Balance of ENSO Processes in Each Model
van Oldenborgh et al 2005
ENSO Variability: Processes and
Feedbacks
Process/Feedback
Impact on ENSO
variability
Mean upwelling and advection
up
Thermocline feedback
up
SST/wind stress (Ekman) feedback
up
Surface zonal advective feedback
no change
Atmospheric damping
down
Atmospheric variability
?
Other processes e.g. TIW
?
Summary and Conclusions
Background Conditions
• The tropical easterly trade winds are likely to weaken
• Models show SSTs warm more on the equator than off
• The equatorial thermocline shoals and the stratification of the
thermocline increases
• Upwelling weakens
Variability
• ENSO variability is controlled by a delicate balance of amplifying
and damping feedbacks, and it is likely that one or more of the major
physical processes that are responsible for determining the
characteristics of ENSO will be modified by climate change
• While the possibility of large changes in ENSO cannot be ruled out,
research conducted to date does not yet enable us to say precisely
whether ENSO variability will be enhanced or damped, or if the
frequency of events will change (because of errors in models)
East-Pacific vs Central Pacific/Modoki
Lee and McPhaden 2010
Opportunities and Questions: I
• Finding statistically significant changes in ENSO under
high levels of greenhouse gases is a mitigation-relevant
scientific problem – what about ENSO characteristics in
the next 10-30 years?
• New project; Tom Russon, Sandy Tudhope, Mat Collins,
Gabi Hegerl, …
• Assess long-term natural variations in ENSO from coral
archives and model simulations
• CMIP5 and Isotope-enabled HadCM3, palaeo-corals
from the Galapagos from last 1000 years
Opportunities and Questions: II
• Are CMIP3 findings confirmed in CMIP5 models?
• Can we design and implement better metrics?
• We expect uncertainties, so how do we deal with them?
• Funding proposal; Mat Collins, Eric Guilyardi, Fei-Fei
Jin, Axel Timmermann, Will Roberts, Geert Jan van
Oldenborgh, …
• New CMIP5 models and simulations, new metrics
• Use ENSO models of intermediate complexity
“ENMICs” to map out a much larger region of possible
ENSO futures and constrain with
observations/(re)analysis products
Opportunities and Questions: III
• Detection and attribution of observed changes (signalto-noise issue)
• Deeper understanding of the role of the ocean
especially clouds
• Reconciling past background and variability changes
from palaeo-archives (Pliocene, mid-Holocene, LGM)
with model simulations
•…
El Niño Conditions
upwelling
© Crown copyright Met Office
HadCM3 Perturbed Physics
Ensemble
• 33 ensemble
members
• Anthro and
natural forcings
• SRES A1B
• Mean ENSO
strength and
frequency is
sensitive to
forcing in 20th
and 21st
centuries
Is El Niño Changing?
• Ensemble mean
change in NINO3 std.
dev. in Hadley Centre
models and
observations
• Increase in ENSO
variability over 20th
Century is seen in
observations and
model simulations
forced by increasing
greenhouse gases
• Future projections
show larger, more
frequent ENSO events
Forced changes in ENSO
variability
5-95%tiles
obs
Natural internal variability
5-95%tiles
obs
Mat Collins, Met Office Hadley Centre
How does ENSO change?
• Switch from
smaller El Nino
events which
propagate EastWest…
• …to larger
events which
develop insitu or
even propagate
West-East
• More frequent
events in the
future
El Niño Southern Oscillation, Climate
Models and Climate Change
• Climate models solve the
dynamical equations of the
atmosphere and ocean on a grid
• Because of restrictions in
computer power, the grid is
relatively coarse and sub-grid scale
processes need to be
parameterised
• Climate models are imperfect
representations of the real world
• Nevertheless, the recent
generation of climate models can
simulate the basic physics and
characteristics of ENSO
Understanding El Niño In Ocean–atmosphere General Circulation Models: Progress And
Challenges Eric Guilyardi, Andrew Wittenberg, Alexey Fedorov, Mat Collins, Chunzai Wang,
Antonietta Capotondi, Geert Jan Van Oldenborgh, And Tim Stockdale, BAMS 2009.