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 Nino 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

Figure 8.5

• 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

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thermocline

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ellin

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Normal Conditions

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Climate Change

thermocline

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thermocline

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Climate Change(SST Anomalies)

• 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 Nino Conditions(SST Anomalies)

thermocline

upwelling

Changes in ENSO Variability

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Changes in ENSO variability 2050-2100 in CMIP3 A1B experiments

van Oldenborgh et al 2005

thermocline

upwelling

© Crown copyright Met Office

El Nino Features and Processes

atmospheric damping

SST response to thermocline anomalies

SST response to wind stress anomalies

upwelling

surface zonal current

zonal advective feedback

external noise

Walker Circulation

Balance of ENSO Processes in Each Model

van Oldenborgh et al 2005

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 ?

ENSO Variability: Processes and Feedbacks

Summary and ConclusionsBackground 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 (signal-to-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

•…

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thermocline

upwelling

El Nino Conditions

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 Nino Changing?

Natural internal variability5-95%tiles

obs

obs

• 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 variability5-95%tiles

Mat Collins, Met Office Hadley Centre

How does ENSO change?

• Switch from smaller El Nino events which propagate East-West…

• …to larger events which develop insitu or even propagate West-East

• More frequent events in the future

El Nino 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 Nino 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.