Part I. Mike McPhaden--What We Know and Whatis Unresolved: An Observational Perspective
Part II. David Battisti--What We Know and What isUnresolved: A Theoretical and ModelingPerspective
Sleeping Lady Mountain Retreat
Leavenworth, WA
19 September 2005
ENSO Dynamics and Global Impacts(“A Benign Problem”)
Sleeping Lady Mountain Retreat
Leavenworth, WA
19 September 2005
Part I. What We Know andWhat is Unresolved:An Observational Perspective
1997-98 El Niño: Global Impacts Fatalities: 23,000Economic Losses: $36 Billion
El and La
El Niño is oftenfollowed by orpreceded by LaNiña: an unusualcooling of thetropical Pacific
Upwelling zones
Western Pacific“warm pool”
El Niño happensroughly every 2-7years, lasts 12-18months, and peaksat the end of thecalendar year
El Nino/Normal
El Nino/Normal
AnomalousGeostrophicDivergence
Every fewyears, thetrade windsweaken…
El Nino/Normal
AnomalousGeostrophicConvergence
Feedbacks
Ocean-Atmosphere Feedback LoopsDuring El Niño
Winds
SST
Fast PositiveFeedback Warms
Feedbacks
Ocean-Atmosphere Feedback LoopsDuring El Niño
Winds
SST
Fast PositiveFeedback Warms
Thermocline Depth
Slow NegativeFeedback Cools
Feedbacks
Ocean-Atmosphere Feedback LoopsDuring La Niña
Winds
SST
Fast PositiveFeedback Cools
Thermocline Depth
Slow NegativeFeedback Warms
NINO3.4 and SOI, 1980-2005
NINO-3.4
Darwin
Tahiti
NINO3.4 and SOI, 1980-2005
Darwin
Tahiti
NINO-3.4
SOI and NINO3.4 Correlation=-0.9(maximum @zero lag)
NINO3.4 and SOI, 1980-2005
Darwin
Tahiti
NINO-3.4
El Niño/Southern Oscillation (ENSO):
Warm phase (El Niño) // Cold Phase (La Niña)
Thermocline Depth (20°C)
Thermocline Depth (20°C)
Thermocline Depth (20°C)
Build up of excess heatcontent along equator is anecessary precondition forEl Niño to occur.
The time between ElNiños is determined by thetime to recharge.
El Niño purges excessheat to higher latitudes,which terminates theevent.
Upper Ocean Heat Content and El Niño(Recharge Oscillator Theory*)
*Wyrtki, 1985; Cane et al, 1986; Jin, 1997
Niño3.4 SST
Janowiak et al (2003)rainfall & ERS windvelocity
Reynolds et al (2003)SST & ERS windstress
Peak Phase, 1997
Peak Phase, 2004
Janowiak et al (2003)rainfall & Quikscatwind velocity relativeto ERS climatology
Reynolds et al (2003)SST & Quikscat windstress relative to ERSclimatology
DRY
Processes Affecting Equatorial SST
EnhancedSurfaceHeat Fluxes
ZonalAdvection
SuppressedUpwelling
Atmospheric CirculationChanges During El Niño
Changes in tropicalrainfall patterns affectthe global atmosphericcirculation via“teleconnections”
Heavyrain
Pacific-North American(PNA) Pressure Pattern
Subtropical Jet Stream in NHSplits, Southern Branch ShiftsSouth and Intensifies
Global Impacts
Impacts on Global Weather Patterns
El Niño shifts the probability of droughts, floods, heat waves, andextreme weather events in large regions of the globe.
Tropical Storms
El Niño tends tosuppress formation ofAtlantic hurricanes.
El Niño tends toincrease intensity andgeographic range ofPacific hurricanes.
Opposite tendenciesoccur during La Niña.
Impacts on Tropical Storms
Global Impacts
El Niño shifts the probability of droughts, floods, heat waves, andextreme weather events in large regions of the globe.
Magnitude of impacts scales with magnitude of Pacific SST anomalies
La Niña impacts roughly opposite to those of El Niño
Impacts on Global Weather Patterns
Global Impacts
El Niño shifts the probability of droughts, floods, heat waves, andextreme weather events in large regions of the globe.
Washington State:
For 9 El Niños between 1941-1995 Expected by Chance
6 warm winters (~2°F increase)3
2 neutral winters3
1 cold winter3
Impacts on Global Weather Patterns
Global Impacts
Social and Economic Consequences
El Niño can affect life,property, andeconomic vitality dueto weather relatedhazards.
1997-98 El Niño: Global Impacts Fatalities: 23,000Economic Losses: $36 Billion
1997-98 El Niño: U.S. Impacts
•Negative
–189 Fatalities
–$4-5 billion in economic losses
•Positive
–850 lives saved
–$20 billion in economic gains
Weather Noise and ENSO Stability
If ENSO is a freely oscillating instability of the ocean-atmospheresystem governed by basin scale dynamics (Schopf & Suarez, 1988:Battisti & Hirst, 1989), weather “noise” is not essential butintroduces irregularity.
If ENSO is a stable or weakly damped oscillator, external forcingin the form of weather noise is essential for initiation anddevelopment of warm events (Penland & Sardeshmukh, 1995;Moore & Kleeman, 1999; Kessler, 2002).
Mean thermocline depth
Stability characteristicsdetermined by strength of ocean-atmosphere coupling and mayvary decadally with changingbackground conditions (Kirtmanand Schopf, 1998; Fedorov andPhilander, 2000)
Fedorov & Philander, 2000
A=1980-90s; B=1960-70s
TAO/TRITON
ATLAS Mooring
TAO/TRITON: A U.S./Japan collaboration
Current ConditionsNear normal conditions prevail
Current Conditions
Thermocline slopes down towest because of trade windforcing.
Cold subsurface (100-200 m)temperature anomalies mayindicate trend towards LaNiña cooling.
Weather Noise and Stochastic Forcing
Episodic westerly windforcing anddownwelling Kelvinwave responses
Weather Noise and Stochastic Forcing
June-July 2004Westerly Wind Burst
BEFORE:
“It is…likely that ENSO-neutral conditions will continuefor the next 3 months (through August 2004).”
NOAA/NCEP10 June 2004
AFTER:
“El Niño conditions are expected to develop during thenext 3 months.”
NOAA/NCEP5 August 2004
Westerly Wind Bursts Amplitude and Phase of ENSO
Westerly Wind Bursts Amplitude and Phase of ENSO
29°C
Stochastic forcing notentirely random
Effects of Westerly Wind Bursts on Equatorial SST
Westerly wind bursts coolthe western Pacific, and warmthe central and eastern PacificOcean via processes similar tothose that operate on longertime scales (Shinoda &Hendon, 1998; Zhang, 2001;McPhaden, 2002).
Nonlinear processes canrectify short time scalevariations into lowerfrequency changes (Lukas andLindstrom, 1991; Kessler et al,1995; Kessler and Kleeman,2000; Waliser et al, 2003)
Spatial structure resembles“optimal perturbations” insome coupled models of ENSO(Moore and Kleeman, 1999).
EnhancedSurfaceHeat Fluxes
ZonalAdvection
SuppressedUpwelling
Sleeping Lady Mountain Retreat
Leavenworth, WA
19 September 2005
Part II. What We Know and Whatis Unresolved: A Theoretical andModeling Perspective
ENSO and Models/Theory: Resolved Issues
•ENSO is the result of coupled atmosphere-ocean physics inthe tropical Pacific.
–Ocean models must be forced by Southern Oscillation to produce ‘El Ninos’;
–Atmosphere models must be forced by ‘El Nino’ SST to produce the SouthernOscillation.
•ENSO is a true mode of the coupled system.
•ENSO is a strong function of the climatological meanstate.
–The annual cycle acts to coordinate the ENSO mode to peak at the end of thecalendar year.
•ENSO is predictable 12 months in advance.
•ENSO affects are teleconnected from the tropical Pacificby well-understood atmosphere and ocean dynamics:
–Atmospheric impact is nearly global in extent.
www.atmos.washington.edu/~david/enso_pcc.pdf
Examples from an intermediateatmosphere/ocean model
Some models of the tropical PacificAtmosphere and Ocean system have realisticENSO cycles
The processes that affect SST during anENSO cycle are time and space dependant
Horizontal advection,vertical mixing,entrainment andsurface heat fluxes areall important forENSO
Nino3
Nino1
SST Tendency
Time ( model years)
Basic elements of ENSO in observations and models
The “delayed oscillator physics” and “recharge oscillator” arecomplimentary toy-model descriptions of the ENSO mode.
Ocean Adjustment & Bjerknes
ENSO is a true eigenmode of the coupledatmosphere/ocean system in the Pacific
The Bjerknes Mechanism and the Ocean Dynamics arefeatures of the ENSO mode
Few Global Climate models have realistic ENSOs
First EOF of tropical PacificSST (all IPCC ‘04 models)
Models w/ realistic ENSOspace/time variability
•Global Climate models can produce realistic ENSO variability:
- Eg, the high resolution GFDL tropical Pacific-global atmosphere model(Philander et al 1992); the paleo-CSM.
•Unfortunately, none of the current generation global climatemodels have realistic ENSO variability. Why?
- lousy mean states in the tropical Pacific (why?); too lowresolution in equatorial ocean; etc.
Only one model used inthe last IPCC assessmentsimulated ENSOvariability that did notviolate the robustobservational constraints.
The ENSO mode (cont).
ENSO exists because of the structure in the annual averagestate of the atmosphere/ocean system in the tropical Pacific,and …
ENSO tends to peak at the end of
the calendar year because of the
annual cycle in the in the basic state.
Spectrum of the pure ENSO mode
Thompson and Battisti 2001
ENSO as a linear, stochastic system
Results from a linear coupled atmosphere-ocean model forcedby white noise
Observed
Model
ENSO peaks at the end of the calendar yearbecause the of the annual cycle in the mean state
Observation
Model
ENSO is Predictable
•Skill depends on knowingwhere you are on the ENSOmode
•The long-lead time for skillfulforecast is due to long periodof the ENSO mode
•Presently, empirically basedforecast models are at least asskillful as coupled GCMS
ENSO is predictable
Empirical models skillfully predict the state of ENSO12 months in advance.
Skill 1965-1993
Persistence
Empirical Models
•Skill depends on the start month.
Empirical Model
Persistence
These plots are from an empirical model using tropicalPacific SST (1981-present).
ENSO is predictable
March Starts
September Starts
Why is ENSO irregular?The Seasonal Footprinting Mechanism
•ENSO is variable, in part, because themode interacts with the annual cycle.
•The Seasonal Footprinting Mechanismaccounts for about 1/4 to 1/3 of ENSOvariance.
•Other factors responsible for irregularity?
- Westerly wind bursts?
Vimont et al. 2001, 02, 03
ENSO: Unresolved Issues
•Is the ENSO mode stable or unstable in thepresent climate? (consensus - stable)
–If stable, what are the sources of energy for ENSO?
–Does stability matter for predictability?
•What is the limit of predictability?
–24 months? 12 months?
•How does ENSO affect the mean state?
•Will ENSO change due to Global Warming?
•What is the cause of the decadal ENSO-likevariability in the present (1900-2005) climate?
ENSO: Unresolved Issues
•The leading pattern of variability in the tropical Pacific has an ENSOlike pattern in SST and atmosphere circulation.
Something new, or the debris that results from averagingover past ENSO events? (see Vimont 2005 if you aren’t alreadyconvinced)
ENSO and Models: Resolved Issues
•ENSO is the result of coupled atmosphere-ocean physics inthe tropical Pacific.
–Ocean models must be forced by Southern Oscillation to produce ‘El Ninos’;
–Atmosphere models must be force by ‘El Nino’ SST to produce the SouthernOscillation.
•ENSO is a true mode of the coupled system.
•ENSO is a strong function of the climatological meanstate.
–The annual cycle acts to coordinate the ENSO mode to peak at the end of thecalendar year.
•ENSO is predictable 12 months in advance.
•ENSO affects are teleconnected from the tropical Pacificby well-understood atmosphere and ocean dynamics:
–Atmospheric impact is nearly global in extent.
www.atmos.washington.edu/~david/enso_pcc.pdf
ENSO: Unresolved Issues
•Is the ENSO mode stable or unstable in thepresent climate? (consensus - stable)
–If stable, what are the sources of energy for ENSO?
–Does stability matter for predictability?
•What is the limit of predictability?
–24 months? 12 months?
•How does ENSO affect the mean state?
•Will ENSO change due to Global Warming?
•What is the cause of the decadal ENSO-likevariability in the present (1900-2005) climate?
Extratropical forcing of ENSO
•Skill depends on the startmonth.
Empirical Model
Persistence
March Starts
September Starts
These plots are from an empirical model usingtropical Pacific SST (1981-present).
ENSO is predictable