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Modes of Tropical Climate Variability
• El Niño / Southern Oscillation• Atlantic Niño• Atlantic and Pacific Meridional Modes • Indian Ocean Dipole
Observational description Theory Representation in climate models Response to climate change
Clara Deser, Climate Analysis Section, NCAR
Deser et al., 2009: Annual Review of Marine Sciences, Vol. 2 www.cgd.ucar.edu/cas/cdeser/
Implications for Data AnalysisSea Surface Temperature Trends, 1900-2008
“HadSST2” Data Set(quality-controlled, bucket/intake corrections applied, no infilling)
% of months with at least 1 measurement
-1°C 1°C0°C
1. Observations
• High resolution satellite data (since ~ 1982)
– Anomalies (departures from the mean seasonal cycle)
Jan 1, 2008
Sea Surface Temperature¼ degree satellite data
Jan 1, 2008
Sea Surface Temperature
Jan 1 Mean 1982-2008
¼ degree satellite data
Jan 1, 2008
Jan 1, 2008 Anomaly
Jan 1 Mean 1982-2008
Mean 1982-2008
(x in1
nN
x)2
N 1
Standard deviation of monthly anomalies
Mean 1982-2008
(x in1
nN
x)2
N 1
Standard deviation of monthly anomalies
PacificEquatorial
“Cold Tongue”
Galapagos Penguin
Sea Surface TemperatureMean Seasonal Cycle
EkmanCurrent
Equatorial Upwelling
Mean Seasonal CycleSea Surface Temperature
EkmanCurrent
2. El Niño / Southern Oscillation (ENSO)
ENSO
Ensō ( 円相 ) is a Japanese word meaning "circle" and a concept strongly associated with Zen. Ensō is one of the most common subjects of Japanese calligraphy even though it is a symbol and not a character. It symbolizes enlightenment, strength, elegance, the universe, and the void.
ENSO
Ensō ( 円相 ) is a Japanese word meaning "circle" and a concept strongly associated with Zen. Ensō is one of the most common subjects of Japanese calligraphy even though it is a symbol and not a character. It symbolizes enlightenment, strength, elegance, the universe, and the void.
EÑSO
Mean 1982-2008
(x in1
nN
x)2
N 1
Standard deviation of monthly anomalies
ENSO SSTanomaly
index“Niño 3”
“Niño 3” SST monthly anomaly index
monthly standard deviation
ENSO
ENSO Composite Evolution (SST, SLP)
ENSO Composite Evolution (SST, SLP)
Southern Oscillation
Annual ENSO Composite
“Southern Oscillation”
“El Niño”
SST
SLP and wind
ENSO Mechanisms
Self-sustained natural oscillatory mode of the coupled tropical Pacific ocean-atmosphere system
(the delayed oscillator / recharge oscillator paradigms)
OR
Stable mode … triggered by stochastic forcing(the noise forced paradigm)
• Both involve positive ocean-atmosphere (“Bjerknes”) feedback and negative oceanic feedbacks
• Dynamical processes and oceanic thermocline are key
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
“Bjerknes Feedback”
positive SST anomaly
negative SLP anomaly
westerly wind anomaly
deepened thermocline
positive SST anomaly
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedbackPoleward Vg
anomaly L H V ~ dp/dx
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
Neutral State
ShallowerMeanThermocline
Poleward Vg
anomaly L H V ~ dp/dx
Ocean Heat Discharge
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
Neutral State
ShallowerMeanThermocline
Poleward Vg
anomaly L H V ~ dp/dx
Ocean Heat DischargeShallow thermocline >Cold SST via upwelling
Cold event
Positive feedback
The “Recharge Oscillator”:A simple paradigm for the ENSO cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
Neutral State
ShallowerMeanThermocline
Poleward Vg
anomaly L H V ~ dp/dx
Ocean Heat DischargeShallow thermocline >Cold SST via upwelling
Cold event
Positive feedback
Equatorward Vg
anomaly H L V ~ dp/dx
The “Recharge Oscillator”:A simple paradigm for the El Niño cycle
(Instability of the tropical Pacific ocean-atmosphere system)
F.-F. Jin, J. Atmos. Sci. 1997
Warm event
ThermoclineDepth anomaly
Wind stressanomaly
Positive feedback
Neutral State
ShallowerMeanThermocline
Poleward Vg
anomaly L H V ~ dp/dx
Ocean Heat DischargeShallow thermocline >Cold SST via upwelling
Cold event
Positive feedback
Equatorward Vg
anomaly H L V ~ dp/dx
Ocean Heat Recharge
SST East PacificZonally-averaged “warm water volume” (thermocline depth)
Does the real ocean behave as a “Recharge Oscillator” ?
TAO Web Site
Excitation of the “Recharge Oscillator”
• Self excitation (self sustaining)
• Stochastic excitation (decaying coupled
oscillator sustained by noise)
ENSO in Models1000 year simulation: GFDL Coupled General Circulation Model
No external forcing other than seasonal earth-sun geometry
Wittenberg, Geophysical Research Letters, 2009
ENSO in Models
ENSO response to increasing CO2
• No consensus among models (period, amplitude)
• Mean Tropical Pacific SST response is El Nino – like
• Even if there were to be a change in ENSO characteristics, it may be difficult to attribute to CO2 due to high level of natural variability
Guilyardi et al., Bulletin of the American Meteorol. Society, 2009
Beyond ENSO:Tropical Pacific Decadal Variability
“Pacific Decadal Oscillation” or “Interdecadal Pacific Oscillation”
e.g., 1976/1977 “climate shift”
Indian Ocean SST
SE Pacific SST
North Pacific SLP
SPCZ Rainfall
Eq Pacific Rainfall
Eq Pacific Cloud
E-W SLP Gradient(Indian – Pac)
1-2-1 smoothedDeser et al. J. Climate 2004
Tro
pica
l Clim
ate
Indi
ces
Tropical Atlantic and Indian Ocean Modes of Variability
Leading Modes of SST Variability in the Tropical Atlantic Ocean
Niño Mode
See Review in Xie and Carton 2004
Meridional Dipole Mode
Leading Modes of SST Variability in the Tropical Atlantic Ocean
Niño Mode
See Review in Xie and Carton 2004
Meridional Dipole Mode
Typical amplitude ~ 0.5°C, period ~ 2-3 years (in some decades)Bjerknes feedback/delayed oscillator mechanism
Leading Modes of SST Variability in the Tropical Atlantic Ocean
Niño Mode
See Review in Xie and Carton 2004
Meridional Dipole Mode
Wind-evaporation-SST feedback mechanism (Chang et al., 1997)Impacts nordeste Brazil rainfall, North Atlantic hurricanes(Vimont and Kossin, 2007)Similar mode in the Pacific (Chiang and Vimont, 2004)
Leading Modes of SST Variability in the Tropical Indian Ocean
Basin Mode
See Schott et al., Reviews of Geophysics, 2009
Zonal Dipole Mode
Modes occur in association with, and independently of, ENSO
ENSO Composite Evolution (SST, SLP)
Indian Ocean Dipole Mode
Indian Ocean Basin Mode
Modes of Tropical Climate Variability
Thank You
Clara Deser, Climate Analysis Section, NCAR
Deser et al., 2009: Annual Review of Marine Sciences, Vol. 2 www.cgd.ucar.edu/cas/cdeser/