Modes of Tropical Climate Variability

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/

1. Observations

• Data Coverage

Data Coverage from Ships of Opportunity

% of months with at least1 observation in a 2° latitude x 2° longitude box

International Comprehensive Ocean-Atmosphere Data Set (ICOADS)

Sea temp,Air temp,Pressure,Wind,Cloudiness,Humidity

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

Wang and Fiedler, Progress in Oceanography, 2006

Oceanic Thermocline

Tropical Atmosphere Ocean (TAO) Buoy Array (since ~1983)

~ 70 moorings Air T, wind, humidityocean currents and temperatures (z)

Oceanic Thermocline

Tropical Atmosphere Ocean (TAO) Buoy Array (since ~1983)

0 m

200 m

400 m

Dep

th

T (z)Oct 2007

wind

thermocline

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/

HadSST2 #OBSHadSST2

MOHMAT4

Kaplan v2

ERSST v3

HadISST

Night-time Marine Air T

Comparison between

Raw Data

and

Optimally Interpolated

Data Sets