Modes of Tropical Climate Variability Clara Deser Climate

Modes of Tropical Climate Variability Clara Deser, Climate Analysis Section, NCAR • • 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 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 International Comprehensive Ocean-Atmosphere Data Set (ICOADS) % of months with at least 1 observation in a 2° latitude x 2° longitude box Sea temp, Air temp, Pressure, Wind, Cloudiness, Humidity

Implications for Data Analysis Sea Surface Temperature Trends, 1900 -2008 “Had. SST 2” Data Set (quality-controlled, bucket/intake corrections applied, no infilling) % of months with at least 1 measurement -1°C 0°C 1°C

1. Observations • High resolution satellite data (since ~ 1982) – Anomalies (departures from the mean seasonal cycle)

Jan 1, 2008 Sea Surface Temperature ¼ degree satellite data

Jan 1, 2008 Jan 1 Mean 1982 -2008 Sea Surface Temperature ¼ degree satellite data

Jan 1, 2008 Jan 1 Mean 1982 -2008 Jan 1, 2008 Anomaly

Mean 1982 -2008 Standard deviation of monthly anomalies

Mean 1982 -2008 Galapagos Penguin Pacific Equatorial “Cold Tongue” Standard deviation of monthly anomalies

Mean Seasonal Cycle Sea Surface Temperature

Sea Surface Temperature Mean Seasonal Cycle Equatorial Upwelling Ekman Current

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 EÑSO 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.

Mean 1982 -2008 Standard deviation of monthly anomalies ENSO SST anomaly index “Niño 3”

ENSO “Niño 3” SST monthly anomaly index monthly standard deviation

ENSO Composite Evolution (SST, SLP)

ENSO Composite Evolution (SST, SLP) Southern Oscillation

Annual ENSO Composite SST “El Niño” SLP and wind “Southern Oscillation”

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, humidity ocean currents and temperatures (z)

Oceanic Thermocline Tropical Atmosphere Ocean (TAO) Buoy Array (since ~1983) Depth T (z) Oct 2007 0 m 200 m thermocline 400 m wind

The “Recharge Oscillator”: A simple paradigm for the El Niño cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Wind stress anomaly Thermocline Depth anomaly Positive feedback F. -F. Jin, J. Atmos. Sci. 1997

The “Recharge Oscillator”: A simple paradigm for the El Niño cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Wind stress anomaly “Bjerknes Feedback” positive SST anomaly Thermocline Depth anomaly Positive feedback negative SLP anomaly westerly wind anomaly deepened thermocline positive SST anomaly F. -F. Jin, J. Atmos. Sci. 1997

The “Recharge Oscillator”: A simple paradigm for the El Niño cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Wind stress anomaly Thermocline Depth anomaly Poleward Vg Positive feedback anomaly L H V ~ dp/dx F. -F. Jin, J. Atmos. Sci. 1997

The “Recharge Oscillator”: A simple paradigm for the El Niño cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Neutral State Wind stress anomaly Thermocline Depth anomaly Poleward Vg Positive feedback anomaly Ocean Heat Discharge L H V ~ dp/dx F. -F. Jin, J. Atmos. Sci. 1997 Shallower Mean Thermocline

The “Recharge Oscillator”: A simple paradigm for the El Niño cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Neutral State Wind stress anomaly Shallower Mean Thermocline Depth anomaly Poleward Vg Positive feedback anomaly Ocean Heat Discharge L H V ~ dp/dx Shallow thermocline > Cold SST via upwelling Positive feedback F. -F. Jin, J. Atmos. Sci. 1997 Cold event

The “Recharge Oscillator”: A simple paradigm for the ENSO cycle (Instability of the tropical Pacific ocean-atmosphere system) Warm event Neutral State Wind stress anomaly Shallower Mean Thermocline Depth anomaly Poleward Vg Positive feedback anomaly Ocean Heat Discharge L H V ~ dp/dx Shallow thermocline > Cold SST via upwelling Positive feedback F. -F. Jin, J. Atmos. Sci. 1997 Cold event 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) Warm event Neutral State Wind stress anomaly Shallower Mean Thermocline Depth anomaly Poleward Vg Positive feedback anomaly Ocean Heat Discharge L H V ~ dp/dx Shallow thermocline > Cold SST via upwelling Positive feedback F. -F. Jin, J. Atmos. Sci. 1997 Cold event Ocean Heat Recharge Equatorward Vg anomaly H L V ~ dp/dx

Does the real ocean behave as a “Recharge Oscillator” ? SST East Pacific Zonally-averaged “warm water volume” (thermocline depth) TAO Web Site

Excitation of the “Recharge Oscillator” • Self excitation (self sustaining) • Stochastic excitation (decaying coupled oscillator sustained by noise)

ENSO in Models 1000 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 CO 2 • 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 CO 2 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”

Deser et al. J. Climate 2004 1 -2 -1 smoothed Tropical Climate Indices North Pacific SLP Indian Ocean SST SE Pacific SST SPCZ Rainfall Eq Pacific Cloud E-W SLP Gradient (Indian – Pac)

Tropical Atlantic and Indian Ocean Modes of Variability

Leading Modes of SST Variability in the Tropical Atlantic Ocean Niño Mode Meridional Dipole Mode See Review in Xie and Carton 2004

Leading Modes of SST Variability in the Tropical Atlantic Ocean Niño Mode Meridional Dipole Mode Typical amplitude ~ 0. 5°C, period ~ 2 -3 years (in some decades) Bjerknes feedback/delayed oscillator mechanism See Review in Xie and Carton 2004

Leading Modes of SST Variability in the Tropical Atlantic Ocean Niño Mode 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) See Review in Xie and Carton 2004

Leading Modes of SST Variability in the Tropical Indian Ocean Basin Mode Zonal Dipole Modes occur in association with, and independently of, ENSO See Schott et al. , Reviews of Geophysics, 2009

ENSO Composite Evolution (SST, SLP) Indian Ocean Basin Mode Indian Ocean Dipole Mode

Modes of Tropical Climate Variability Clara Deser, Climate Analysis Section, NCAR Thank You Deser et al. , 2009: Annual Review of Marine Sciences, Vol. 2 www. cgd. ucar. edu/cas/cdeser/

Had. SST 2 #OBS Had. SST 2 Comparison between Raw Data and Had. ISST Kaplan v 2 MOHMAT 4 ERSST v 3 Optimally Interpolated Data Sets Night-time Marine Air T