El Ni d L Ni A li d A d ENSOEl Nino and La Nina Amplitude Asymmetry and ENSO Interaction with Higher-frequency Perturbations

Tim LiIPRC University of HawaiiIPRC, University of Hawaii

1. El Nino and La Nina amplitude asymmetry

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Su et al., 2010: Amplitude asymmetry of El Nino and La Nina in the eastern equatorial Pacific. J. Climate, 23, 605–617.

Asymmetric climate effect between El Nino and La Nina

Composite DJF mean precipitation anomalies (mm/day) for (a) eight El ( y) ( ) gNino events and (b) seven La Nino events during 1958-2001. The data is derived from Chinese 560-station observation for the period of 1958-2001.

Mixed-layer heat budget analysis using SODA 2.0.2 data (Su et al. 2010)

QT ⎛ ⎞⎛ ⎞ ⎛ ⎞∂0.2

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QT uT vT wTt c hρ⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠

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Light bar: El Nino

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dTdt adv HFLX Sum

Dark bar: La Nina

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Different from An and Jin (2004) major contribution of El

Light bar: El Nino; Dark bar: La Nina

Different from An and Jin (2004), major contribution of El Nino/La Nina amplitude asymmetry arises from nonlinear horizontal advection, not nonlinear vertical advection !

B t h th li h i t l d ti t ib t t thBut how the nonlinear horizontal advection contributes to the positive skewness in the eastern equatorial Pacific?

Zonal-vertical section of T and current anomalies along the equator

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Meridional-vertical section of T and current anomalies over the eastern Pacific

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Note that maximum SSTA is located south of the equator, while the maximum precipitation anomaly and the wind convergence anomaly appear north of the equator.

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Why is the zonal current anomaly against the wind anomaly in the far eastern equatorial Pacific?

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Thermocline anomaly induced geostrophic zonal currents dominate in the eastern equatorial Pacific Ocean.

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Red bar: El Nino composite

Bl b L Ni itBlue bar: La Nina composite

In the eastern equatorial Pacific, the mixed-layer zonal current anomaly is controlled by thermocline variation, while the meridional current is attributed to wind-driven Ekman current.

SummarySummary

The diagnosis of the SODA2.0.2, SODA1.4.2, and GODASThe diagnosis of the SODA2.0.2, SODA1.4.2, and GODAS data reveals that the nonlinear horizontal temperature advection is essential to cause the El Nino and La Nina amplitude asymmetry in the eastern equatorial Pacific whileamplitude asymmetry in the eastern equatorial Pacific, while the nonlinear vertical advection has an opposite effect.

The local zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth

l hil th idi l t l i d t i danomaly, while the meridional current anomaly is determined by the Ekman current.

ENSO - A multi-scale interaction systemRong et al (2011 QJRMS)

At h i HFVBackground

Rong et al. (2011, QJRMS)

MJO 2-7 years

Atmospheric HFV SSTCirculation 10-90 days

Large-scale control

Background Transientfeedback WWB ENSO

y

Synoptic Eddies2-10 days Surface wind stress

Upscale feedback

Ocean nonlinearityAtmospheric eddy momentum transport

Rong, X., R. Zhang, T. Li, and J. Su, 2011: Upscale feedback of high-frequency winds to ENSO Q J R Meteorol Soc 137 894 907frequency winds to ENSO. Q. J. R. Meteorol. Soc., 137, 894-907.

Time series of the HF zonal wind (green line) at 170˚E, 0˚N and the LF (red line) and the climatological mean (blue solid) zonal wind fields averaged

o er 160˚E 180˚ 5˚S 5˚Nover 160 E-180 , 5 S-5 N.

Questions:1. How does the ENSO modulate HF atmospheric activities?2. To what extent do the HF atmospheric activities further feed back to the p

ENSO variability?

Characteristics of HF surface zonal wind in the tropics

Skewness of HF zonal winds during El Niño and La Niña

HF westerly wind events during El Nino onset and developing phases are not stronger than those during La Nina phases !

El Niño La Niña

Sh d d LF l i dShaded: LF zonal wind anomaly (units: m s-1)

Contour: HF zonal wind variance

HF wind variance is ENSO-phase dependent !

ENSO-phase dependent HF wind variabilityp p y

Variance of the HF zonal wind over 5˚S-5˚N in JJASON composed for El Niño (red bars), La Niña (blue bars) and normal (green bars) years.

How does the phase-dependent HF variability feed back to ENSO? 1: Enhance the amplitude of interannual wind stress anomaly 2: Modify the skewness of the wind stress anomaly

uVC D ||ρτ =2: Modify the skewness of the wind stress anomaly

τLF (pure effect of LF wind):lc uuu +=

τHF (include effect of HF wind):hlc uuuu ++=

CLIM

Green: nonlinearly rectified LF wind stress anomaly due to eddy-eddy and eddy-mean flow interactions

El Nino

L Ni

(a) Time series of zonal wind stress anomalies (Units: dyne/cm2) averaged over 160˚E-180˚, 5˚S-5˚N

La Nina

(a) Time series of zonal wind stress anomalies (Units: dyne/cm ) averaged over 160 E 180 , 5 S 5 N. Red and blue lines denote τHF and τLF respectively, and green shaded denotes the difference.

(b) Skewness of τHF (red bars) and τLF (blue bars) averaged over 5˚S-5˚N.

Both the observational data and simple model calculations demonstrate that LF wind anomalies associated with ENSO favor a negative zonal wind stress skewness in the central equatorial Pacific.q

ENSO-state dependent HF winds tend to 1) enhance the amplitude of LF wind stress anomaly, and 2) switch the y )sign of anomalous wind stress skewness from a negative to a positive one.

How do the nonlinearly rectified LF wind stress anomalies by HF wind further affect interannual SSTA?

OGCM simulations

Time evolution ofTime evolution of SSTA along the equator:

(a) GISST observation

(b) OGCM simulation with C+LF wind forcingforcing

(c) OGCM simulation with C+LF+HFwith C+LF+HF wind forcing

(d) Difference(d) Difference between (c) and (b)

Change of the skewness of LF surface wind stress anomaly in theChange of the skewness of LF surface wind stress anomaly in the central equatorial Pacific and SST anomaly in the eastern equatorial Pacific due to HF-induced nonlinear wind stress rectification

Blue: C + LF

Pink: C + LF + HF

Nonlinear rectification of surface latent heat flux anomaly due to HF winds

El Nino compositeEl Nino composite

Left: LHF calculated based on C+LF motion (positive value means warming the ocean surface)

Right: LHF difference between total (C+LF+HF) motion

d C LF tiand C+LF motion

SSTA structure changes due to the nonlinear rectification ofSSTA structure changes due to the nonlinear rectification of LF surface wind stress and heat flux by HF wind

Red: El Nino composite

Blue: La Nina composite

HF wind effect re-shapes the El Nino zonal structure and leads to a stronger and more positively skewed SSTA !

Relative role of ocean nonlinearity vs. nonlinearly rectified LF wind stress forcing

STD of simulated SSTASTD of wind stress STD of simulated SSTASTD of wind stress

HF wind HF wind stress component

LF wind stress componentLF wind stress component

Conclusion:1. HF wind induced (nonlinearly rectified) LF wind stress anomaly

forces a significant interannual SST anomaly in the equatorial Pacific.f g f y q

2. Whereas the LF wind leads to a negative zonal wind stress skewness in the western-central equatorial Pacific the state-dependent HF windin the western central equatorial Pacific, the state dependent HF wind tends to reverse the negative skewness to a positive skewness.

3 OGCM simulations reveals that the combined dynamic and3. OGCM simulations reveals that the combined dynamic and thermodynamic effects by the HF wind re-shape the ENSO structure and lead to a greater and more positively skewed SSTA in the eastern g p yequatorial Pacific.

4. Nonlinear rectification of surface wind stress plays a more important4. Nonlinear rectification of surface wind stress plays a more important role in affecting SSTA than the ocean nonlinearity.

ThanksThanksThanksThanksDiamond Head