enso stability sam ferrett
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ENSO Stability Sam Ferrett. What is El Nino Southern Oscillation (ENSO)? Bjerknes Stability Index Perturbed physics ensemble results Enso stability and climate change. What is ENSO?. - PowerPoint PPT PresentationTRANSCRIPT
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ENSO Stability
Sam Ferrett
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1. What is El Nino Southern Oscillation (ENSO)?
2. Bjerknes Stability Index
3. Perturbed physics ensemble results
4. Enso stability and climate change
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What is ENSO?
• Naturally occurring variation in climate in tropical Pacific region between abnormally cold (La Nina) and abnormally warm (El Nino) conditions.
• Has a large effect on weather in other parts of the world so important to understand ENSO
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SSTA's during El Nino and La Nina conditions
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Using SSTA's to assess ENSO
Nino 3, 4 and 3.4 SST Anomalies Amplitude - s.d Nino 3.4 SSTA Period - Nino 3 SSTA Power Spectra
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Bjerknes Stability Index
• The BJ index is used to assess the growth rate of ENSO in models and observations.
• Assessed using coefficients representing processes in ENSO and variable (e.g. h, Q, T…) mean states.
• IBJ<0 the leading ENSO mode is damped,
and for IBJ>0 the leading mode is unstable.
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BJ index terms
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BJ = - Mean Current Damping (CD)
– Thermodynamic Damping (TD)
+ Zonal Advective
Feedback (ZA)
+ Thermocline Feedback (TC)
+ Ekman (upwelling)
Feedback (EK)
- Ocean Adjustment Damping (OA)
BJ index: 1984-2000
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DataPerturbed Physics Ensemble, 33 members.Uses fully coupled version of HadCM316 perturbations to parameters in
atmosphere components16 perturbations to parameters in ocean
components.
SODA 2.0.2 reanalysis, 1984-2000 OAFlux, 1984-2000
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BJ Index Results 1986-2015
Thermocline feedback largest contributing component to positive index. Thermodynamic damping most sensitive to perturbations and weak when compared to observation value.
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BJ index v. ENSO Amplitude
BJ index feedbacks (y-axis) against Nino 3.4 SSTA sd. (x-axis) for 1986-2015
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Zonal Adv (ZA) feedback, thermocline (TC) feedback, ZA+TC and ZA/TC as a function of time. Increase in ZA/TC, but no difference in ZA+TC suggests a possible change in contributing ENSO feedbacks that does not impact stability.
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Conclusions• Ensemble members show a large range
of ENSO behaviour
• 20th century conditions show thermocline feedback as largest contributor to ENSO
• Changes in thermodynamic damping and thermocline feedback impact stability the most
• Changes occurring in ENSO processes don’t impact stability but may suggest changed to El Nino development.