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ENSO Stability Sam Ferrett

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?.

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ENSO Stability Sam Ferrett

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  1. ENSO StabilitySam Ferrett

  2. What is El Nino Southern Oscillation (ENSO)? Bjerknes Stability Index Perturbed physics ensemble results Enso stability and climate change

  3. 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

  4. SSTA's during El Nino and La Nina conditions

  5. 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

  6. 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.

  7. BJ index terms

  8. BJ index: 1984-2000 BJ = - Mean Current Damping (CD) – Thermodynamic Damping (TD) + Zonal Advective Feedback (ZA) + Thermocline Feedback (TC) + Ekman (upwelling) Feedback (EK) - Ocean Adjustment Damping (OA)

  9. Data • Perturbed Physics Ensemble, 33 members. • Uses fully coupled version of HadCM3 • 16 perturbations to parameters in atmosphere components • 16 perturbations to parameters in ocean components. • SODA 2.0.2 reanalysis, 1984-2000 • OAFlux, 1984-2000

  10. 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.

  11. BJ index v. ENSO Amplitude BJ index feedbacks (y-axis) against Nino 3.4 SSTA sd. (x-axis) for 1986-2015

  12. 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.

  13. 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.

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