1. ENSO Stability Sam Ferrett1

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 3

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

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 5

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. 6

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) 8

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 reanalysis,
OAFlux, 9

10. BJ Index Results
Thermocline feedback largest contributing component to positive index.
Thermodynamic damping most sensitive to perturbations and weak when compared to observation value. 10

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

12. 12

13. 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

14. 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. 14