Slide 1

ENSO Stability Sam Ferrett Slide 2 1. What is El Nino Southern Oscillation (ENSO)? 2. Bjerknes Stability Index 3. Perturbed physics ensemble results 4. Enso stability and climate change Slide 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 Slide 4 SSTA's during El Nino and La Nina conditions Slide 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 Slide 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. I BJ 0 the leading mode is unstable. Slide 7 BJ index terms Slide 8 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 Slide 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 Slide 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. Slide 11 BJ index v. ENSO Amplitude BJ index feedbacks (y-axis) against Nino 3.4 SSTA sd. (x-axis) for 1986-2015 Slide 12 Slide 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. Slide 14 Conclusions Ensemble members show a large range of ENSO behaviour 20 th 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 dont impact stability but may suggest changed to El Nino development.