ENSO Stability

Sam Ferrett

1. What is El Nino Southern Oscillation (ENSO)?

2. Bjerknes Stability Index

3. Perturbed physics ensemble results

4. Enso stability and climate change

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

SSTA's during El Nino and La Nina conditions

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

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.

BJ index terms

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

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

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.

BJ index v. ENSO Amplitude

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

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.

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.