weakened el nino predictability in the early 21 century · occurrences of el nino/la nina play...
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The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Weakened El Nino Predictability in the Early 21st Century Mei Zhao, Harry Hendon, Oscar Alves, Guo Liu, Guomin Wang
CAWCR/BoM Melbourne Australia
Paper in review: Nature Geosciences
Occurrences of El Nino/La Nina play primary role in Australian climate variability especially in SE during winter/spring
Capability to make extended range prediction of climate stems largely from capability to predict El Nino/La Nina
Barnston et al. 2012: reported forecast skill for range of models decreased for 2002-2011 compared to what was achieved in1980's and 90's
Why ? Chance reduction of activity or role for change in background climate?
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Conundrum: Drop in skill despite forecast systems continued to improve, especially initial conditions (ARGO >2000) and improved atmospheric reanalyses
Postulate due to decrease of ENSO activity (lower signal to noise)
Results possibly skewed by inclusion of different model versions and real-time limitations of initial conditions
Revisit with a single forecast system, using research quality-controlled initial conditions
Barnston et al. 2012
9yr sliding correlation Nino3 Lt 3 (thick) and 6 (thin) mnth
9yr sliding Nino3 amplitude 1985 2005
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1960-1979 1980-1999 2000-2010
Results from BoM POAMA2.4 hindcasts initialized with PEODAS using quality controlled obs data, 33 member ensemble from 1st of each month
Confirms that skill varies with amplitude. But what caused change in amplitude? Jan Jul Dec
Annual cycle Nino3 Amplitude
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Recharge-discharge paradigm weakened post 1999 • Lag with Nino3 weakened and shortened (predictive capability diminished) • WWV variability reduced • Implies reduced impact of thermocline feedback for ENSO development
change in background climate contributed to drop in predictability
Basic behaviour of ENSO changed as well Lag regression Nino3 with WWV (after McPhaden 2012)
1960-1979 1980-1999 2000-2010
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See also Xiang et al. 2012 Hu et al. 2013
England et al: Recent change in background climate, characterized by a swing to the cold phase of the IPO after 1999, also caused the hiatus in global surface warming
Trends 1992-2011
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Trend well depicted as epochal difference (2000-2013) minus (1981-1999)
Based on PEODAS reanalysis
CMAP
NCEP reanal
Differences are small compared to interannual anomalies but significant
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Design simple forecast experiment to assess impacts of background climate change on ENSO predictability and variability Circumvent possibility due to random reduction of ENSO variability Initialize earlier/stronger events with cold IPO and later/weaker events see warm IPO Use observed anomalies and mean state changes in forecast model with proven skill Let X be an observed (control) initial condition in early epoch (1985/95) and Y in later epoch (2000/10): Xc(0) = X′(0) + X�c(0)
Yc(0) = Y′(0) + Y�c(0)
Xe(0) = X′(0) + Y�c(0) = Xc(0) + ∆
Ye(0) = Y′(0) + X�c(0) = Yc(0) − ∆
∆= Y�c − X�c
Create new experiment initial conditions by making earlier epoch see mean climate of later period and vice versa
Alter initial condition for 3-d fields of T-S (ocean) and u,v,T,q (atmosphere) (as per previous slide) Rerun forecasts initialized with Xe and Ye and compare to forecasts with Xc and Yc 10 member ensemble Control and Experiment from 1st of each mnth (~120 starts, 1200 forecasts)
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Change in Nino3 (Experiment minus Ctl for each epoch)
Predictability Rowell et al= (ensemble mean variance)/(ensemble mean variance + spread variance)
Observed difference (00/10 minus 85/95)
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LT 1 mnth
LT 3 mnth
LT 6 mnth
Obs difference (00/10) minus (85/95)
Exp-Ctl (85/95) Exp-Ctl (00/10) Difference in standard deviation of SST
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Exp-Ctl 85/95 Exp-Ctl 00/10
El Nino and La Nina equally affected In response to imposed cold IPO, El Nino and La Nina anomalies "weaken" in early period (and vice versa in later period) Regression slope is difference in growth rate (deg/mnth)
Scatter of difference in predicted N3difference (Exp-Ctl) after 1 mnth versus observed N3 at initial time
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𝜕𝜕𝜕′𝜕𝜕𝜕≈ −w� 𝜕𝜕𝜕′
𝜕𝜕𝜕− u′∂T�/∂x
Δw� 1 = we 1 − wc 1 Δ𝜕𝜕T′ 1𝜕𝜕𝜕 =
𝜕𝜕Te′ 1𝜕𝜕𝜕 −
𝜕𝜕Tc′ 1𝜕𝜕𝜕
− Δw� 1𝜕𝜕Tc′ 1𝜕𝜕𝜕
+ we 1 Δ𝜕𝜕T′ 1𝜕𝜕𝜕
− uc′ 1 Δ𝜕𝜕T� 1𝜕𝜕𝜕
+ Δu′ 1𝜕𝜕T�e 1𝜕𝜕𝜕
Δ𝜕𝜕T′
𝜕𝜕𝜕 =Te′ 1 − Tobs′ 0
Δ𝜕−
Tc′ 1 − Tobs′ 0Δ𝜕
=Te′ 1 − Tc′ 1
Δ𝜕=
Linear SST growth during ENSO is primarily due to thermocline and zonal advective feedback:
Heat budget (Experiment minus Control) for first month of forecast (following Dinezio et al 2012):
Delta operator for means and perturbations defined as:
Dinezio couldn’t quantify these terms
Regress tendencies onto Nino3 at initial time to reveal contribution to initial dispersion as a result of imposed background climate change Find that rhs well approximated by (a) and (b) in main Nino region
a b c
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Tendency difference (total solid; sum of three terms dotted)
Equatorial tendency differences at first month (deg/mnth) Red Exp-Ctl (2000-2010) Green Exp-Ctl (1985-1995)
Zonal advective feedback due to imposed change in mean zonal gradient
Zonal advective feedback due to change in perturbation current during forecast
Thermocline feedback due to imposed change in upwelling velocity
Difference in stand dev SST after 1 month (Exp-Ctl)
Exp-Ctl (85-95) Exp-Ctl (00-10)
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Regress surface zonal wind onto Nino3
Results from a combination of colder (warmer) local SST and strengthened (weakened) Walker circulation Cold SST and sinking motion weaken coupling Warm SST and upward motion strengthen coupling
Cold "IPO" acts to weaken wind (and current) response to SST' Warm "IPO" acts to strengthen
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Why did 2014 El Nino fizzle? Operational Forecasts for El Nino 2014
165 Ensemble (33 from 16, 20, 23,27,30 March)
What would have happened If 2014 saw warm IPO as in 85-95? Event would have been much stronger and more "predictable" Cold IPO resulted in El Nino 2014 that was more susceptible to noise and less predictable
Ensemble-mean warmer than verification, (especially for earlier forecasts) but weak event captured by a few members
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Conclusions/Discussion • Swing to Cold "IPO" after 1999 acted to weaken ENSO predictability
First demonstration that observed mean state changes have affected predictability
Resulted from weakened coupling at heart of ENSO in east Pac
Offered as explanation for fickle behavior of 2014 El Nino
• What caused swing in "IPO" (opposite to expected change due to anthropogenic forcing, but can't rule out ocean thermostat) and there are two-way feedbacks ?
Need to think about how to address possible feedback
• Predictability of decadal variations of ENSO implied (due to connection with mean state change) no indication yet of any predictability of decadal variations of ENSO (Wittenberg et al. 2014) : but studies done with flawed models
• Will high ENSO skill scores achieved during the 1980-90s ever be achieved again?
ENSO variability was historically high in 1980's-90's
no consensus on how ENSO variability might change in future
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extras
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11 yr running mean: IPO index , central Pac wind stress, Nino3 amplitude in 11 yr window
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The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
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