increasing frequency of extreme el niño events due to ......5 . supplementary figure 4| full...

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Supplementary material for:

Increasing frequency of extreme El Niño events due to greenhouse warming

Wenju Cai1,2, Simon Borlace1, Matthieu Lengaigne3, Peter van Rensch1, Mat Collins4, Gabriel Vecchi5, Axel Timmermann6, Agus Santoso7, Michael J. McPhaden8, Lixin Wu2, Matthew England7, Guojian

Wang2,1, Eric Guilyardi3,9, and Fei-Fei Jin10

1. CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia 2. Physical Oceanography Laboratory, Qingdao Collaborative Innovation Center of Marine

Science and Technology, Ocean University of China, Qingdao, China 3. Laboratoire d’Océanographie et du Climat: Expérimentation et Approches Numériques

(LOCEAN), IRD/UPMC/CNRS/MNHN, Paris, France 4. College of Engineering Mathematics and Physical Sciences, Harrison Building, Streatham

Campus, University of Exeter, Exeter, UK 5. Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, New Jersey, USA 6. IPRC, Department of Oceanography, SOEST, University of Hawaii, Honolulu, Hawaii, USA 7. Climate Change Research Centre and ARC Centre of Excellence for Climate System Science,

University of New South Wales, Sydney, Australia 8. NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington, USA 9. NCAS-Climate, University of Reading, Reading, UK 10. Department of Meteorology, SOEST, University of Hawaii, Honolulu, Hawaii, USA

Wenju Cai: [email protected]

Simon Borlace: [email protected]

Matthieu Lengaigne: [email protected]

Peter van Rensch: [email protected]

Mat Collins: [email protected]

Gabriel Vecchi: [email protected]

Axel Timmermann: [email protected]

Agus Santoso: [email protected]

Michael J. McPhaden: [email protected]

Lixin Wu: [email protected]

Matthew England: [email protected]

Guojian Wang: [email protected]

Eric Guilyardi: [email protected]

Fei-Fei Jin: [email protected]

Increasing frequency of extreme El Niño events due to greenhouse warming

© 2014 Macmillan Publishers Limited. All rights reserved.

http://www.nature.com/doifinder/10.1038/nclimate2100

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Supplementary Figure 1 | Composites of moderate and extreme El Niño in observations1, 2. a and b, time and longitude plots of SST anomalies (colour, °C) and rainfall anomalies (black contours, in percentage of climatology) over the equatorial Pacific latitude band (2.5°S–2.5°N) for moderate El Niño and extreme El Niño events, respectively, showing the evolution over a two-year period. The 28 °C isotherm (purple curve) and 5 mm per day isopleth (green curve) are superimposed. c and d, spatial patterns of observed Austral summer rainfall anomalies (in percentage of climatology) for moderate El Niño and extreme El Niño events, respectively. The anomalies are referenced to the mean over the full period.


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Supplementary Figure 2 | Composites of total SST (colour, °C) and rainfall (black contour, mm per day) in austral summer for moderate and extreme El Niño. a and c, observations, and b and d for selected CGCMs as described in Supplementary Tables 1 and 2. Purple thick curves indicate the 28 °C isotherm, and green thick curves indicate the rainfall isopleth of 5 mm per day. Panels c and d show an eastward and equatorward shift of atmospheric convection such that the warm pool covers the entire equatorial Pacific, and rainfall in the eastern equatorial Pacific is several times that during moderate events.


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Supplementary Figure 3 | Observed relationship between meridional and zonal SST gradients. The meridional SST gradient is defined as the average SST over the eastern off-equatorial region (5°N–

10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–

90°W). The zonal SST gradient is calculated as the SST difference over the 150°E–80°W longitude range, obtained after applying a linear regression with respect to longitudes to meridionally averaged SSTs over 5°S–5°N. The figure shows that an increased east-minus-west zonal SST gradient is associated with a reduced meridional SST gradient. Red, green, and blue dots indicate extreme El Niño, moderate El Niño, and La Niña and neutral events, respectively.


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Supplementary Figure 4| Full circulation fields associated with the 1991/92 El Niño events. a, SST, b, rainfall, c, vertical velocity at 500 mb (negative upward), and d, vertical velocity averaged over the Niño3 region vs the northern off-equatorial minus equatorial meridional SST gradient over the Niño3 longitudes. Although it is a zonal SPCZ event during 1991/92, the warm pool (28 °C isotherm, purple contour in a) and the convection (black contour in c) did not extend across the eastern Pacific, the ITCZ is situated north of the equator, and there is a positive Niño3 vertical velocity (descending motion) and a positive meridional SST gradient (meaning warmest SST in the off-equatorial region), indicating that it is not an extreme El Niño event.


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Supplementary Figure 5 | Differentiation between independent zonal SPCZ events (green dots, number of events indicated on the side), independent extreme El Niño events (purple dots), and concurrent events (red dots). Shown is an aggregate over 20 selected models over the full 200 years. a, Scatter plot indicating all zonal SPCZ events identified through an empirical orthogonal function analysis on rain as in Ref. 5, and extreme El Niño events in a scatter plot of Niño3 rainfall vs the north off-equatorial minus equatorial meridional SST gradients over the Niño3 longitudes. Neither type is a subset of the other, and independent extreme El Niño or zonal SPCZ can occur, though the two types often take place concurrently. These features exist regardless of the definition of extreme El Niño events, because many zonal SPCZ events occur with very low Niño3 rainfall and large meridional SST gradients over the Niño3 longitudes. b and c, Composite of quadratically detrended SST and rainfall anomalies associated with independent zonal SPCZ and extreme El Niño events, respectively; d and e, Composite of quadratically detrended rainfall associated with independent zonal SPCZ and extreme El Niño events, respectively.


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Supplementary Figure 6 | Assessment of CMIP3 model performance. Nine selected CMIP3 CGCMs are shown in terms of their ability to (1) produce the nonlinear ocean-atmosphere coupling with austral summer rainfall skewness greater than 1, and (2) generate an austral rainfall average over the Niño3 region greater than 5 mm per day in at least one summer over a 200-year period (1891–

2090). The multi-model average skewness is 2.90, comparable to the observed skewness since 1979 of 2.76. Other statistical properties are listed in Supplementary Table 1. The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W). Red, green, and blue dots indicate extreme El Niño, moderate El Niño, and La Niña and neutral events, respectively.


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Supplementary Figure 7 | CMIP3 model performance. Shown are CMIP3 CGCMs excluded because of their lack of ability to either (1) produce the nonlinear ocean-atmosphere coupling with austral summer rainfall skewness greater than 1, or (2) to generate an austral summer rainfall average over the Niño3 region greater than 5 mm per day in at least one summer, over a 200-year period (1891–

2090). The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W).


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Supplementary Figure 8 | Assessment of CMIP5 model performance. Eleven selected CMIP5 CGCMs are shown in terms of their ability to (1) produce the nonlinear ocean-atmosphere coupling with austral summer rainfall skewness greater than 1, and (2) to generate an austral rainfall average over the Niño3 region greater than 5 mm per day in at least one summer, over a 200-year period (1891–2090). The multi-model average skewness is 2.90, comparable with the observed skewness since 1979 of 2.76. Other statistical properties are listed in Supplementary Table 2. The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W). Red, green, and blue dots indicate extreme El Niño, moderate El Niño, and La Niña and neutral events, respectively.


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Supplementary Figure 9 | CMIP5 model performance. Shown are CMIP5 CGCMs excluded because of their lack of ability to either (1) produce the nonlinear ocean-atmosphere coupling with austral summer rainfall skewness greater than 1, or (2) generate an austral summer rainfall average over the Niño3 region greater than 5 mm per day in at least one summer, over a 200-year period (1891–

2090). The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W).


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Supplementary Figure 10 | Assessment of CMIP3 model performance. Nine selected CMIP3 CGCMs are shown in terms of the relationship between Niño3 (5°S–5°N, 150°W–90°W) SST and Niño3 rainfall, two panels for each model, left for the Control period, and right for the Climate Change period. All selected models have seasonal mean Niño3 SST in the convective threshold range (26 °C-28 °C) in the Control period. In the Climate Change period, the rainfall profile appears to shift with rising SSTs. Red, green, and blue dots indicate extreme El Niño, moderate El Niño, and La Niña and neutral events, respectively. Two panels are shown for each model, one for the Control period and one for the Climate Change period, as illustrated in the blue box for the first model.


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Supplementary Figure 11 | Assessment of CMIP5 model performance. Eleven selected CMIP5 CGCMs are shown in terms of the relationship between Niño3 (5°S–5°N, 150°W–90°W) SST and Niño3 rainfall, two panels for each model, left for the Control period, and right for the Climate

Change period. All selected models have seasonal mean Niño3 SST in the convective threshold range (26 °C-28 °C) in the Control period. In the Climate Change period, the rainfall profile appears to shift with rising SSTs. Red, green, and blue dots indicate extreme El Niño, moderate El Niño, and La Niña and neutral events, respectively. Two panels are shown for each model one for the Control period and one for the Climate Change period as illustrated in the blue box for the first model.


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Supplementary Figure 12 | Assessment of CMIP3 model performance. Ten CMIP3 CGCMs not selected are shown in terms of the relationship between Niño3 (5°S–5°N, 150°W–90°W) SST and Niño3 rainfall, two panels for each model, left for the Control period, and right for the Climate

Change period. Many models that do not have a seasonal mean rainfall total greater than 5 mm per day in the Control period and tend to have low Niño3 SSTs (indicated by a green cross) still fail to produce such a rainfall total after greenhouse warming increases Niño3 SSTs to the convective threshold range in the Climate Change period (indicated by a red cross).


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Supplementary Figure 13 | Assessment of CMIP5 model performance. Ten CMIP5 CGCMs not selected are shown in terms of the relationship between Niño3 (5°S–5°N, 150°W–90°W) SST and Niño3 rainfall, two panels for each model, left for the Control period, and right for the Climate

Change period. Many models that do not have a seasonal mean rainfall total greater than 5 mm per day in the Control period and tend to have low Niño3 SSTs (indicated by a green cross) still fail to produce such a rainfall total after greenhouse warming increases Niño3 SSTs to the convective threshold SST range in the Climate Change period (indicated by a red cross).


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Supplementary Figure 14 | Relationship between meridional and zonal SST gradient anomalies aggregated over the 20 selected CGCMs (Supplementary Tables 1 and 2). a, Control period, and b, Climate Change period. The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W). The zonal SST gradient (°C) is calculated as the SST difference over the 150°E–80°W longitude range obtained after applying a linear regression with respect to longitudes to meridionally averaged SSTs over 5°S–5°N. Anomalies are then constructed referencing to the mean over the Control period. Red, green, and blue dots indicate extreme El Niño (defined as one for which austral summer rainfall is greater than 5 mm per day), moderate El Niño (defined as events with SST anomalies greater than 0.5 standard deviation of the Control period that are not extreme El Niño events), and La Niña and neutral events, respectively. The figure shows that an increased east-minus-west zonal SST gradient is associated with a reduced meridional SST gradient.


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Supplementary Figure 15 | Relationship between Niño3 rainfall and meridional SST gradient in the PPE using the HadCM3 CGCM3. See Supplementary Table 4 for more details. In these PPE experiments, perturbations are made to uncertain physical parameters within a single model structure. a, The Control period, and b, Climate Change period. The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W). Red, green, and blue dots indicate extreme El Niño (defined as one for which austral summer rainfall is greater than 5 mm per day), moderate El Niño (defined as events with SST anomalies greater than 0.5 standard deviation of the Control period that are not extreme El Niño events), and La Niña and neutral events, respectively. There is an increase of over 300% in the extreme El Niño events from the Control to the Climate Change period using HadCM3 CGCM, in which biases are corrected through a fixed flux adjustment3. The climate change experiments are forced with a 1% per year CO2 increase.


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Supplementary Figure 16 | Mean state changes for selected CGCMs. The difference between the ensemble average over the Climate Change and the Control period. a, SST (°C), b, rainfall (mm per day), and c, mean sea level pressure (SLP, hPa). The result shows that the eastern equatorial Pacific is warming faster than the western equatorial Pacific and the off-equatorial Pacific region, and a weakening in the Walker circulation.


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Supplementary Figure 17 | Quadratically detrended rainfall (mm per day) time series in selected CGCMs. a, Control period, and b, Climate Change period. Occurrences of rainfall exceeding a given threshold value increase markedly from the Control to the Climate Change period.


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Supplementary Figure 18 | Multi-model statistics of detrended rainfall and SST anomalies for austral summer. a and b, sensitivity of Niño3 rainfall anomalies to Niño3 SST anomalies during El Niño events only, both quadratically detrended, showing an increase from the Control to the Climate

Change period. The increase is statistically significant above the 95% confidence level, based on a t-test comparing the two slopes. c, comparison highlighting the difference between a and b.


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Supplementary Figure 19 | Response patterns of grid-point rainfall to Niño3 rainfall using extreme El Niño samples only. a and b, Regression for the Control and Climate Change periods, respectively. Areas with statistically significant correlations are shown in colour. White areas indicate correlation below the 95% confidence level.


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Supplementary Table 1 | Statistics of CMIP3 models. Niño3 rainfall skewness that measures the nonlinearity of the equatorial Pacific; climatological rainfall over the Niño3 region; standard deviation of Niño3 SST anomalies after removing variances on time scales longer than nine years, including climate change signals; and frequency of extreme El Nino events. Nine CGCMs (in blue) are selected for their ability to (1) produce the nonlinear ocean-atmosphere coupling with an austral summer rainfall skewness greater than 1, and (2) generate austral rainfall over the Niño3 region greater than 5 mm per day in at least one summer over the 200-year period (1891–2090). Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate Change period (1991–2090). There is no consistent change in Niño3 SST standard deviation among the selected models, but the consensus is strong for an increase in extreme El Niño events. Models marked with a red star are able to generate zonal SPCZ event4. The CGCMs selected for this study are not the same as those selected for the zonal SPCZ study, because different criteria are used and not all zonal SPCZ is concurrent with an extreme El Niño.

Model

Niño3 rainfall

skewness, full period

Niño3 rainfall

climatology, (1891–1990)/ (1991–2090) mm per day

Niño3 SST interannual

standard deviation (1891–1990)/ (1991–2090)

°C

Frequency of extreme El Niño

events (1891–1990)/ (1991–2090)

bccr_bm2_0 1.66 0.88 / 1.02 0.98 / 1.24 cccma_cgcm3_1 0.97 1.02 / 1.57 0.34 / 0.36 csiro_mk3_0 0.28 2.56 / 2.78 0.81 / 0.78 giss_model_e_r 0.25 5.99 / 6.73 0.21 / 0.18 ingv_echam4 1.95 0.7 / 0.87 0.93 / 0.93 inmcm3_0 0.92 1.37 / 1.61 0.71 / 0.67 miroc3_2_medres 0.17 3.24 / 3.51 0.47 / 0.41 ncar_ccsm3_0 0.96 2.21 / 2.39 0.86 / 0.72 ncar_pcm1 6.0 0.91 / 0.88 0.88 / 0.87 ukmo_hadgem1 1.11 2.19 / 2.26 0.59 / 0.72 cnrm_cm3* 2.16 1.71 / 2.48 1.93 / 1.85 5 / 15 csiro_mk3_5* 2.26 2.76 / 3.17 0.93 / 0.80 2 / 7 gfdl_cm2_0* 3.71 2.45 / 3.32 0.93 / 1.24 2 / 11 gfdl_cm2_1* 4.8 1.85 / 2.30 1.10 / 1.31 3 / 4 ipsl_cm4* 1.48 2.25 / 2.72 1.08 / 1.02 4 / 8 miub_echo_g* 2.9 1.31 / 1.46 1.05 / 1.06 3 / 8 mpi_echam5* 3.72 0.60 / 0.95 1.33 / 1.40 1 / 1 mri_cgcm2_3_2a* 3.34 1.62 / 2.55 0.68 / 1.17 2 / 11 ukmo_hadcm3* 1.74 2.75 / 2.98 1.03 / 0.99 5 / 6 Average for all models 2.10 2.02 / 2.40 0.84 / 0.89

Average for selected models 2.90 1.92 / 2.43 1.12 / 1.20 27 / 71

Observed (1979-2013) 2.7 2.05 1.05 2


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Supplementary Table 2 | Statistics of CMIP5 models. Niño3 rainfall skewness that measures the nonlinearity of the equatorial Pacific; climatological rainfall over the Niño3 region; standard deviation of Niño3 SST anomalies after removing variances on time scales longer than nine years, including climate change signals; and frequency of extreme El Nino events. Eleven CGCMs (in blue) are selected for their ability to (1) produce the nonlinear ocean-atmosphere coupling with an austral summer rainfall skewness greater than 1 and (2) generate austral rainfall over the Niño3 region greater than 5 mm per day in at least one summer over the 200-year period (1891–2090). CGCMs marked with a red star are able to generate zonal SPCZ event4. The CGCMs selected for this study are not the same as those selected for the zonal SPCZ study, because different criteria are used. Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate

Change period (1991–2090). There is no consistent change in Niño3 SST standard deviation among the selected models, but the consensus is strong for an increase in extreme El Niño events.

Model

Niño3 rainfall


Niño3 rainfall

climatology, (1891–1990)/ (1991–2090) (mm per day)



(°C)


events (1891–1990)/ (1991–2090)

GISS-E2-R 0.91 4.78 / 7.28 0.61 / 0.58 CSIRO-Mk3-6-0* 0.84 1.49 / 1.75 0.70 / 0.64 IPSL-CM5A-MR 0.55 2.04 / 2.30 0.75 / 0.52 MIROC-ESM 0.34 3.30 / 4.00 0.48 / 0.49 MIROC-ESM-CHEM 0.39 3.25 / 4.06 0.48 / 0.44 Inmcm4 0.48 1.85 / 1.90 0.62 / 0.48 IPSL-CM5A-LR 0.67 1.92 / 2.17 0.72 / 0.62 HadGEM2-ES 0.96 1.46 / 1.91 0.88 / 0.82 ACCESS1-0* 0.82 3.85 / 5.15 0.91 / 0.97 ACCESS1-3* 0.95 3.30 / 4.39 0.84 / 0.80 CanESM2 1.08 3.00 / 4.03 1.14 / 1.02 10 / 23 CCSM4* 1.83 2.89 / 3.49 1.20 / 0.98 16 / 16 CESM1-BGC* 1.88 2.45 / 3.35 0.99 / 0.89 11 / 17 CMCC-CM* 4.01 0.64 / 1.03 0.62 / 0.71 0 / 1 CNRM-CM5* 1.63 2.94 / 3.39 1.11 / 1.11 12 / 22 GFDL-CM3 * 2.06 3.00 / 3.68 1.23 / 1.41 8 / 16 GFDL-ESM2M * 2.70 2.63 / 2.88 1.56 / 1.50 10 / 8 HadGEM2-CC 1.30 1.55 / 1.99 1.01 / 0.93 2 / 10 IPSL-CM5B-LR* 2.06 2.75 / 3.70 0.71 / 0.86 2 / 17 MIROC5 * 3.93 1.66 / 2.44 1.24 / 1.45 3 / 7 MRI-CGCM3* 4.23 1.27 / 2.03 0.64 / 0.88 0 / 4 Average for all models 1.6 2.47 / 3.18 0.87 / 0.88

Average for selected models 2.4 2.25 / 2.91 1.04 / 1.06 74 / 141

Observed (1979- 2.7 2.05 1.05 2


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2013)

Supplementary Table 3 | Sensitivity of frequency of extreme El Niño events to varying definitions. The results shown were obtained using rainfall alone for the selected models as described in Supplementary Tables 1 and 2. Numbers in red type indicate a decrease or no change from the Control period (1891–1990) to the Climate Change period (1991–2090).

Definitions of extreme El Niño

Model

Niño3 rainfall > 5 mm per day, (1891–1990)/ (1991–2090)

Niño3 rainfall > 6 mm per day, (1891–1990)/ (1991–2090)

Niño3 rainfall > 50% of the West

Pacific (10°S–10°N,

140°E–160°E, 1891–1990)

mean rainfall, (1891–1990)/ (1991–2090)

Niño3 rainfall > 75% of the West

Pacific (10°S–10°N,

140°E–160°E, 1891–1990)

mean rainfall, (1891–1990)/ (1991–2090)

cnrm_cm3 5 / 15 5 / 13 13 /21 6 / 18 csiro_mk3_5 2 / 7 0 / 3 2 / 9 0 / 1 gfdl_cm2_0 2 / 11 0 / 8 1 / 10 0 / 6 gfdl_cm2_1 3 / 4 3 / 4 8 / 5 6 / 5 ipsl_cm4 4 / 8 1 / 6 8 / 16 1 / 6 miub_echo_g 3 / 8 2 / 4 6 / 9 1 / 4 mpi_echam5 1 / 1 1 / 0 1 / 2 1 / 0 mri_cgcm2_3_2a 2 / 11 0 / 9 3 / 12 0 / 7 ukmo_hadcm3 5 / 6 0 / 4 26 / 32 5 / 6 CCSM4 16 / 16 13 / 12 16 / 21 13 / 12 MRI-CGCM3 0 / 4 0 / 2 0 / 4 0 / 2 MIROC5 3 / 7 3 / 6 3 / 7 3 / 6 IPSL-CM5B-LR 2 / 17 0 / 12 3 / 19 0 / 8 GFDL-CM3 8 / 16 6 / 13 10 / 21 6 / 10 GFDL-ESM2M 10 / 8 9 / 8 11 / 11 9 / 8 CNRM-CM5 12 / 22 10 / 14 15 / 25 9 / 13 CESM1-BGC 11 / 16 8 / 12 11 / 18 7 / 11 CanESM2 10 / 23 6 / 13 14 / 28 2 / 6 HadGEM2-CC 2 / 10 0 / 0 2 / 2 0 / 0 CMCC-CM 0 / 1 0 / 0 0 / 1 0 / 0 Total (% increase)

101 / 212 (111 %) 67 / 143 (113 %) 117 / 192 (64 %) 63 / 127 (102 %)


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Supplementary Table 4 | Sensitivity of frequency of extreme El Niño events to varying definitions. The results shown were obtained using Niño3 total rainfall and the eastern equatorial Pacific meridional SST gradient combined for selected models as described in Supplementary Tables 1 and 2. Number in red type indicates a decrease or no change from the Control period (1891–1990) to the Climate Change period (1991–2090). Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate Change period (1991–2090). The meridional SST gradient (°C) is defined as the average SST over the eastern off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the eastern equatorial region (2.5°S–2.5°N, 150°W–90°W).


Model

Niño3 rainfall > 5 mm per day & meridional SST gradient <1 °C (1891–1990)/ (1991–2090)

Niño3 rainfall > 5 mm per day & meridional SST

gradient < 0.5°C (1891–1990)/ (1991–2090)

Niño3 rainfall > 5 mm per day & meridional SST gradient < 0 °C (1891–1990)/ (1991–2090)

cnrm_cm3 5 / 15 5 / 15 5 / 15 csiro_mk3_5 2 / 7 2 / 7 0 / 6 gfdl_cm2_0 2 / 11 2 / 10 0 / 9 gfdl_cm2_1 3 / 4 3 / 4 3 / 4 ipsl_cm4 4 / 8 4 / 8 4 / 6 miub_echo_g 3 / 7 3 / 3 0 / 0 mpi_echam5 1 / 1 1 / 1 0 / 0 mri_cgcm2_3_2a 2 / 11 2 / 11 2 / 10 ukmo_hadcm3 0 / 4 0 / 3 0 / 0 CCSM4 16 / 16 16 / 16 13 / 12 MRI-CGCM3 0 / 4 0 / 3 0 / 3 MIROC5 3 / 7 3 / 6 3 / 5 IPSL-CM5B-LR 2 / 17 2 / 17 2 / 12 GFDL-CM3 8 / 16 8 / 15 6 / 9 GFDL-ESM2M 10 / 8 10 / 8 10 / 8 CNRM-CM5 12 / 22 12 / 22 12 / 22 CESM1-BGC 11 / 17 7 / 16 0 / 11 CanESM2 10 / 23 8 / 20 2 / 7 HadGEM2-CC 2 / 1 2 / 0 0 / 0 CMCC-CM 0 / 1 0 / 1 0 / 1 Total (% increase) 96 / 200 (108 %) 90 / 186 (107 %) 62 / 140 (125 %)


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Supplementary Table 5 | Statistics for CMIP3 models not selected. Shown are Niño3 rainfall skewness that measures the nonlinearity of the equatorial Pacific; climatological rainfall over the Niño3 region; standard deviation of Niño3 SST anomalies after removing variances on time scales longer than nine years, including climate change signals; and frequency of extreme El Nino event. Here we discard the criterion of “an austral summer rainfall skewness greater than 1” but retain the

criterion of “austral rainfall over the Niño3 region greater than 5 mm per day in at least one summer” over the 200-year period (1891–2090). This is to test the sensitivity to our criteria. Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate

Change period (1991–2090). Models in purple indicate they generate an overly large Niño3 rainfall. Number in red type indicates a decrease or no change from the Control period (1891–1990) to the Climate Change period (1991–2090). When these CMIP3 models are included, there is still a large increase in the occurrences of extreme El Niño events.

Model

Niño3 rainfall


Niño3 rainfall




°C


events (1891–1990)/ (1991–2090)

bccr_bm2_0 1.66 0.88 / 1.02 0.98 / 1.24 0/0 cccma_cgcm3_1 0.97 1.02 / 1.57 0.34 / 0.36 0/0 csiro_mk3_0 0.28 2.56 / 2.78 0.81 / 0.78 0/0 giss_model_e_r 0.25 5.99 / 6.73 0.21 / 0.18 61/99 ingv_echam4 1.95 0.7 / 0.87 0.93 / 0.93 0/0 inmcm3_0 0.92 1.37 / 1.61 0.71 / 0.67 0/0 miroc3_2_medres 0.17 3.24 / 3.51 0.47 / 0.41 0/0 ncar_ccsm3_0 0.96 2.21 / 2.39 0.86 / 0.72 5/2 ncar_pcm1 6.0 0.91 / 0.88 0.88 / 0.87 0/0 ukmo_hadgem1 1.11 2.19 / 2.26 0.59 / 0.72 0/0 Observed (1979-2013) 2.7 2.05 1.05 2


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Supplementary Table 6 | Statistics for CMIP5 models not selected. Shown are Niño3 rainfall skewness that measures the nonlinearity of the equatorial Pacific; climatological rainfall over the Niño3 region; standard deviation of Niño3 SST anomalies after removing variances on time scales longer than nine years, including climate change signals; and frequency of extreme El Nino event. Here we discard the criterion of “an austral summer rainfall skewness greater than 1” but retain the

criterion of “austral rainfall over the Niño3 region greater than 5 mm per day in at least one

summer” over the 200-year period (1891–2090). This is to test the sensitivity to our criteria. Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate

Change period (1991–2090). Models in purple indicate they generate an overly large Niño3 rainfall. When these CMIP3 models are included, there is still a large increase in the occurrences of extreme El Niño events.

Model

Niño3

rainfall skewness, full period

Niño3

rainfall

climatology, (1891–1990)/

(1991–2090)

(mm per day)


standard deviation

(1891–1990)/

(1991–2090)

(°C)

Frequency of s extreme El Niño

events

(1891–1990)/

(1991–2090)

GISS-E2-R 0.91 4.78 / 7.28 0.61 / 0.58 51/89 CSIRO-Mk3-6-0 0.84 1.49 / 1.75 0.70 / 0.64 0/0

IPSL-CM5A-MR 0.55 2.04 / 2.30 0.75 / 0.52 0/0

MIROC-ESM 0.34 3.30 / 4.00 0.48 / 0.49 0/0

MIROC-ESM-CHEM 0.39 3.25 / 4.06 0.48 / 0.44 0/10

Inmcm4 0.48 1.85 / 1.90 0.62 / 0.48 0/0

IPSL-CM5A-LR 0.67 1.92 / 2.17 0.72 / 0.62 0/0

HadGEM2-ES 0.96 1.46 / 1.91 0.88 / 0.82 0/0

ACCESS1-0 0.82 3.85 / 5.15 0.91 / 0.97 39/58 ACCESS1-3 0.95 3.30 / 4.39 0.84 / 0.80 10/24 Observed (1979-2013) 2.7 2.05 1.05 2


27

Supplementary Table 7 | Statistics of perturbed physics ensembles. Experiments in blue are selected for their ability to (1) produce the nonlinear ocean-atmosphere coupling with austral summer rainfall skewness greater than 1 and (2) generate austral summer rainfall over the Niño3 region greater than 5 mm per day in at least one summer over the 200-year period (1891–2090). Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate

Change period (1991–2090). Experiments marked with a red star are able to generate zonal SPCZ events4.

Model

Niño3 rainfall


Niño3 rainfall



standard deviation (1891–1990)/ (1991–2090) mm per day


events (1891–1990)/ (1991–2090)

0* 4.31 1.41 / 2.07 0.85 / 1.52 2 / 6 1* 4.22 1.42 / 1.74 1.13 / 1.55 2 / 5 2* 6.11 1.46 / 2.07 0.99 / 1.51 1 / 4 3* 2.56 1.27 / 1.48 0.82 / 1.16 4* 5.90 1.46 / 2.05 0.86 / 1.46 1 / 4 5* 3.70 1.37 / 1.51 1.11 / 1.39 2 / 4 6* 2.49 1.13 / 1.54 0.70 / 1.22 7 1.05 1.13 / 1.59 0.49 / 1.13 8* 2.03 0.99 / 1.29 0.54 / 1.20 9* 6.06 1.12 / 1.79 0.60 / 1.44 0 / 4 10* 3.48 1.19 / 1.65 0.74 / 1.39 0 / 2 11* 1.17 1.36 / 1.72 0.52 / 0.99 12* 1.93 0.97 / 1.40 0.82 / 1.38 13 1.45 1.23 / 1.67 0.45 / 1.10 14* 4.63 0.97 / 1.24 0.77 / 1.29 0 / 1 15* 0.76 1.26 / 1.55 0.52 / 1.28 16* 4.96 0.91 / 1.24 0.73 / 1.28 0 / 2 17* 3.81 1.62 / 2.60 1.27 / 1.78 4 / 10 18* 4.90 1.12 / 2.15 0.80 / 1.60 0 / 8 19 5.37 1.32 / 2.06 0.82 / 1.45 1 / 6 20* 3.78 1.31 / 2.47 0.96 / 1.75 2 / 10 21* 4.35 1.44 / 2.37 0.86 / 1.49 1 / 9 22* 4.87 1.44 / 1.98 0.89 / 1.50 0 / 4 23* 3.08 1.91 / 2.70 1.36 / 1.76 7 / 12 24 6.44 1.40 / 1.94 0.83 / 1.51 0 / 4 25* 4.19 1.29 / 2.52 0.91 / 1.73 1 / 10 26* 5.46 1.27 / 1.83 0.87 / 1.43 0 / 5 27* 3.11 1.79 / 2.75 1.36 / 1.85 7 / 13 28* 3.82 1.63 / 2.56 1.20 / 1.78 3 / 12 29* 3.77 1.71 / 2.92 1.13 / 1.77 3 / 12 30* 4.15 1.44 / 2.53 1.03 / 1.67 3 / 11 31* 4.44 1.36 / 2.10 0.91 / 1.49 0 / 8 32* 4.46 1.45 / 2.19 1.04 / 1.64 3 / 7 Average for all 3.82 1.33 / 1.97 0.87 / 1.46 Average for selected 4.54 1.34 / 2.12 0.96 / 1.56 43 / 173 (302 %)


28

experiments

Supplementary Table 8 | Sensitivity of frequency of extreme El Niño events to varying definitions. The results shown were obtained using quadratically detrended Niño3 rainfall anomalies for selected models as described in Supplementary Tables 1 and 2. Numbers in red type indicate a decrease or no change from the Control period (1891–1990) to the Climate Change period (1991–2090). Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate Change period (1991–2090).


Model

Detrended Niño3 rainfall

anomaly > 2 mm per day (1891–1990)/ (1991–2090)







cnrm_cm3 11 / 18 6 / 14 5 / 13 2 / 10 csiro_mk3_5 2 /5 0 / 1 0 / 1 0 / 1 gfdl_cm2_0 2 / 10 0 / 8 0 / 6 0 / 6 gfdl_cm2_1 5 / 5 3 / 4 3 / 4 2 / 4 ipsl_cm4 6 / 9 3 / 6 1 / 3 0 / 0 miub_echo_g 7 / 10 5 / 8 2 / 7 1 / 4 mpi_echam5 2 / 9 1 / 2 1 / 1 1 / 0 mri_cgcm2_3_2a 3 / 12 2 / 10 2 / 6 0 / 5 ukmo_hadcm3 5 / 6 0 / 4 0 / 2 0 / 1 CCSM4 16 / 15 15 / 11 12 / 8 9 / 6 MRI-CGCM3 0 / 4 0 / 2 0 / 2 0 / 2 MIROC5 4 / 7 3 / 6 3 / 6 3 / 5 IPSL-CM5B-LR 3 / 12 0 / 9 0 / 5 0 / 2 GFDL-CM3 7 / 15 6 / 11 6 / 7 5 / 7 GFDL-ESM2M 11 / 8 10 / 8 9 / 8 9 / 8 CNRM-CM5 14 / 19 10 / 14 9 / 9 6 / 5 CESM1-BGC 12 / 15 10 / 10 7 / 5 1 / 3 CanESM2 9 / 11 7 / 6 2 / 3 0 / 2 HadGEM2-CC 6 / 7 2 / 3 1 / 1 0 / 0 CMCC-CM 1 / 1 0 / 1 0 / 1 0 / 0 Total (% increase) 126 / 198 (57 %) 83 / 138 (66 %) 63 / 98 (56 %) 39 / 71 (82 %)


29

Supplementary Table 9 | Sensitivity of frequency of extreme El Niño events to varying definitions. The results shown were obtained using quadratically detrended Niño3 SST for selected models as described in Supplementary Tables 1 and 2. Numbers in red type indicate a decrease or no change from the Control period (1891–1990) to the Climate Change period (1991–2090). Numbers in bold type indicate an increase from the Control period (1891–1990) to the Climate Change period (1991–

2090).


Model

Normalised, detrended Niño3 SST

greater than 1.5 standard

deviations (1891–1990)/ (1991–2090)


greater than 2 standard deviations

(1891–1990)/ (1991–2090)


greater than 2.5 standard

deviations (1891–1990)/ (1991–2090)


greater than 3 standard deviations

(1891–1990)/ (1991–2090)

cnrm_cm3 6 / 4 0 / 0 0 / 0 0 / 0 csiro_mk3_5 6 / 5 3 / 1 0 / 1 0 / 0 gfdl_cm2_0 6 / 13 4 / 10 1 / 7 1 / 3 gfdl_cm2_1 8 / 10 6 / 5 1 / 4 1 / 3 ipsl_cm4 7 / 5 2 / 4 0 / 0 0 / 0 miub_echo_g 6 / 9 0 / 1 0 / 0 0 / 0 mpi_echam5 9 / 12 2 / 6 1 / 1 1 / 0 mri_cgcm2_3_2a 6 / 16 5 / 12 1 / 8 1 / 6 ukmo_hadcm3 8 / 6 4 / 3 0 / 2 0 / 0 CCSM4 6 / 1 1 / 1 0 / 0 0 / 0 MRI-CGCM3 19 / 17 11 / 11 8 / 10 3 / 9 MIROC5 4 / 7 3 / 6 3 / 4 2 / 1 IPSL-CM5B-LR 17 / 17 10 / 12 4 / 8 2 / 5 GFDL-CM3 6 / 5 2 / 0 0 / 0 0 / 0 GFDL-ESM2M 4 / 1 0 / 0 0 / 0 0 / 0 CNRM-CM5 7 / 6 1 / 0 0 / 0 0 / 0 CESM1-BGC 10 / 3 2 / 2 0 / 1 0 / 0 CanESM2 3 / 2 0 / 0 0 / 0 0 / 0 HadGEM2-CC 4 / 1 2 / 0 0 / 0 0 / 0 CMCC-CM 12 / 17 11 / 12 5 / 9 5 / 6 Total (% increase) 154 / 157 (2 %) 69 / 86 (25 %) 24 / 55 (129 %) 16 / 33 (106 %)


30

Supplementary Table 10 | Total number of zonal SPCZ events and extreme El Niño events, and their overlaps from an aggregate of 20 models, each covering a 200-year period giving 3400 years of virtual climate. Shown are for the case in which an extreme El Niño is defined as when Niño3 rainfall is greater than 5 mm per day.

Total number

of Extreme El Niño

Total

number of Extreme El

Niño coinciding with zonal SPCZ event

Total

number of Extreme El Niño not

coinciding with zonal

SPCZ events

Total

number of Zonal SPCZ event not coinciding

with extreme El

Niño

Total

number of Zonal SPCZ

event coinciding

with extreme El

Niño

Total

number of Zonal SPCZ

event

313 85% of the

total number of zonal SPCZ

events

252 80% of the

total number of extreme El Niño events

60 20% of the

total number of extreme El

Niño events

168 40% of the

total number of zonal SPCZ

events

252 60% of the total zonal

SPCZ events

420 134% of the

total number of extreme El

Niño events

References

1. Rayner, N. A. et al. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res. 108, 4407, (2003).

2. Adler, R. F. et al. The version 2 Global Precipitation Climatology Project (GPCP) monthly precipitation analysis (1979–present). J. Hydrometeor. 4, 1147–1167 (2003).

3. Johnson, N. C., & Xie, S.-P. Changes in the sea surface temperature threshold for tropical

convection. Nature Geosci. 3, 842-845 (2010).

4. Xie, S. P. et al. Global warming pattern formation: sea surface temperature and rainfall. J.

Clim. 23, 966–986 (2010).

5. Cai, W. et al. More extreme swings of the South Pacific convergence zone due to greenhouse warming. Nature 488, 365–369 (2012).

6. Collins, M. et al. A comparison of perturbed physics and multi-model ensembles: Model errors, feedbacks and forcings. Clim. Dyn. 36, 1737–1766. (2011).


increasing frequency of extreme el niño events due to ......5 . supplementary figure 4| full...

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