The El Niño–Southern Oscillation (ENSO) drives substantial variability in rainfall1, 2, 3, severe weather4, 5, agricultural production3, 6, ecosystems7 and disease8 in many parts of the world. Given that further human-forced changes in the Earth’s climate system seem inevitable9, 10, the possibility exists that the character of ENSO and its impacts might change over the coming century. Although this issue has been investigated many times during the past 20 years, there is very little consensus on future changes in ENSO, apart from an expectation that ENSO will continue to be a dominant source of year-to-year variability9, 11, 12. Here we show that there are in fact robust projected changes in the spatial patterns of year-to-year ENSO-driven variability in both surface temperature and precipitation. These changes are evident in the two most recent generations of climate models13, 14, using four different scenarios for CO2 and other radiatively active gases14, 15, 16, 17. By the mid- to late twenty-first century, the projections include an intensification of both El-Niño-driven drying in the western Pacific Ocean and rainfall increases in the central and eastern equatorial Pacific. Experiments with an Atmospheric General Circulation Model reveal that robust projected changes in precipitation anomalies during El Niño years are primarily determined by a nonlinear response to surface global warming. Uncertain projected changes in the amplitude of ENSO-driven surface temperature variability have only a secondary role. Projected changes in key characteristics of ENSO are consequently much clearer than previously realized.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Leading patterns (standardized, first EOFs) of interannual variability in surface temperature (ST) and precipitation. (482 KB)
Observations29, 30 (top: a, ST; b, precipitation), CMIP5 models (c, ST; d, precipitation) and CMIP3 models (e, ST; f, precipitation). The model patterns are MMAs of the first EOF of each individual model. All data were spectrally filtered to remove variability with periods greater than 13 years before EOF analysis and are standardized as described in Methods.
- Extended Data Figure 2: Scatter plot showing the amplitude of EOF1 time series in the twenty-first century (y axis) and the twentieth century (x axis) in individual models under the four scenarios. (134 KB)
Dots above the line (indicated by the dashed line) are projected increases; dots below the line y = x are projected decreases. The amplitude is measured here using the standard deviation. The time series for each model was re-scaled as described in the text. The vertical (dotted) line gives an observational estimate. The figures in the bottom right of the plot give the percentage of models above the diagonal; that is, with a projected increase.
- Extended Data Figure 3: The SST anomalies (SSTAs) used in the AGCM experiments. (624 KB)
a, SST_EN; b, SSTA_GW (RCP8.5); c, SSTA_GW (A2); d, ΔSSTA_EN (RCP8.5); e, ΔSSTA_EN (A2). SSTA_EN is the observed SST anomaly averaged over all El Niño events between 1978 and 2009, SSTA_GW is the change in background SST projected for the late twenty-first century, and ΔSSTA_EN is the projected multi-model average change in the filtered El Niño SST pattern obtained from climate models.
- Extended Data Figure 4: Multi-model average (MMA) of projected change in mean SST under the four scenarios. (1,062 KB)
Note the high degree of similarity in the spatial structures between the scenarios and the enhanced equatorial warming towards the east.
- Extended Data Figure 5: Impact of bias correction (or ‘shifting’) on the MMA of ΔEOF1 of surface temperature and precipitation for the four different twenty-first-century scenarios. (488 KB)
Each model’s first EOF was shifted east to maximize the spatial correlation coefficient with the observed first EOF. On average, roughly 20° (CMIP3 models) and 14° (CMIP5 models) of shifting was required. Some models did not require adjustment; others required eastward shifts up to 22.5°. The MMA of both unadjusted EOFs (‘raw’) and bias-corrected EOFs (‘shifted’) are shown. The dateline is indicated by the red dashed vertical line in each panel for reference only. ST: a–d, raw; e–h, shifted. Precipitation: h–l, raw; m–p, shifted. Stippling is applied if more than 70% of models agree on the sign of change. See Methods for further details.
- Extended Data Figure 6: Precipitation along the Equator over the Pacific in AGCM experiments 6 and 7. (460 KB)
See Methods and Extended Data Table 2 for more details on these experiments. Values in the top two panels represent differences between precipitation with α ≥ 1 and the corresponding experiment with α = 0. The twentieth-century figures, for example, are differences relative to precipitation in the experiment with α = 0 under twentieth-century conditions. The twenty-first-century figures are differences relative to precipitation in the experiment with α = 0 and SSTA_GW, and either twentieth-century CO2 concentrations (dotted lines) or late twenty-first-century CO2 concentrations (dashed lines) from experimental sets 6 and 7. a, RCP8.5; b, A2. The lower two panels depict the impact of warming only (21C − 20C, solid lines) and the impact of increasing CO2 concentrations only (21C SST&CO2 − 21C, dotted lines). c, RCP8.5; d, A2. The results show that the SST changes are primarily responsible for the precipitation response (a, b), and that the impact of CO2 change—over and above the impact it has as a result of the changes in SST it causes—is small compared with the nonlinear response (c, d).
- Extended Data Figure 7: Contribution of PTH, PMCD, PCOV and E to the response in the AGCM. (182 KB)
a, RCP8.5; b, A2. α = 1 and 4 only. Symbols are defined in Methods.
- Extended Data Figure 8: Ability of CMIP5 models to simulate the spatial structure of observed EOF2 of ST, and projected changes in the relative frequency of central Pacific and eastern Pacific El Niños. (389 KB)
a, Observed EOF2 of ST. b, Spatial correlation coefficient between observed EOF2 and both EOF2 and EOF3 in each model. c, Change in the relative frequency of central Pacific and eastern Pacific El Niños under the RCP8.5 scenario.