globalchange  > 气候变化事实与影响
DOI: doi:10.1038/nclimate2100
论文题名:
Increasing frequency of extreme El Niño events due to greenhouse warming
作者: Wenju Cai
刊名: Nature Climate Change
ISSN: 1758-1438X
EISSN: 1758-7558
出版年: 2014-01-19
卷: Volume:4, 页码:Pages:111;116 (2014)
语种: 英语
英文关键词: Projection and prediction
英文摘要:

El Niño events are a prominent feature of climate variability with global climatic impacts. The 1997/98 episode, often referred to as ‘the climate event of the twentieth century’1, 2, and the 1982/83 extreme El Niño3, featured a pronounced eastward extension of the west Pacific warm pool and development of atmospheric convection, and hence a huge rainfall increase, in the usually cold and dry equatorial eastern Pacific. Such a massive reorganization of atmospheric convection, which we define as an extreme El Niño, severely disrupted global weather patterns, affecting ecosystems4, 5, agriculture6, tropical cyclones, drought, bushfires, floods and other extreme weather events worldwide3, 7, 8, 9. Potential future changes in such extreme El Niño occurrences could have profound socio-economic consequences. Here we present climate modelling evidence for a doubling in the occurrences in the future in response to greenhouse warming. We estimate the change by aggregating results from climate models in the Coupled Model Intercomparison Project phases 3 (CMIP3; ref. 10) and 5 (CMIP5; ref. 11) multi-model databases, and a perturbed physics ensemble12. The increased frequency arises from a projected surface warming over the eastern equatorial Pacific that occurs faster than in the surrounding ocean waters13, 14, facilitating more occurrences of atmospheric convection in the eastern equatorial region.

The 1982/83 and 1997/98 extreme El Niño events were characterized by an exceptional warming, with sea surface temperatures (SSTs) exceeding 28°C extending into the eastern equatorial Pacific2, 3. This led to an equatorward shift of the intertropical convergence zone (ITCZ), and hence intense rainfall in the equatorial eastern Pacific where cold and dry conditions normally prevail. This major reorganization of atmospheric convection severely disrupted global weather patterns and spurred major natural disasters. Catastrophic floods occurred in the eastern equatorial region of Ecuador and northern Peru3, 7, and neighbouring regions to the south and north experienced severe droughts (Supplementary Fig. 1). The anomalous conditions caused widespread environmental disruptions, including the disappearance of marine life and decimation of the native bird population in the Galapagos Islands15, 16, and severe bleaching of corals in the Pacific and beyond4, 5. The impacts extended to every continent, and the 1997/98 event alone caused US$35–45 billion in damage and claimed an estimated 23,000 human lives worldwide17.

The devastating impacts demand an examination of whether greenhouse warming will alter the frequency of such extreme El Niño events. Although many studies have examined the effects of a projected warming on the Pacific mean state, El Niño diversity and El Niño teleconnections18, 19, 20, 21, the issue of how extreme El Niños will change has not been investigated. Here we show that greenhouse warming leads to a significant increase in the frequency of such events.

We contrast the characteristics between the extreme and moderate El Niño events using available data sets22, 23, focusing on December–January–February (DJF), the season in which El Niño events peak. During moderate events, which include canonical and Modoki El Niño24, the eastern boundary of the warm pool (indicated by the 28°C isotherm, purple, Fig. 1a) and the atmospheric convective zone move eastwards to just east of the Date Line. The ITCZ lies north of the Equator25, and the rainfall anomaly over the eastern equatorial Pacific is small (Fig. 1a).

Figure 1: Evolution and nonlinear characteristics of observed extreme El Niño events.
Evolution and nonlinear characteristics of observed extreme El Nino events.

a,b, Time–longitude diagram for composite moderate and extreme El Niño events, respectively, of equatorial SST anomalies (colour scale) and rainfall anomalies (contour, at intervals of 3mm per day), 28°C isotherm (purple curve) and total rainfall 5mm per day isopleth (green curve). c,d, Relationship of eastern equatorial Pacific (Niño3 area: 5°S–5°N, 150°W–90°W) DJF total rainfall with DJF Niño3 SST and meridional SST gradients in the Niño3 longitude range. The meridional SST gradient is defined as the average SST over the off-equatorial region (5°N–10°N, 150°W–90°W) minus the average over the equatorial region (2.5°S–2.5°N, 150°W–90°W). Extreme El Niño (defined as events for which austral summer rainfall is greater than 5mm per day), moderate El Niño (defined as events with SST anomalies greater than 0.5 s.d. of that over the period since 1979 that are not extreme El Niño events), and La Niña and neutral events, are indicated by red, green and blue dots respectively. During extreme El Niño, the meridional SST gradient diminishes, or reverses, shifting the ITCZ to the eastern equatorial Pacific.

Diagnosis of extreme El Niño events.

We use rainfall data in the satellite era (1979–present)23, and SSTs from a global reanalysis22. DJF rainfall averaged over the Niño3 region (150°W–90°W, 5°S–5°N) and meridional SST gradient in the eastern Pacific (150°W–90°W), calculated as the difference between the average over the off-equatorial (5°N–10°N) and equatorial box (2.5°S–2.5°N) regions, are used as atmospheric and oceanic indices to characterize extreme El Niño events. Rainfall increases nonlinearly with Niño3 SST, or with the meridional gradient. The nonlinearity is measured by the skewness of Niño3 precipitation, which is 2.75 in observations. DJF Niño3 rainfall greater than 5mm per day defines an extreme El Niño event.

Selection of models.

DJF Niño3 rainfall greater than 5mm per day and rainfall skewness greater than 1 are used as criteria for model selection from a total of 19 CMIP3 (ref.  10) and 21 CMIP5 (ref. 11) CGCMs. One experiment (the first simulation) from each model is used, covering the period 1891–2090 using historical anthropogenic and natural forcings to 2000 for CMIP3 and 2005 for CMIP5, and then a future emission scenario SRESA2 for CMIP3 and the RCP8.5 for CMIP5. In addition, 33 SST-bias-corrected PPE experiments, conducted with the HadCM3 CGCM forced with historical radiative perturbations and a 1% per year CO2 increase12 for the future climate change runs, each covering a 200-year period, are used. Only 9 CMIP3 and 11 CMIP5 CGCMs meet the criteria (Supplementary Tables 1 and 2), yielding a mean skewness close to the observed (Supplementary Tables 1 and 2). The skewness criterion filters out models with an overly wet or cold and dry model east equatorial Pacific (Supplementary Figs 10 and 11). These biases generally reduce the skewness, and are associated with SSTs well below or above the convective threshold range of 26–28°C (ref. 29), leading to overly subdued or active Niño3 rainfall variability. Out of 33 PPE experiments, 25 meet the skewness criterion. We derive changes in the frequency of extreme El Niño events by comparing the first 100 years (control period) to the later (climate change period) years. We also test the sensitivity of our results to varying definitions (Supplementary Tables 3–6).

Contrasts between extreme El Niño and zonal SPCZ events.

Neither zonal SPCZ nor extreme El Niño is a subset of the other (Supplementary Fig. 5a). This is because zonal SPCZ events are more closely associated with the south off-equatorial minus the equatorial meridional SST gradients over the central Pacific longitudes (Supplementary Fig. 5b), instead of the north off-equatorial minus the equatorial SST gradients over the eastern Pacific longitudes, which characterize extreme El Niño (Fig. 2c,d and Supplementary Fig. 5c). An aggregation over the 20 selected CGCMs (Supplementary Figs 6–13 and Tables 1–2) and over 200 years shows that about 40% of all zonal SPCZ events are independent from extreme El Niño events (green dots, Supplementary Fig. 5a and Table 10), analogous to the 1991/92 event, with generally lower Niño3 rainfall and larger north off-equatorial minus equatorial SST gradients in the eastern Pacific, in contrast to those during extreme El Niño events that can occur without zonal SPCZ events (about 20%, purple dots in Supplementary Fig. 5a). Supplementary Fig. 5b–e further contrasts the SST and rainfall anomaly patterns associated with independent zonal SPCZ events from those during extreme El Niño events in which the anomalies extend farther east into the Niño3 region.

Total rainfall change.

The total rainfall change in the eastern equatorial Pacific under greenhouse warming (ΔRaintotal) contains contributions from a change in the annual cycle (ΔRainannual-cycle), a long-term trend (ΔRainlong-term), and a change in the response of rainfall to changing El Niño/Southern Oscillation (ENSO; ΔRainENSO). For a given season, the ΔRainannual-cycle and ΔRainlong-term terms can be combined to a total long-term trend, ΔRaintotal-long-term, such that

As ENSO is seasonally phase-locked, peaking in austral summer, if there is a trend due to the response to ENSO, the total rainfall trend will include the contribution from ΔRainENSO, which would be at least partially removed by the detrending process. To understand how the distribution of rainfall anomalies will change, rainfall is quadratically detrended. The detrending process might partially remove the rainfall increase due to the increased frequency of extreme El Niño events.

Statistical significance test.

We use a bootstrap method27 to examine whether the change in frequency of the extreme El Niño events is statistically significant. The 2,000 DJF samples from the 20 selected CGCMs in the control period are re-sampled randomly with replacement to construct 10,000 realizations. The standard deviation of the extreme El Niño frequency in the inter-realization is 9.8 events per 2,000 years, far smaller than the difference between the control and the climate change periods at 111 events per 2,000 years (Fig. 2c,d), indicating statistical significance of the difference.

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  8. Vincent, E. M. et al. Interannual variability of the South Pacific Convergence Zone and implications for tropical cyclone genesis. Clim. Dynam. 36, 18811896 (2011). URL:
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资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/5258
Appears in Collections:气候变化事实与影响
科学计划与规划
气候变化与战略

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Wenju Cai. Increasing frequency of extreme El Niño events due to greenhouse warming[J]. Nature Climate Change,2014-01-19,Volume:4:Pages:111;116 (2014).
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