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WIREs Clim Change
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El Niño and our future climate: where do we stand?

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Abstract El Niño and La Niña comprise the dominant mode of tropical climate variability: the El Niño and Southern Oscillation (ENSO) phenomenon. ENSO variations influence climate, ecosystems, and societies around the globe. It is, therefore, of great interest to understand the character of past and future ENSO variations. In this brief review, we explore our current understanding of these issues. The amplitude and character of ENSO have been observed to exhibit substantial variations on timescales of decades to centuries; many of these changes over the past millennium resemble those that arise from internally generated climate variations in an unforced climate model. ENSO activity and characteristics have been found to depend on the state of the tropical Pacific climate system, which is expected to change in the 21st century in response to changes in radiative forcing (including increased greenhouse gases) and internal climate variability. However, the extent and character of the response of ENSO to increased in greenhouse gases are still a topic of considerable research, and given the results published to date, we cannot yet rule out possibilities of an increase, decrease, or no change in ENSO activity arising from increases in CO2. Yet we are fairly confident that ENSO variations will continue to occur and influence global climate in the coming decades and centuries. Changes in continental climate, however, could alter the remote impacts of El Niño and La Niña. Copyright © 2010 John Wiley & Sons, Ltd. This article is categorized under: Paleoclimates and Current Trends > Detection and Attribution

Tropical Pacific climatology, El Niño, and El Niño impacts. Upper panels show sea surface temperature (SST, shaded) and precipitation (contoured) for (a) the annual average and (b) monthly anomalies averaged June–December for five recent El Niño events (1982, 1987, 1991, 1997, 2002). SST is shown in units of degree Celsius and is computed from Ref 12, precipitation is shown in units of millimeter per day and is computed using the Ref 15 dataset. Dashed contours in (b) indicate regions of reduced rainfall. Also indicated are the NIÑO3 index region (150°W–90°W, 5°S–5°N) and the source location of fossil corals recovered from Palmyra Island (Ref 13 and Figure 2). Lower panels (courtesy of the United States National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center) are schematic representations of the typical climate response to El Niño during (c) austral winter and (d) boreal winter.

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Multimodel variability of surface temperature and rainfall, and the projected sensitivity of the variability. Top panels show a multimodel estimate of interannual standard deviation of (a) surface temperature and (b) precipitation. Lower panels show the fractional change in interannual standard deviation in (c) surface temperature and (d) precipitation projected from a mid‐range emissions scenario after stabilization. In the lower panels, blue colors indicate a reduction in variability, orange and yellow shading indicate an increase in variability. Figure adapted from Ref 65. Notice the strongest increase (reduction) in tropical rainfall variability in panel (d) occurs in regions where the mean rainfall increases (decreases) most strongly (Figure 4(b)). (Reprinted with permission from the American Meteorological Society).

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Twenty‐first century projected changes in El Niño characteristics. (a) Multimodel projections of changes in tropical Pacific sea level pressure mean state (horizontal axis) versus change in El Niño sea surface temperature variability (vertical axis); the ‘mean state’ change in each model is characterized by its similarity to the pattern of El Niño variability. Changes in mean state and El Niño are computed by comparing the end of the 21st century projections with the end of the 20th century from the analysis of Ref 32. (b) Change in El Niño amplitude (vertical axis) versus the meridional (north–south) width of the preindustrial near‐equatorial westerly wind anomalies associated with El Niño, in response to increasing levels of atmospheric CO2, from the CMIP3 ensemble of global climate models; different symbols indicate character of model response as characterized in Ref 58. The three models highlighted by green Ref 47 arrows have 20th century El Niño variations that are much weaker than the observed and are considered less reliable. (Left panel adapted with permission from Ref 48. Copyright 2009; Right panel adapted with permission from Ref 60. Copyright 2006 American Meteorological Society).

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Twenty‐first century projected changes in climatology due to increasing greenhouse gases. Multimodel averages of the (a) change in surface temperature and (b) fractional change in precipitation in the 21st century relative to the late‐20th century, using the 21 general circulation models that participated in the CMIP3 intercomparison. In both panels, the changes have been normalized by each model's global‐mean surface temperature change prior to averaging across models. (Adapted with permission from Ref 46. Copyright 2007 American Meteorological Society).

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Simulated decadal and centennial variations in El Niño in the absence of radiative forcing changes. Running annual‐mean values of NIÑO3 sea surface temperature (Figure 1) from a 2000‐year simulation using a ‘state‐of‐the‐art’ global climate model with invariant radiative forcing (i.e., no changes in greenhouse gases or insolation, and so on). Red (blue) shading indicates El Niño (La Niña) events. Notice the strong internally generated variations in the character of El Niño on multidecadal and centennial timescales. (Adapted with permission from Ref 22. Copyright 2009).

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Instrumental and coral‐based records of El Niño/La Niña. Upper time series shows the monthly NIÑO3 sea surface temperature (SST) anomaly index from Ref 12 (blue line, left scale), the logarithm of the number of SST observations per year in the NIÑO3 region based on Ref 11 (yellow shading, right scale), and the era in which satellite estimates of SST are available (red horizontal line). Lower time series show the 2–7‐year filtered ratio of Oxygen‐18 to Oxygen‐16 isotope concentrations from corals taken from Palmyra Island—with positive values indicating warmer, wetter conditions associated with El Niño—after Ref 13. See Figure 1(a,b) for location of Palmyra Island and the NIÑO3 region. (Lower panels are reprinted with permission from Ref 13. Macmillan Publishers Ltd. Copyright 2003).

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