Will There Be a Significant Change to El Niño in the Twenty-First Century?

Abstract

The El Niñ o-Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 18 nominal resolution Community Climate System Model, version 4 (CCSM4) Coupled Model Intercomparison Project phase 5 (CMIP5) simulations: twentieth-century ensemble, preindustrial control, twenty-first-century projections, and stabilized 2100-2300 ''extension runs.'' ENSO variability weakens slightly with CO 2 ; however, various significance tests reveal that changes are insignificant at all but the highest CO 2 levels. Comparison with the 1850 control simulation suggests that ENSO changes may become significant on centennial time scales; the lack of signal in the twentieth-versus twenty-first-century ensembles is due to their limited duration. Changes to the mean state are consistent with previous studies: a weakening of the subtropical wind stress curl, an eastward shift of the tropical convective cells, a reduction in the zonal SST gradient, and an increase in vertical thermal stratification take place as CO 2 increases. The extratropical thermocline deepens throughout the twenty-first century, with the tropical thermocline changing slowly in response. The adjustment time scale is set by the relevant ocean dynamics, and the delay in its effect on ENSO variability is not diminished by increasing ensemble size. The CCSM4 results imply that twenty-first-century simulations may simply be too short for identification of significant tropical variability response to climate change. An examination of atmospheric teleconnections, in contrast, shows that the remote influences of ENSO do respond rapidly to climate change in some regions, particularly during boreal winter. This suggests that changes to ENSO impacts may take place well before changes to oceanic tropical variability itself become significant.

Linear trends in sea surface temperature of the tropical Pacific Ocean and implications for the El Niño-Southern Oscillation

“…The wintertime (November-February) season is not associated with significant trends in the zonal SST gradient. While ENSO represents a coupled mode of atmosphereocean variability, it is reasonable to presume that significant positive SST trends across much of the equatorial Pacific will impact ENSO categorizations based on SSTs, even if it isn't clear how the aggregate characteristics of the ENSO phenomenon will be affected by these trends (DiNezio et al 2009;Collins et al 2010;Stevenson et al 2011). The NOAA Climate Prediction Center (CPC) primarily uses ERSST.v3b values of the Niño-3.4 index to categorize El Niño and La Niña events.…”

Mean Biases, Variability, and Trends in Air–Sea Fluxes and Sea Surface Temperature in the CCSM4

“…The need for improvement is not inconsequential for understanding climate change, as the seasonal cycle and interannual variability are strongly correlated with societal and climatological interest (Stine et al 2009). Annual variability is substantially in error in virtually all regions with the likelihood of robust CCSM4-CORE disagreement, based on the WPA analysis of Stevenson et al (2010), almost always above 90%. Using WPA, representation of interannual variability is better with all of the regions showing likely agreement.…”

“…Furthermore, the CORE dataset involves interpolation and is thus likely to be inaccurate on a gridpoint by gridpoint level. For a quantitative estimate of significance of variance errors, we employ another statistical measure, the wavelet probability analysis (WPA) hypothesis testing procedure (Stevenson et al 2010). WPA was designed for use with climate index data, where the variability is generally not normally distributed (e.g., El Niñ o indices), nor are the variances distributed according to chisquared statistics (although WPA will reproduce these distributions if they do occur).…”

“…This attribution was based on numerical experiments involving select substitution of model fluxes and related parameters with observations. Stine et al (2009) point out the poor quality of the annual cycle in Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) GCMs, and improving interannual variability has also received recent attention in CCSM (Neale et al 2008;Stevenson et al 2010Stevenson et al , 2012. We examine flux variability in CCSM4 on these time scales, a first for CCSM because having the CORE fluxes only now makes this comparison meaningful.…”

Reef cores as records of Holocene water temperatures of the southern Great Barrier Reef: Geochemical analysis of traditional (Porites) and non-traditional (Acropora) reef building corals

“…Numerical climate models currently disagree on the ENSO response to the current trend of climate change, with predictions for an enhanced, reduced or unchanged system [e.g., DiNezio et al, 2012;Rashid et al, 2016;Stevenson et al, 2012].…”