Abstract
AbstractThe impact of a warming climate on El Niño–Southern Oscillation (ENSO) is investigated in large-ensemble simulations of the Community Earth System Model (CESM1). These simulations are forced by historical emissions for the past and the RCP8.5-scenario emissions for future projections. The simulated variance of the Niño-3.4 ENSO index increases from 1.4°C2 in 1921–80 to 1.9°C2 in 1981–2040 and 2.2°C2 in 2041–2100. The autocorrelation time scale of the index also increases, consistent with a narrowing of its spectral peak in the 3–7-yr ENSO band, raising the possibility of greater seasonal to interannual predictability in the future. Low-order linear inverse models (LIMs) fitted separately to the three 60-yr periods capture the CESM1 increase in ENSO variance and regularity. Remarkably, most of the increase can be attributed to the increase in the 23-month damping time scale of a single damped oscillatory ENSO eigenmode of these LIMs by 5 months in 1981–2040 and 6 months in 2041–2100. These apparently robust projected increases may, however, be compromised by CESM1 biases in ENSO amplitude and damping time scale. An LIM fitted to the 1921–80 observations has an ENSO eigenmode with a much shorter 8-month damping time scale, similar to that of several other eigenmodes. When the mode’s damping time scale is increased by 5 and 6 months in this observational LIM, a much smaller increase of ENSO variance is obtained than in the CESM1 projections. This may be because ENSO is not as dominated by a single ENSO eigenmode in reality as it is in the CESM1.
Highlights
As the dominant mode of tropical interannual variability with global teleconnections, El Niño–Southern Oscillation (ENSO) is the leading source of forecast skill on seasonal and interannual time scales and plays an important role in the global dynamics of climate change
Given its prominent role in climate variability, it is of great interest to determine how ENSO may change in a warming climate
The spectrum of the modified linear inverse models (LIMs) have more power in the ENSO band, the peaks are slightly lower than those for LIMs directly fitted to period 2 (P2) and period 3 (P3) (Figs. 8a,b). These results suggest that an increase from 23 to 28 months and from 23 to 29 months in the damping time scale of the single ENSO mode can by itself explain almost all of the changes in the CESM1 spectra
Summary
As the dominant mode of tropical interannual variability with global teleconnections, El Niño–Southern Oscillation (ENSO) is the leading source of forecast skill on seasonal and interannual time scales and plays an important role in the global dynamics of climate change. Capotondi and Sardeshmukh (2017) and Aiken et al (2013) studied the change of ENSO in the observational record and found a general increase of ENSO variability, as well as of SST spectral power in the 3–7-yr band. Their studies were limited by the length of their observational record of about 50 years. They partly addressed this limitation, when assessing the statistical significance of their conclusions, by generating large synthetic dynamically consistent samples using linear inverse models (LIMs) fitted to different segments of the observational record
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