Does ENSO regularity increase in a warming climate?

As leading source of seasonal to interannual predictability, El Nino Southern Oscillation (ENSO) plays a central role in the climate system. NCAR scientists in collaboration with University of Colorado/CIRES and University of Oxford, UK, analyzed ensemble simulations with the Community Earth System Model (CESM1) to address the question how ENSO regularity and variability change in a warming climate. These simulations are forced by historical emissions for the past and the RCP8.5-scenario emissions for future projections.

The simulated variance of the Nino3.4 ENSO index increases from 1.4C2 in 1921-1980 to 1.9 C2 in 1981-2040 and 2.2 C2 in 2041-2100 (Fig. a,b). The autocorrelation timescale of the index also increases (Fig. c), consistent with a narrowing of its spectral peak in the 3- to 7-yr ENSO band, raising the possibility of greater seasonal to interannual predictability in the future.

Utilizing low-order linear inverse models (LIMs) it is shown that most of the change can be attributed to an increase in the 23-month damping timescale of a single damped oscillatory ENSO eigenmode of these LIMs by 5 months in 1981-2040 and 6 months in 2041-2100.

These findings were reported in the Journal of Climate, a Journal of the American Meteorological Society.

a) Distribution of ENSO-index for 33 ensemble members for time periods 1921-1980 (blue), 1981- 2040 (green) and 2041-2100 (red). b) Distribution of the variance of the ENSO-index (in C2). The mean variance for each time period is given by the thick vertical lines. The observed variance for the period 1921-1980 is given as black dashed line. c) Autocorrelation function as function of lag (in months) for each member (thin lines). Mean autocorrelation function averaged over all members is given by the thick solid lines.
Figure: a) Distribution of ENSO-index for 33 ensemble members for time periods 1921-1980 (blue), 1981- 2040 (green) and 2041-2100 (red). b) Distribution of the variance of the ENSO-index (in C2). The mean variance for each time period is given by the thick vertical lines. The observed variance for the period 1921-1980 is given as black dashed line. c) Autocorrelation function as function of lag (in months) for each member (thin lines). Mean autocorrelation function averaged over all members is given by the thick solid lines.