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.