Examining ENSO Statistics in CESM-LE

An analysis of the probability density function (PDF) of the Southern Oscillation Index (SOI) in the CESM-WACCM4 control run, 150 Tg US-Russia nuclear war scenario, and CESM-LE 1850 PI run highlights a potential issue with our 60-year (720 month) control run compared to the CESM-LENS PI run. The PDF of the SOI in the CESM-LENS PI run (200 year sample) is far more narrow compared to the 60 years of the CESM-WACCM4 control run. The 150 Tg US-Russia run is skewed to the left as a result of the Nuclear Niño.

I examined 10 random 720 month chunks of the CESM-LE to test whether using sample sizes similar to the 60-year CESM-WACCM4 control run could explain the difference between the CESM-LENS PI and CESM-WACCM4 control run. Unfortunately, this is not really the case. There are minor variations to the left and right but no single chunk is as wide as the CESM-WACCM4.

Potential explanations for differences:

1. CESM-WACCM4 uses year 2000 CO2 concentrations. It’s possible that in a warmer climate, CESM simulates a higher amplitude ENSO signal.

2. CESM-WACCM4 uses a high-top model for its atmospheric component compared to CESM-LE which uses CAM5. The higher-top model better simulates stratospheric dynamics which could influence ENSO.

Next step: I could use the 35 ensembles available with the CESM-LE and use the 1990-2005 period (15 years * 35 ensembles) for 525 years to see if it resembles the CESM-WACCM4 more.
OR I can figure out if anyone has used WACCM4 to run a longer control run which I could use as comparison.
OR I could do nothing and just word any comparisons of Nuclear Niño to the historical record more carefully.