CESM2 Large Ensemble Community Project

We are pleased to announce the launch of the CESM2 Large Ensemble in partnership with the IBS Center for Climate Physics in South Korea. When completed, the CESM2 Large Ensemble will consist of 100 members at 1 degree spatial resolution covering the period 1850-2100 under CMIP6 historical and SSP370 future radiative forcing scenarios. Data sets from this ensemble will be made available via the Climate Data Gateway on June 14th, 2021.

As part of the CESM2 Large Ensemble project, it was decided to use two separate sets of biomass burning emissions forcing files, with the first 50 members having been forced with CMIP6 protocols identical to those used in Danabasoglu et al. (2020) in the presentation paper for CESM2. For the second set of 50 ensemble members, the pertinent species for biomass burning fluxes from the CMIP6 protocols were smoothed with an 11-year running mean filter, impacting the fluxes over the years 1990-2020. The first set of 50 ensemble members is denoted CMIP6, and the second set of 50 SMBB, for smoothed biomass burning. All smoothing was done only after projecting fields onto the CESM2 grid. The biomass burning fluxes are represented in Fig. S2, and the difference in ensemble mean response in Northern Hemisphere sea ice and surface temperature in Fig. S3, of the preprint of the presentation paper. The enhanced biomass burning for the CMIP6 versus SMBB groupings has an impact on large-scale climate over 1990-2020, as indicated through the acceleration of Arctic sea ice loss during September as well as through warming over the Northern Hemisphere and the tropical Pacific.

Unlike the CESM1 Large Ensemble, the CESM2 Large Ensemble will use a combination of different oceanic and atmospheric initial states to create ensemble spread as follows.

  1. Members 1-10: These will begin from years 1001, 1021, 1041, 1061, 1081, 1101, 1121, 1141, 1161, and 1181 of the 1400-year pre-industrial control simulation. This segment of the control simulation was chosen to minimize drift.
  2. Members 11-90: These begin from 4 pre-selected years of the pre-industrial control simulation based on the phase of the Atlantic Meridional Overturning Circulation (AMOC). For each of the 4 initial states, there will be 20 ensemble members created by randomly perturbing the atmospheric temperature field by order 10^-14K. The chosen start dates (model years 1231, 1251, 1281, and 1301) sample AMOC and Sea Surface Height (SSH) in the Labrador Sea at their maximum, minimum and transition states.
  3. Members 91-100: These will begin from years 1011, 1031, 1051, 1071, 1091, 1111, 1131, 1151, 1171, and 1191 of the 1400-year pre-industrial control simulation. This set will include the extensive "MOAR" output, which can be used to drive regional climate models.

The initialization design will allow assessment of oceanic (AMOC) and atmospheric contributions to ensemble spread, and the impact of AMOC initial-condition memory on the global earth system.

When presenting results based on the CESM2 Large Ensemble in either oral or written form, please acknowledge the CESM2 Large Ensemble Community Project and supercomputing resources provided by the IBS Center for Climate Physics in South Korea, and reference the DOI of the Rodgers et al. (2021) - preprint.

Project Leads, in alphabetical order