Increasing Earth System Sensitivity in mid-Pliocene simulations from
CCSM4 to CESM2
Abstract
Three new equilibrium Mid-Pliocene (MP) simulations are implemented with
the Community Climate System Model version 4 (CCSM4), Community Earth
System Model version 1.2 (CESM1.2), and 2 (CESM2). All simulations are
carried out with the same boundary and forcing conditions following the
protocol of Pliocene Model Intercomparison Project Phase 2. These
simulations reveal amplified MP climate change relative to preindustrial
going from CCSM4 to CESM2, seen in global mean and polar amplification
of surface warming, sea ice reduction in both Arctic and Antarctic, and
weakened Hadley circulation. The enhanced global mean warming arises
from both enhanced Earth System Sensitivity (ESS) and Equilibrium
Climate Sensitivity (ECS) to CO forcing. ESS is amplified by up to 70%
in CCSM4, and up to 100% in CESM1.2 and CESM2 relative to ECSs of
respective models. Simulations also agree on the strengthened Atlantic
Meridional Overturning Circulation, but disagree on several other
climate metrics. Compared to preindustrial, CCSM4 features small
increase in both low and high cloud cover and no change in the mean
climate state of the equatorial Pacific. Whereas, both CESM1.2 and 2
show reduction of cloud cover at all heights, and an anomalous El
Niño-like state of the equatorial Pacific. The performances of MP
simulations are assessed with a new compilation of paleo-observations of
sea surface temperature (SST). CESM1.2 and 2 show better skills than
CCSM4 in simulating MP global mean warming and amplified SST warming in
the northern middle and high latitudes, supporting the amplified ESS
compared to the CCSM4.