The variability determining the initial conditions for ensemble spread in the ZEC (CESM2) scenario

In this study, we analyze the results of a multiple ensemble experiment using a single model (CESM2) for the Zero Emission Commitment (ZEC) scenario, where atmospheric CO2 emissions are initially increased as in a warming scenario and then reduced to zero. We found a significant increase in the ensemble spread of global temperature during the Zero Emission period following the warming phase. 
Ensembles which initially have the relatively higher salinity in the North Atlantic during the early Zero Emission period show the higher North Atlantic temperatures and salinity, along with less Arctic sea ice distribution, in later (ZEC) periods. Conversely, ensembles with initially lower salinity displayed opposite characteristics. We propose that the initial conditions of the Zero Emission period are associated to long-period internal variability that occurred during the previous period of positive CO2 emission fluxes (the warming period). The increase in ensemble spread in the Northern Atlantic is due to the the Atlantic Meridional Overturning Circulation (AMOC) salinity feedback becoming elongated due to strong ocean stratification. This suggests a prolonged period for this feedback mechanism, associated with the internal variability in AMOC.