Stochastic Parameterizations and the Climate Response to External Forcing: An Experiment With EC-Earth

The impacts of including stochastic physics schemes (SPS) in the atmospheric component of the EC-Earth climate model is studied by analyzing coupled simulations from 1850 to 2160. Sea ice in the Arctic retreats more slowly when SPS are included. An abrupt loss of winter sea ice is simulated 10 years later if the SPS are activated. However, this abrupt collapse is associated with a global surface air temperature (GSAT) of 17.4 ± 0.35 °C in all the cases. While the transient climate sensitivity is smaller if SPS are included up to the end of the 21st century, the opposite occurs in a warmer climate and in the absence of Arctic sea ice all the year-round. These results are explained by the skewed interaction between stochastic perturbations and the process of condensation. Under a warmer climate, tropical convection is enhanced with SPS leading to the formation of high-level clouds, which further increases GSAT.