Do Stochastic Parameterizations Modify the Climate Response to External
Forcing? An Experiment with EC-Earth.
Stochastic physics (SP) schemes provide a more realistic representation
of the unresolved scales in global circulation models by improving both
mean climate and climate variability. We study the impact of including
an SP scheme in the atmospheric component of EC-Earth on the simulated
climate. In particular, we analyze the evolution of the sea-ice extent
in the Arctic during long-term simulations covering the historical and
future periods. The experiments consist of coupled climate simulations
in which three ensemble members constitute the control runs (base) and
three ensemble members include stochastic physics (stoc). For the
latter, the Stochastically Perturbed Parametrization Tendencies (SPPT)
scheme is incorporated in the atmospheric component of EC-Earth. The
original experiments, that are part of the SPHINX project, span from
1850 to 2100. We have additionally extended each simulation for 60
years; the future scenario corresponds to the CMIP5 RCP8.5 set up. We
compare both sets of experiments to investigate the climate response to
a perturbed atmosphere. The simulated Arctic sea-ice extent in September
and March display an overall decrease. The sea-ice loss results faster
in the base experiments than in the stoc ones. The model simulates an
abrupt sea-ice loss in March that takes place about 10 years earlier in
the base experiments than in the stoc ones. The evolution of the global
annual mean surface air temperature differs if the SP is on or off.
Curves start separating by the second half of the 20th century; reach
the maximum difference in 2100 and become almost indistinguishable
around 2110. Our results suggest that the transient climate sensitivity
is lower when the SP is on than when it is off during the 21st century.
However, the opposite occurs when the Arctic is free of sea ice along
the whole year. This behavior might be the consequence of the asymmetric
effect of stochastic perturbations on the process of condensation. We
are now investigating the differences in the albedo and cloud feedbacks
between both sets of experiments and the possible influence of the mean
state on the model climate sensitivity.