  Coupled climate model simulations designed to isolate the effects of Arctic
sea-ice loss often apply artificial heating, either directly to the ice or
through modification of the surface albedo, to constrain sea-ice in the absence
of other forcings. Recent work has shown that this approach may lead to an
overestimation of the climate response to sea-ice loss. In this study, we
assess the spurious impacts of ice-constraining methods on the climate of an
idealised aquaplanet general circulation model (GCM) with thermodynamic
sea-ice. The true effect of sea-ice loss in this model is isolated by inducing
ice loss through reduction of the freezing point of water, which does not
require additional energy input. We compare results from freezing point
modification experiments with experiments where sea-ice loss is induced using
traditional ice-constraining methods, and confirm the result of previous work
that traditional methods induce spurious additional warming. Furthermore,
additional warming leads to an overestimation of the circulation response to
sea-ice loss, which involves a weakening of the zonal wind and storm track
activity in midlatitudes. Our results suggest that coupled model simulations
with constrained sea-ice should be treated with caution, especially in boreal
summer, where the true effect of sea-ice loss is weakest but we find the
largest spurious response. Given that our results may be sensitive to the
simplicity of the model we use, we suggest that devising methods to quantify
the spurious effects of ice-constraining methods in more sophisticated models
should be an urgent priority for future work.
