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Abstract

In this dissertation, I explore the large differences in Arctic sea-ice evolution
between climate models and observations, and among individual climate models.
First, I investigate the drivers of the long-term Arctic Ocean warming in a
multi-model ensemble. I find that there is no consensus between the models about
whether the excess energy is gained by the ocean through the net atmospheric
surface flux or through the meridional oceanic heat flux. However, all models agree
on the magnitude of the projected warming. The warming is small compared to the
anomalies in the energy fluxes. This is because most of the energy gained through
one energy flux is lost through the other energy flux due to a relationship between
the magnitude of the increase in oceanic heat inflow and the increase in turbulent
heat loss to the atmosphere.
Second, I explore the feasibility of an observation operator for the Arctic Ocean.
An observation operator translates the Arctic Ocean climate simulated by a climate
model into a brightness temperature. The brightness temperature is the quantity
directly measured by satellites from space. Hence, an observation operator enables
us to circumvent the observational uncertainty currently inhibiting reliable climate
model evaluation. Sea-ice brightness temperatures at 6.9 GHz are driven by the
liquid water fraction profile inside the ice and snow, which is not resolved in most
climate models. I show that in winter this profile can be described reasonably well
by a linear temperature profile and a salinity profile prescribed as a self-similar
function of depth. In summer, the melt-pond fraction is more important for the
simulation of the brightness temperature than the internal structure of the ice.
Third, I develop an Arctic Ocean Observation Operator for 6.9 GHz based on
these findings. I compare brightness temperatures simulated from the output of an
Earth System Model to brightness temperatures measured by satellites. The
differences between simulated and measured brightness temperatures can mainly
be explained by the uncertainty in the simulated state of the sea-ice concentration,
the assimilation process, and the melt-pond parametrization. Differences
attributable to biases in the observation operator itself are small. The operator is
therefore a suitable method for climate model evaluation.
In summary, I show different perspectives on the large differences in Arctic
sea-ice evolution. On the one hand, I point out that the multi-model ensemble
mean is not always representative for the simulated Arctic climate and should be
interpreted with care. On the other hand, I introduce and develop an
unconventional tool providing new opportunities for future climate model
evaluation.