Due to record-breaking Arctic sea ice extent minima in the last decade, a lot of attention has been paid to the atmospheric large-scale circulation response to decreased Arctic sea ice area. However, also a substantial thinning of Arctic sea ice has been observed which modifies the heat flux through the ice. We focus on the fast atmospheric response to a sudden thinning of Arctic sea ice to disentangle the role of various different processes. Overall the response of the atmospheric large-scale circulation is relatively small with up to 2 hPa in the Arctic mean sea level pressure and even smaller changes in the mid-latitudes during the first 15 days; the quasi-equilibrium response reached in the second and third month of the integration is about twice as large. During the first few days the response tends to be baroclinic in the whole Arctic. Already after a few days an anti-cyclonic equivalent-barotropic response develops over north-western Siberia and north-eastern Europe. The structure resembles very much that of the atmospheric equilibrium response indicating that fast tropospheric processes such as fewer quasi-barotropic cyclones entering this continental area are key opposed to slower processes such as those involving, for example, stratosphere-troposphere interaction.