Adjoint analysis of the 2007 all time Arctic sea-ice minimum

The past two decades saw a steady decrease of summer Arctic sea ice extent. The 2007 value was yet considerably lower than expected from extrapolating the long‐term trend. We present a quantitative analysis of this extraordinary event based on the adjoint of a coupled ocean‐sea ice model. This new approach allows to efficiently assess the sensitivity of the ice‐covered area in September 2007 with respect to any potential influence factor. We can trace back 86% of the ice area reduction to only four of these factors: May and June wind conditions, September 2‐meter temperature, and March ice thickness. Two thirds of the reduction are determined by factors that are already known at the end of June, suggesting a high potential for an early prediction.