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. The recent Arctic sea ice reduction comes with an increase in the ice-freeseason duration, with comparable contributions of earlier ice retreat andlater advance. CMIP5 models all project that the trend towards later advanceshould progressively exceed and ultimately double the trend towards earlierretreat, causing the ice-free season to shift into autumn. We show that suchashift is a basic feature of the thermodynamic response of seasonal ice towarming. The detailed analysis of an idealised thermodynamicice–ocean modelstresses the role of two seasonal amplifying feedbacks. Thesummer feedback generates a 1.6-day-later advance in response to a 1-day-earlierretreat. The underlying physics are the property of the upper ocean toabsorb solar radiation more efficiently than it can release heat right before ice advance.The winter feedback is comparatively weak, prompting a 0.3-day-earlierretreat in response to a 1-day shift towards later advance. This is because ashorter growth season implies thinner ice, which subsequently melts awayfaster. However, the winter feedback is dampened by the relatively long icegrowth period and by the inverse relationship between ice growth rate andthickness. At inter-annual timescales, the thermodynamic response of iceseasonality to warming is obscured by inter-annual variability. Nevertheless,in the long term, because all feedback mechanisms relate to basic and stableelements of the Arctic climate system, there is little inter-modeluncertainty on the projected long-term shift into autumn of the ice-freeseason.

Arctic sea-ice-free season projected to extend into autumn