Abstract. The recent Arctic sea ice reduction comes with an increase in the ice-free
season duration, with comparable contributions of earlier ice retreat and
later advance. CMIP5 models all project that the trend towards later advance
should progressively exceed and ultimately double the trend towards earlier
retreat, causing the ice-free season to shift into autumn. We show that such
a
shift is a basic feature of the thermodynamic response of seasonal ice to
warming. The detailed analysis of an idealised thermodynamic
ice–ocean model
stresses the role of two seasonal amplifying feedbacks. The
summer feedback generates a 1.6-day-later advance in response to a 1-day-earlier
retreat. The underlying physics are the property of the upper ocean to
absorb solar radiation more efficiently than it can release heat right before ice advance.
The winter feedback is comparatively weak, prompting a 0.3-day-earlier
retreat in response to a 1-day shift towards later advance. This is because a
shorter growth season implies thinner ice, which subsequently melts away
faster. However, the winter feedback is dampened by the relatively long ice
growth period and by the inverse relationship between ice growth rate and
thickness. At inter-annual timescales, the thermodynamic response of ice
seasonality to warming is obscured by inter-annual variability. Nevertheless,
in the long term, because all feedback mechanisms relate to basic and stable
elements of the Arctic climate system, there is little inter-model
uncertainty on the projected long-term shift into autumn of the ice-free
season.
The influence of the albedo-temperature feed-back on climate sensitivity