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

Abstract

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.

A fundamental shift in the melt processes of Arctic sea ice

“…In summer, these changes have led to the sea ice edge retreating further northwards, and a more fractured state for the sea ice that remains (5). These changes are consistent with an Arctic Ocean transitioning from a perennial sea-ice cover to a seasonal ice regime (6). The Beaufort Sea epitomizes this transition as it now has a predominately seasonal ice cover, and in the spring and summer large expanses of the Beaufort are subject to low concentrations of sea ice, or even ice-free conditions in its southernmost regions.…”

The Seasonal and Regional Transition to an Ice‐Free Arctic

“…This seasonal asymmetry in sea ice loss could be linked to a strong ocean thermal feedback and upper ocean warming in autumn in response to sea ice loss and ice‐albedo feedback in summer (Stammerjohn et al., 2012; Steele & Dickinson, 2016), and enhanced wintertime atmospheric warming as a result of sea ice thinning (Labe et al., 2018; Lang et al., 2017). The faster future sea ice decline in autumn than in spring for the Chukchi and Beaufort Seas (Figure 3; Lebrun et al., 2019; Wang & Overland, 2015) could also be related to the seasonality of ocean heat transport through the Bering Strait (Serreze et al., 2016).…”

Preparing the New Phase of Argo: Scientific Achievements of the NAOS Project

“…This approach offers considerable potential, as shown by our scientific results, but some major technological challenges remain to be tackled before BGC-Argo floats can be deployed in larger numbers in ice-covered sectors of the Arctic and Antarctic Oceans. Due to recent marked increases in the size of the seasonal ice zone and the duration of the ice-free season (Arrigo and Van Dijken, 2015), further expected to grow significantly (Notz and Stroeve, 2018;Lebrun et al, 2019), the use of BGC-Argo floats for real-time operations has and will become more and more feasible in Arctic seas. On the other hand, their use under sea ice will require appropriate deployment strategies.…”