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Mark England, Alexandra Jahn, and Lorenzo Polvani

1. Introduction The rapid loss of Arctic sea ice over the last 50 years has been one of the most alarming signals of a changing climate. September sea ice extent has decreased by roughly 50% since 1979 ( Comiso et al. 2017 ; Stroeve and Notz 2018 ). Current model projections show that a summer ice-free Arctic before 2100 is very likely unless future warming is limited to 1.5°C or less ( Jahn 2018 ; Niederdrenk and Notz 2018 ; Sigmond et al. 2018 ) and is likely to occur by the middle of this

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Amélie Desmarais and L. Bruno Tremblay

1. Introduction Uncertainties in the timing of a seasonally ice-free Arctic come from model errors, missing or unresolved physics, uncertainties in the forcings, and natural variability of the Arctic climate system. Mechanisms for long-term variability in Arctic sea ice extent (SIE) can be linked with the ocean, with the atmosphere, and with local feedbacks amplifying the sea ice response (e.g., Kashiwase et al. 2017 ). For instance, the Atlantic meridional overturning circulation (AMOC) and

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Amélie Simon, Guillaume Gastineau, Claude Frankignoul, Clément Rousset, and Francis Codron

1. Introduction The Arctic is a region of pronounced climate change. Since the mid-twentieth century, the Arctic has warmed more than twice as fast as the rest of the planet (e.g., Blunden and Arndt 2012 ), a phenomenon referred to as Arctic amplification. The Intergovernmental Panel on Climate Change (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate ( Meredith et al. 2019 ) concluded that over the 1979–2018 period the Arctic sea ice extent has shrunk in all months of the

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Dorian S. Abbot, Chris C. Walker, and Eli Tziperman

1. Introduction Sea ice plays a crucial role in Arctic climate, particularly during winter, when it insulates the atmosphere from the relatively warm ocean. This allows the atmosphere to drop to extremely low temperatures during polar night, which can affect lower latitudes when cold fronts of Arctic air penetrate southward. Significant loss of sea ice would put immediate strain on Arctic biota ( Smetacek and Nicol 2005 ), could accelerate the melting of the Greenland ice sheet ( Lemke et al

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Jed E. Lenetsky, Bruno Tremblay, Charles Brunette, and Gianluca Meneghello

1. Introduction Skillful seasonal predictions of Arctic sea ice on a regional scale are important for the safe navigation of Arctic waters and for local indigenous communities who use sea ice for hunting, fishing, and recreational activities ( Pearce et al. 2015 ; United States Navy 2014 ). State-of-the-art coupled ocean–ice–atmosphere models have been shown to provide skillful predictions of September pan-Arctic sea ice extent with lead times up to five months, both in seasonal forecast

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Jan Sedlacek, Jean-François Lemieux, Lawrence A. Mysak, L. Bruno Tremblay, and David M. Holland

1. Introduction Sea ice dynamics plays an important role in shaping the sea ice cover in polar regions. In the last few decades, several models have been developed to represent the dynamics of sea ice. Crucial to the model representation of dynamics is the formulation of the rheology, that is, the relationship between applied stresses and the resulting deformations. In these models, sea ice has been modeled either as a continuum (e.g., Coon et al. 1974 ) or as a collection of discrete

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S. Bathiany, D. Notz, T. Mauritsen, G. Raedel, and V. Brovkin

1. Introduction The recent rapid retreat of Arctic summer sea ice has raised the question of whether global warming can bring Arctic sea ice to a so-called tipping point ( Lindsay and Zhang 2005 ; Winton 2006 ; Notz 2009 ). This term implies that at a certain level of warming, sea ice loss would accelerate substantially in contrast to the more gradual change of the forcing. Such rapid loss could have severe consequences for the Arctic climate and ecosystems. In case of large positive

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Louis-Philippe Nadeau, Raffaele Ferrari, and Malte F. Jansen

those time scales (e.g., Broecker 1982 ; Toggweiler 1999 ; Brovkin et al. 2007 ; Adkins 2013 ). Most hypotheses for explaining the changes in deep-ocean stratification and circulation between glacial and interglacial periods have focused on changes in North Atlantic convection and shifts in Southern Hemisphere westerlies (e.g., de Boer et al. 2007 ; Toggweiler 2009 ; Anderson et al. 2009 ). However, increasing attention has been given to the potential role of buoyancy fluxes and sea ice at

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Charles Brunette, Bruno Tremblay, and Robert Newton

1. Introduction Adaptation to a changing Arctic climate relies in part on our capacity to predict sea ice. Since the start of satellite monitoring of the Arctic in the late 1970s, observations have shown a decrease of the average September sea ice extent (SIE) at a rate of 13.3% per decade ( Fetterer et al. 2017 , updated daily). The retreat of the pack ice and the transition to a sea ice–free summer in the Arctic have important implications for marine ecosystems ( Arrigo et al. 2008 ; Frey et

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Megan C. Kirchmeier-Young, Francis W. Zwiers, and Nathan P. Gillett

1. Introduction Sea ice extent (SIE) in the Arctic has decreased throughout the satellite record ( Vaughan et al. 2013a ). Loss of Arctic sea ice has implications in many areas ( IPCC 2014 ; Serreze et al. 2007 ), such as ecosystems, transportation, fisheries/commerce, and Arctic communities. Arctic SIE reached a minimum of 4.28 × 10 6 km 2 in September 2007 ( Stroeve et al. 2008 ), during a period of strong Arctic sea ice decline ( Stroeve et al. 2007 ; Comiso et al. 2008 ; Serreze et al

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