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Kyle C. Armour, Cecilia M. Bitz, LuAnne Thompson, and Elizabeth C. Hunke

1. Introduction Sea ice can be viewed in two distinct area categories: first-year (FY) ice that was formed since the summer minimum in the previous September and multiyear (MY) ice that has survived at least one summer melt season (see Fig. 1 ). Recent estimates of the FY and MY ice area by direct observation and by model estimates of sea ice age ( Johannessen et al. 1999 ; Comiso 2002 ; Rigor and Wallace 2004 ; Nghiem et al. 2007 ; Maslanik et al. 2007 ; Kwok et al. 2009 ; Hunke and

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Edward Blanchard-Wrigglesworth and Cecilia M. Bitz

1. Introduction Over the last few years, interest in Arctic sea ice predictability has grown mainly as a consequence of the recent decline in Arctic sea ice. Stakeholders include groups as diverse as resource extraction, shipping, and local traditional hunting industries. The extreme melt in 2007 triggered an organization of yearly summer forecasts called the Sea Ice Outlook project under the auspices of the Study for Environmental Arctic Change (SEARCH). The outlook project has become an

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Christof König Beatty and David M. Holland

1. Introduction Landfast ice is sea ice that forms and remains fixed along a coast, where it is either attached to the shore or held between shoals or grounded icebergs. It covers Arctic shelves during large portions of the year, normally starting to form in October and reaching its widest extent during April and May ( Volkov et al. 2002 ; Barber and Hanesiak 2004 ; Divine et al. 2005 ). Because of its lack of mobility, landfast ice fundamentally modifies the momentum exchange between

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Ingrid H. Onarheim, Tor Eldevik, Lars H. Smedsrud, and Julienne C. Stroeve

1. Introduction The rapid decline of Arctic sea ice is one of the clearest indicators of ongoing climate change ( Serreze and Barry 2011 ). Along with reduced sea ice cover in both extent and thickness ( Kwok and Rothrock 2009 ; Cavalieri and Parkinson 2012 ), the multiyear ice cover is decreasing ( Maslanik et al. 2007 ; Nghiem et al. 2007 ), the melt season is extending ( Stroeve et al. 2014 ), and drift speeds and deformation rates are increasing ( Rampal et al. 2009 ). The current Arctic

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Alexander V. Wilchinsky and Daniel L. Feltham

1. Introduction Sea ice forms from the freezing of polar waters and covers a significant fraction, up to almost 10%, of the earth’s oceans. Sea ice is well recognized as an important component of the earth’s climate system and, as a result, sea ice models are routinely incorporated into global climate models (GCMs). Although sea ice affects polar and global climate through its impact on the thermohaline budget (e.g., through its high albedo compared to seawater), its insulating effect on polar

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Yong-Fei Zhang, Mitchell Bushuk, Michael Winton, Bill Hurlin, Xiaosong Yang, Tom Delworth, and Liwei Jia

1. Introduction Arctic sea ice has undergone rapid changes in recent decades, which imposes threats on the wildlife and local people whose habitats largely rely on sea ice. Meanwhile, it brings economic opportunities including marine fishing, more direct shipping routes through the Arctic, and petroleum extraction. Predicting Arctic sea ice, especially in the summertime, has great implications for environmental protection, human activity regulations, and stakeholder decision making. Recent work

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R. C. Frew, D. L. Feltham, P. R. Holland, and A. A. Petty

1. Introduction Satellite observations have shown Antarctic sea ice to be expanding over the past four decades ( Parkinson and Cavalieri 2012 ). Although this increasing trend is modest, it is in stark contrast to the well-documented rapid Arctic sea ice decline. The small net increase is the result of stronger, opposing regional and seasonal trends ( Holland 2014 ), though a rapid decline in Antarctic sea ice was observed in 2016/17 ( Stuecker et al. 2017 ; Turner et al. 2017 ). There is no

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Claire L. Parkinson

1. Introduction Sea ice has received considerable attention in recent years, largely because of significant decreases in the Arctic sea ice cover ( Parkinson et al. 1999 ; Rothrock et al. 1999 ; Kwok and Rothrock 2009 ; Cavalieri and Parkinson 2012 ; Stroeve et al. 2012 ) and the fact that those decreases are an important indicator of climate change ( Walsh 2013 ) and have important consequences for climate ( Screen et al. 2013 ; Walsh 2013 ; Vihma 2014 ), for the Arctic ecosystem ( Post

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R. W. Lindsay, J. Zhang, A. Schweiger, M. Steele, and H. Stern

1. Introduction Arctic sea ice retreated dramatically in the summer of 2007, shattering the previous record low ice extent set in 2005 by 23% ( Stroeve et al. 2008 ; Comiso et al. 2008 ). Figure 1 shows the extent of the Arctic sea ice each September (the month of minimum extent) since the beginning of the satellite data record in 1979. The extent in 2007 falls 4 standard deviations of the residuals (4 σ ) below the downward linear trend for 1979–2006. What caused this precipitous drop? The

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Jinlun Zhang, Rebecca Woodgate, and Richard Moritz

1. Introduction The Bering Sea supports one of the world’s most productive and valuable fisheries, immense populations of marine birds and mammals, and the subsistence activities of native communities ( ARCUS 2004 ). This great biological productivity takes place in a dynamic ocean with substantial seasonal ice cover. Significant changes in the Bering ecosystem have been measured in recent decades. Observations indicate that large-scale ecological “regime shifts” occurred in the Bering Sea

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