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Ian Eisenman, Tapio Schneider, David S. Battisti, and Cecilia M. Bitz

1. Introduction The extent of sea ice covering the ocean in the high northern latitudes varies between about 7 Mm 2 at summer minimum and 16 Mm 2 at winter maximum in today’s climate (with 1 Mm 2 = 10 6 km 2 ). During recent decades, Arctic sea ice has been rapidly retreating. The year-to-year retreat of sea ice extent has been considerably more rapid at summer minimum than at winter maximum (e.g., Serreze et al. 2007 ), with an associated increase in the amplitude of the seasonal cycle

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Robert M. Graham, Lana Cohen, Nicole Ritzhaupt, Benjamin Segger, Rune G. Graversen, Annette Rinke, Von P. Walden, Mats A. Granskog, and Stephen R. Hudson

1. Introduction Temperatures in the Arctic are rising twice as fast as the Northern Hemisphere as a whole, and Arctic sea ice is retreating in all seasons ( Serreze and Francis 2006 ; Bekryaev et al. 2010 ; Stroeve et al. 2012 ; Boisvert and Stroeve 2015 ; Stroeve and Notz 2018 ). Many studies documenting and attributing these ongoing changes in the Arctic rely heavily on atmospheric reanalyses ( Screen and Simmonds 2012 ; Screen et al. 2013 ; Mortin et al. 2016 ; Overland and Wang 2016

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Brian E. J. Rose, David Ferreira, and John Marshall

, physically self-consistent look at the coupled atmosphere–ocean–ice processes involved in the growth and retreat of extensive sea ice caps, and an opportunity to diagnose causality in concomitant shifts in ocean circulation and ice cover. They may therefore provide insight into mechanisms for observed large and abrupt climate shifts of the past and guidance in the interpretation of proxy data. One inspiration for our experiments is millennial-scale climate variability of the last ice age, particularly

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B. L. Mueller, N. P. Gillett, A. H. Monahan, and F. W. Zwiers

1. Introduction One of the best quantified aspects of climate change in the Arctic is the change in the spatial extent of the sea ice as measured by passive-microwave sensors on board satellite systems since 1978. Satellite observations of the Arctic show a negative trend in sea ice concentration (SIC) in all seasons and all Arctic subregions except the Bering Sea for the 1979–2012 period ( Serreze et al. 2007 ; Comiso et al. 2008 ; Stroeve et al. 2012b ). The minimum sea ice extent (SIE

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Pawel Schlichtholz

1. Introduction Through the past few decades, the Arctic Ocean sea ice has undergone spectacular changes ( Serreze et al. 2007 ; Kwok and Rothrock 2009 ; Comiso and Hall 2014 ; Stroeve et al. 2014 ). Climate warming has led to a prolonged and more intense melt season, resulting in diminished sea ice cover at the end of summer. In September, the linear trend in Arctic sea ice extent estimated from satellite passive microwave data over the period 1979–2017 amounts to −13% decade −1 ( Serreze

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J. E. Kay, K. Raeder, A. Gettelman, and J. Anderson

1. Motivation For 30 years, climate models have projected amplified Arctic warming and sea ice loss in response to increased greenhouse gas forcing ( Manabe and Stouffer 1980 ). While the sign of the Arctic response is known, the magnitude has not been constrained by climate models. Mean sea ice thickness, winter cloud increases, and enhanced poleward ocean heat transport have all been identified as factors that explain intermodel spread in the Arctic response to the greenhouse gas forcing

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Hyo-Seok Park, Sukyoung Lee, Seok-Woo Son, Steven B. Feldstein, and Yu Kosaka

1. Introduction As the solar insolation in high latitudes rapidly weakens in late fall, sea ice over the Arctic Ocean gradually thickens and extends farther southward until it reaches its maximum extent in early March. The processes that drive sea ice variability during the Arctic winter have not received as much attention as those for the summer, when ice–albedo feedback is thought to play an important role. However, Arctic warming has been most rapid during the winter ( Bekryaev et al. 2010

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Hyo-Seok Park, Sukyoung Lee, Yu Kosaka, Seok-Woo Son, and Sang-Woo Kim

1. Introduction As the sunlight in high latitudes weakens in the fall, Arctic sea ice thickens and extends southward until it reaches its maximum extent in late February. This seasonal march shows large interannual variations presumably due to vigorous wintertime atmospheric and oceanic circulations. For example, the interannual variability of ocean heat flux convergence over the Barents Sea is large and is correlated with sea ice concentration in the winter ( Årthun et al. 2012 ). Moreover, it

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Cynthia Garcia-Eidell, Josefino C. Comiso, Max Berkelhammer, and Larry Stock

-scale freshening of surface waters has been observed in the Southern Ocean ( Purich et al. 2018 ; Durack and Wijffels 2010 ; Jacobs et al. 2002 ), which according to Haumann et al. (2016) is a result of sea ice transport, increased runoff from ice sheet and shelf melt ( Bintanja et al. 2013 , 2015 , and an increase in precipitation minus evaporation ( P − E ) due to the positive trend in southern annular mode (SAM) and its associated poleward shift in the storm tracks and extratropical westerly wind jet

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Melanie F. Fitzpatrick and Stephen G. Warren

1. Introduction The Southern Ocean around Antarctica is a large region of potential importance for global climate change. This region is seasonally covered by sea ice with variable thickness and variable snow cover. The sea ice at its maximum extent occupies an area of ocean larger than the Antarctic continent, but about 85% of the ice melts away by the end of summer each year. Sea ice albedo has been identified as a major positive feedback in climate change ( Manabe and Stouffer 1980

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