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Marika M. Holland, David A. Bailey, Bruce P. Briegleb, Bonnie Light, and Elizabeth Hunke

1. Introduction Sea ice is a sensitive indicator of climate change and reductions in Arctic ice cover have been considerable over the satellite record since 1979 (e.g., Serreze et al. 2007 ). Climate models project that Arctic sea ice loss will continue into the future with the possibility of ice-free summers occurring within this century (e.g., Holland et al. 2006 ; Wang and Overland 2009 ; Boe et al. 2009 ). Accompanying this rapid sea ice loss is an amplified warming in Arctic regions (e

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S. Close, M.-N. Houssais, and C. Herbaut

1. Introduction The climate of the Arctic has been reported to have undergone substantial change over recent decades, manifest notably in increasing air temperature (e.g., Serreze et al. 2009 ) and decreasing sea ice extent (e.g., Maslanik et al. 2007 ; Comiso et al. 2008 ), particularly in summer. While the winter sea ice loss has thus far been much less dramatic than that of summer, the changes occurring in this season are nevertheless important, both because of their link to large

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Jinlun Zhang

, satellite passive microwave images display a significant increase in Antarctic sea ice concentration and extent since 1979 when quality space-based observations are available ( Cavalieri and Parkinson 2003 ; Liu et al. 2004 ). The increase in the observed sea ice extent is 0.027 × 10 12 m 2 yr −1 (0.22% yr −1 ) during 1979–2004 ( Fig. 2d ; Table 1 ), based on the Hadley Centre global sea ice concentration data (HADISST; Rayner et al. 2003 ). This positive trend exceeds the 95% confidence level

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Takahiro Toyoda, Nariaki Hirose, L. Shogo Urakawa, Hiroyuki Tsujino, Hideyuki Nakano, Norihisa Usui, Yosuke Fujii, Kei Sakamoto, and Goro Yamanaka

1. Introduction Sea ice is an important component of the polar ocean climate system that greatly affects air–sea heat exchange. For example, the high albedos of sea ice and of snow on top of the ice, compared with the open ocean, reduce absorption of shortwave radiation by the ocean–sea ice system. Latent heat release to the atmosphere is also greatly reduced when the ocean surface is covered by sea ice with a large dry surface area, and the sensible heat flux and upward longwave radiation may

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Zachary Labe, Gudrun Magnusdottir, and Hal Stern

1. Introduction Climate in the Arctic is undergoing rapid change, as the Arctic mean surface temperature is rising at twice the rate of the global mean surface temperature. Accompanying this Arctic amplification is a widespread loss of Arctic sea ice. Quality observations of sea ice concentration (SIC) and, therefore, total sea ice extent (SIE) are available from satellites covering the entire Arctic from 1979. Observations of sea ice thickness (SIT) are very scant by comparison (e.g., Lindsay

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Teresa Valkonen, Timo Vihma, and Martin Doble

1. Introduction The Antarctic sea ice zone covers approximately 19 × 10 6 km 2 in winter and 3.5 × 10 6 km 2 in summer ( Parkinson 2004 ). In situ observations of the atmosphere over this vast area have been rare, restricted to ship observations (mostly summertime; Andreas 1985 ; Wendler et al. 2005 ), wind and temperature measurements from drifting buoys ( Kottmeier and Sellman 1996 ), and detailed boundary layer observations from two drifting ice stations, in 1992 ( Andreas et al. 2000

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Paul R. Holland, Nicolas Bruneau, Clare Enright, Martin Losch, Nathan T. Kurtz, and Ron Kwok

1. Introduction Arctic sea ice extent has declined rapidly in recent decades (−52 × 10 3 km 2 yr −1 for 1979–2010), but Antarctic sea ice extent has slowly increased (+17 × 10 3 km 2 yr −1 ) over the same period ( Cavalieri and Parkinson 2012 ; Comiso and Nishio 2008 ; Parkinson and Cavalieri 2012 ; Zwally et al. 2002 ), raising fundamental questions of why the two poles have evolved so differently in the context of climate change. The small overall Antarctic increase in ice area is

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Mitchell Bushuk, Rym Msadek, Michael Winton, Gabriel A. Vecchi, Rich Gudgel, Anthony Rosati, and Xiaosong Yang

1. Introduction The rapid loss of Arctic sea ice has the potential to influence the climate system across a broad range of spatial and temporal scales. These impacts include changes in the global energy balance via the sea ice–albedo feedback ( Budyko 1969 ; Curry et al. 1995 ), potential influence on midlatitude weather ( Screen and Simmonds 2013 ), and many human-related consequences, including the livelihoods of northern communities and Arctic wildlife, the opening of trans-Arctic shipping

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Harold D. B. S. Heorton, Nikhil Radia, and Daniel L. Feltham

1. Background Leads and polynyas are ice-free areas within the sea ice cover in which the ocean is in contact with the cold atmosphere in winter. They can form due to warm-water upwelling (sensible heat polynya), katabatic winds or ocean currents that drive newly formed ice away (latent heat polynyas), or when the ice breaks due to internal stresses (leads). Leads are typically long thin features 10 m to 1 km wide and up to 100 km long ( Wilchinsky et al. 2015 ), whereas polynyas are defined as

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J. Ono, H. Tatebe, and Y. Komuro

1. Introduction The Arctic summer sea ice extent (SIE) has markedly decreased since satellite observations began in the late 1970s. For September Arctic sea ice, the 10 lowest minimum extents have occurred since 2000 ( NSIDC 2017 ). In particular, in the summers of 2007 and 2012, extreme sea ice loss was observed. The September average SIE in 2007 and 2012 was 4.27 and 3.57 million km 2 (−2.14 and −2.84 million km 2 anomaly from the 1981–2010 average), which have been ranked as the second

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