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James A. Screen

1. Introduction Satellites have routinely measured Arctic sea ice since the late 1970s. Since then, the sea ice cover has significantly reduced in all calendar months, with the largest trend in September—the month of the annual minimum ( Simmonds 2015 ). The September sea ice extent has declined by 40% and its volume by an estimated 65% ( IPCC 2013 ). Paleoclimate records suggest the sea ice cover is now lower than at any time in the previous 1450 yr ( Kinnard et al. 2011 ). This decline in

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Clara Deser, Robert A. Tomas, and Shiling Peng

1998 ), the leading structure of internal wintertime atmospheric variability over the Northern Hemisphere. It remains to be seen whether the adjustment time in a three-level quasigeostrophic model is indicative of that in a more complex atmospheric GCM. Deser et al. (2004 , hereafter D04 ) and the companion study of Magnusdottir et al. (2004 , hereafter M04 ) used an atmospheric GCM to examine the equilibrium circulation response to two types of boundary forcing: sea ice and SST. These boundary

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Paul R. Holland and Noriaki Kimura

1. Introduction Satellites have played a key role in monitoring decadal changes in the sea ice cover, most notably in the passive microwave record of near-daily ice concentration fields since 1978. During this period, Antarctic sea ice has expanded slightly while Arctic sea ice has contracted dramatically ( Parkinson 2014 ). These high-profile changes raise many questions: Are they anthropogenic or natural? What is the role of ice–climate feedbacks? Why are the two poles so different? Are the

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J. J. Day, S. Tietsche, and E. Hawkins

1. Introduction The rapid reduction in Arctic summer sea ice has led to a large increase in demand for forecasts of sea ice conditions at seasonal to interannual time scales ( Eicken 2013 ). This is important information for end users, including those interested in marine accessibility for routing ships (e.g., Stephenson et al. 2011 ). This interest has led to the development of a number of operational seasonal sea ice prediction systems (e.g., Sigmond et al. 2013 ; Chevallier et al. 2013

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Mitchell Bushuk and Dimitrios Giannakis

1. Introduction Arctic sea ice extent (SIE) has declined precipitously over the satellite era at a rate of roughly −14% decade −1 ( Serreze et al. 2007 ; Stroeve et al. 2014 ). In addition to this decrease in areal coverage, submarine, satellite, and in situ measurements indicate that Arctic sea ice is becoming thinner ( Rothrock et al. 1999 ; Kwok and Rothrock 2009 ), transitioning from multiyear to first-year ice ( Rigor and Wallace 2004 ; Maslanik et al. 2011 ) and experiencing longer

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Ivana Cerovečki, Andrew J. S. Meijers, Matthew R. Mazloff, Sarah T. Gille, Veronica M. Tamsitt, and Paul R. Holland

). Haumann et al. (2016) have attributed the freshening observed over the last several decades to an increase in wind-driven northward freshwater transport by Antarctic sea ice. This increase was strongest and most robust in the Pacific sector of the Southern Ocean. In this sector, freshening was caused by increased southerly winds over the Ross Sea region, enhancing northward sea ice advection since the early 1990s ( Haumann et al. 2014 ; Holland and Kwok 2012 ). Strengthening of southerly winds over

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Zhicong Yin, Huijun Wang, and Xiaohui Ma

and land surface radiation, the relationship between Eurasian snow cover and December haze in North China significantly strengthened after the mid-1990s ( Yin and Wang 2018 ). During the recent decades, the Arctic region has warmed about 2 times more than the global average, and the Arctic sea ice (ASI) has melted rapidly ( Cohen et al. 2014 ). The autumn ASI showed significant impacts on the Eurasian winter climate ( Wang and Liu 2016 ; He 2015 ; Li et al. 2015 ). The role of ASI with regard to

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Edward Blanchard-Wrigglesworth, Lettie A. Roach, Aaron Donohoe, and Qinghua Ding

1. Introduction Antarctic sea ice is characterized by large interannual variability and a small long-term increase in areal coverage since 1979 (e.g., Comiso et al. 2017 ; Eisenman et al. 2014 ) despite an abrupt reduction in sea ice cover since 2016 ( Parkinson 2019 ). The observed record represents the combined effects of natural modes of atmosphere, ocean, and cryosphere internal variability, the response to stratospheric ozone depletion, and the response to global warming, and much work

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Mengyuan Long, Lujun Zhang, Siyu Hu, and Shimeng Qian

1. Introduction The Arctic is covered by a large amount of sea ice all year round. Since the 1980s, the sea ice coverage area in the Arctic has rapidly decreased as global warming continues to develop ( Cavalieri et al. 2003 ; Comiso et al. 2008 ; Stephenson et al. 2013 ). The Arctic sea ice cover is retreating at a rate 3 times as large as the rate of sea ice expanding in the Antarctic ( Eisenman et al. 2014 ). Deng (2014) pointed out that the sea ice concentration (SIC) changed

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Andrew Rhines and Peter J. Huybers

), in order to be consistent with reconstructed glacier equilibrium line altitudes. Denton et al. (2005) also used similar methods to show that glacial winters in Europe were colder than those of the Holocene by a far greater margin than summers, indicating that more extreme seasonality was not limited to Greenland. Sea ice cover is a well-recognized influence on the seasonal cycle of accumulation in Greenland ( Steig et al. 1994 ; Krinner et al. 1997 ; Werner et al. 2000 , 2001 ; Krinner and

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