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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
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
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
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
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
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
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
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
1. Introduction Arctic sea ice is important for shipping, oil and gas, fisheries, wildlife, and indigenous communities. For each of these stakeholders, predicting sea ice concentration (SIC) and thickness (SIT) can be beneficial. Recent declines in Arctic SIC and SIT can be attributed to two major categories of causes. The first of these is natural and anthropogenic forcings, such as emissions of greenhouse gases ( Kay et al. 2011 ; Notz and Stroeve 2016 ; Polvani et al. 2020 ; Zhang 2010
1. Introduction Arctic sea ice is important for shipping, oil and gas, fisheries, wildlife, and indigenous communities. For each of these stakeholders, predicting sea ice concentration (SIC) and thickness (SIT) can be beneficial. Recent declines in Arctic SIC and SIT can be attributed to two major categories of causes. The first of these is natural and anthropogenic forcings, such as emissions of greenhouse gases ( Kay et al. 2011 ; Notz and Stroeve 2016 ; Polvani et al. 2020 ; Zhang 2010
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
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
1. Introduction The Arctic sea ice extent has declined substantially in the past several decades under greenhouse warming ( Comiso 2012 ; Gao et al. 2015 ). According to the National Snow and Ice Data Center (NSIDC) sea ice index ( Fetterer et al. 2017 ), the Arctic sea ice fell to its second-lowest September extent on record on 15 September 2020, just 0.4 × 10 6 km 2 larger than the minimum record of 3.39 × 10 6 km 2 in 2012 ( Francis 2013 ). With the relative amount of first year
1. Introduction The Arctic sea ice extent has declined substantially in the past several decades under greenhouse warming ( Comiso 2012 ; Gao et al. 2015 ). According to the National Snow and Ice Data Center (NSIDC) sea ice index ( Fetterer et al. 2017 ), the Arctic sea ice fell to its second-lowest September extent on record on 15 September 2020, just 0.4 × 10 6 km 2 larger than the minimum record of 3.39 × 10 6 km 2 in 2012 ( Francis 2013 ). With the relative amount of first year
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
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
1. Introduction The rapid decline in Arctic summer sea ice threatens local communities and ecosystems and also creates economic opportunities for marine fishing, shipping and resource extraction. Subseasonal-to-seasonal forecasts of Arctic sea ice hence can aid mitigation of impacts and provide valuable planning information. The current generation of dynamical climate prediction systems have been shown to skillfully predict Arctic sea ice cover (e.g., Sigmond et al. 2013 ; Wang et al
1. Introduction The rapid decline in Arctic summer sea ice threatens local communities and ecosystems and also creates economic opportunities for marine fishing, shipping and resource extraction. Subseasonal-to-seasonal forecasts of Arctic sea ice hence can aid mitigation of impacts and provide valuable planning information. The current generation of dynamical climate prediction systems have been shown to skillfully predict Arctic sea ice cover (e.g., Sigmond et al. 2013 ; Wang et al
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
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
