Abstract
1. Abstract The recognized importance of the annual cycle of sea ice in the Arctic to heat budgets, human behavior, and ecosystem functions, requires consistent definitions of such key events in the ice cycle as break-up and freeze-up. An internally consistent and reproducible approach to characterize the timing of these events in the annual sea-ice cycle is described. An algorithm was developed to calculate the start and end dates of freeze-up and break-up and applied to time series of satellite-derived sea-ice concentration from 1979 to 2013. Our approach builds from discussions with sea-ice experts having experience observing and working on the sea ice in the Bering, Chukchi and Beaufort Seas. Applying the algorithm to the 1979–2013 satellite data reveals that freeze-up is delayed by two weeks per decade for the Chukchi coast and one week per decade for the Beaufort coast. For both regions, break-up start is arriving earlier by 5–7 days per decade and break-up end is arriving earlier by 10–12 days per decade. In the Chukchi Sea, “early” break-up is arriving earlier by one month over the 34-year period and alternates with a “late” break-up. The calculated freeze-up and break-up dates provide information helpful to understanding the dynamics of the annual sea-ice cycle and identifying the drivers that modify this cycle. The algorithm presented here, and potential refinements, can help guide future work on changes in the seasonal cycle of sea ice. The sea-ice phenology of freeze-up and break-up that results from our approach is consistent with observations of sea-ice use. It may be applied to advancing our understanding and prediction of the timing of seasonal navigation, availability of ice as a biological habitat, and assessment of numerical models.
Highlights
The annual cycle of Arctic sea ice, from freeze-up to break-up, currently contracts and expands across an area ranging from 7 to 16 x 106 km2 (Serreze et al, 2007)
We focus in this paper on the northeastern Chukchi Sea and southern Beaufort Sea coasts near Barrow, Alaska
Shown are the transition dates and Estimating Arctic sea-ice freeze-up and break-up melt season lengths for the same period in the “Chukchi/ Beaufort seas” region examined by Markus et al (2009)
Summary
The annual cycle of Arctic sea ice, from freeze-up to break-up, currently contracts and expands across an area ranging from 7 to 16 x 106 km (Serreze et al, 2007). Based on satellite data from 1979 to 2000, the perennial Arctic sea-ice cover is declining at −7% per decade. Trends in satellite-derived surface brightness temperature suggest a lengthening of the melt season (Comiso, 2002; Parkinson, 2014). Melt onset is affected by early season atmospheric storms that bring warm air at the same time that incident solar energy increases in May (Bitz et al, 1996; Perovich and Polashenski, 2012). Late spring atmospheric forcing, at the onset of melt, influences low-frequency variability sometimes for decades through nonlinearities in sea-ice volume (thickness and extent), based on energy balance models forced by anomalous melt onset dates (Bitz et al, 1996). Using an energy balance model, Bitz and Roe (2004) found that variations in season length had a “nontrivial” influence on sea-ice thickness even though the sea-ice growth-thickness relationship, where thin ice grows faster thermodynamically than thick ice, dominates the overall ice behavior
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