Abstract
Arctic sea ice extent has been utilized to monitor sea ice changes since the late 1970s using remotely sensed sea ice data derived from passive microwave (PM) sensors. A 15% sea ice concentration threshold value has been used traditionally when computing sea ice extent (SIE), although other threshold values have been employed. Does the rapid depletion of Arctic sea ice potentially alter the basic characteristics of Arctic ice extent? In this paper, we explore whether and how the statistical characteristics of Arctic sea ice have changed during the satellite data record period of 1979–2017 and examine the sensitivity of sea ice extents and their decadal trends to sea ice concentration threshold values. Threshold choice can affect the timing of annual SIE minimums: a threshold choice as low as 30% can change the timing to August instead of September. Threshold choice impacts the value of annual SIE minimums: in particular, changing the threshold from 15% to 35% can change the annual SIE by more than 10% in magnitude. Monthly SIE data distributions are seasonally dependent. Although little impact was seen for threshold choice on data distributions during annual minimum times (August and September), there is a strong impact in May. Threshold choices were not found to impact the choice of optimal statistical models characterizing annual minimum SIE time series. However, the first ice-free Arctic summer year (FIASY) estimates are impacted; higher threshold values produce earlier FIASY estimates and, more notably, FIASY estimates amongst all considered models are more consistent. This analysis suggests that some of the threshold choice impacts to SIE trends may actually be the result of biased data due to surface melt. Given that the rapid Arctic sea ice depletion appears to have statistically changed SIE characteristics, particularly in the summer months, a more extensive investigation to verify surface melt impacts on this data set is warranted.
Highlights
Arctic sea ice coverage has been monitored since the late 1970s by using remotely sensed sea ice data derived from passive microwave (PM) sensors. a threshold value of 15% sea ice concentration (SIC) has been used traditionally when computing the sea ice extent (SIE) [1]
This choice of threshold is associated with the accuracy of sea ice concentration retrieval algorithms and based on the studies that the ice extent is well represented by the choice of 15% SIC threshold (e.g., [2,3,4])
This evolution of these changes brings up a number of scientific questions: Is 15% truly a robust threshold choice to represent sea ice extent decadal trends? Is the remaining sea ice (SIE with SIC of 0–N%, where N% is the SIC threshold) statistically significant to potentially alter the characteristics of distributions? With the potential of the Arctic becoming nearly ice-free, represented by the Arctic sea ice extent falling below 1 mil square kilometers, in the coming decades, what is the sensitivity of the statistical first ice-free Arctic summer year (FIASY) projections to sea ice edge thresholds? the question we strive to answer is: does the choice of threshold have an impact on Arctic sea ice extent decadal trends?
Summary
Arctic sea ice coverage has been monitored since the late 1970s by using remotely sensed sea ice data derived from passive microwave (PM) sensors. a threshold value of 15% sea ice concentration (SIC) has been used traditionally when computing the sea ice extent (SIE) [1]. Arctic sea ice loss has accelerated in the last half of the satellite data record (e.g., [9]) and multi-year ice is depleting faster than ever [10,11]. This evolution of these changes brings up a number of scientific questions: Is 15% truly a robust threshold choice to represent sea ice extent decadal trends? We use the National Oceanic and Atmospheric Administration (NOAA)/NSIDC sea ice concentration climate data record to examine how threshold choice may impact the timing of annual SIE minimums and maximums, magnitudes of these extrema, data distributions, and projected FIASY values
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