Abstract
Abstract. The Arctic sea ice cover of 2016 was highly noteworthy, as it featured record low monthly sea ice extents at the start of the year but a summer (September) extent that was higher than expected by most seasonal forecasts. Here we explore the 2016 Arctic sea ice state in terms of its monthly sea ice cover, placing this in the context of the sea ice conditions observed since 2000. We demonstrate the sensitivity of monthly Arctic sea ice extent and area estimates, in terms of their magnitude and annual rankings, to the ice concentration input data (using two widely used datasets) and to the averaging methodology used to convert concentration to extent (daily or monthly extent calculations). We use estimates of sea ice area over sea ice extent to analyse the relative “compactness” of the Arctic sea ice cover, highlighting anomalously low compactness in the summer of 2016 which contributed to the higher-than-expected September ice extent. Two cyclones that entered the Arctic Ocean during August appear to have driven this low-concentration/compactness ice cover but were not sufficient to cause more widespread melt-out and a new record-low September ice extent. We use concentration budgets to explore the regions and processes (thermodynamics/dynamics) contributing to the monthly 2016 extent/area estimates highlighting, amongst other things, rapid ice intensification across the central eastern Arctic through September. Two different products show significant early melt onset across the Arctic Ocean in 2016, including record-early melt onset in the North Atlantic sector of the Arctic. Our results also show record-late 2016 freeze-up in the central Arctic, North Atlantic and the Alaskan Arctic sector in particular, associated with strong sea surface temperature anomalies that appeared shortly after the 2016 minimum (October onwards). We explore the implications of this low summer ice compactness for seasonal forecasting, suggesting that sea ice area could be a more reliable metric to forecast in this more seasonal, “New Arctic”, sea ice regime.
Highlights
A dramatic indicator of global climate change is the accelerated loss of Arctic sea ice (Stroeve et al, 2012; Serreze and Stroeve, 2015; Notz and Stroeve, 2016)
The monthly estimates of Arctic sea ice extent (SIE) calculated from the NASA Team and Bootstrap sea ice concentration (SIC) data are shown as box-andwhisker plots in Fig. 3 (2000–2015, 2016 indices highlighted by crosses)
It is interesting to note that the 2016 August and September SIEs, the months which have seen the strongest long-term Arctic sea ice declines, are significantly above the previous monthly record lows, which were both set in 2012
Summary
A dramatic indicator of global climate change is the accelerated loss of Arctic sea ice (Stroeve et al, 2012; Serreze and Stroeve, 2015; Notz and Stroeve, 2016). Over the last several decades, Arctic sea ice extent (SIE) has declined across all seasons, with the strongest decline observed in September; the end of the summer melt season The 10 lowest months of September Arctic SIE have all occurred within the last 10 years. Global climate models (GCMs) and observations suggest the Arctic will become ice free in summer sometime during the middle of the century Petty et al.: The Arctic sea ice cover of 2016
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