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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/tc-2018-151
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2018-151
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Aug 2018

Research article | 08 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

Seasonal sea ice prediction based on regional indices

John E. Walsh1, J. Scott Stewart2, and Florence Fetterer2 John E. Walsh et al.
  • 1Alaska Center for Climate Assessment and Policy, University of Alaska, Fairbanks, AK 99709 USA
  • 2National Snow and Ice Data Center, University of Colorado, Boulder, CO 80303 USA

Abstract. Basic statistical metrics such as autocorrelations and across-region lag correlations of sea ice variations provide benchmarks for the assessments of forecast skill achieved by other methods such as more sophisticated statistical formulations, numerical models, and heuristic approaches. However, the strong negative trend of sea ice coverage in recent decades complicates the evaluation of statistical skill by inflating the correlation of interannual variations of pan-Arctic and regional ice extent. In this study we provide a quantitative evaluation of the contribution of the trend to the predictive skill of monthly and seasonal ice extent on the pan-Arctic and regional scales. We focus on the Beaufort Sea where the Barnett Severity Index provides a metric of historical variations in ice conditions over the summer shipping season. The variance about the trend line differs little among various methods of detrending (piecewise linear, quadratic, cubic, exponential). Application of the piecewise linear trend calculation indicates an acceleration of the trend during the 1990s in most of the Arctic subregions. The Barnett Severity Index as well as September pan-Arctic ice extent show significant statistical predictability out to several seasons when the data include the trend. However, this apparent skill largely vanishes when the data are detrended. No region shows significant correlation with the detrended September pan-Arctic ice extent at lead times greater than a month or two; the concurrent correlations are strongest with the East Siberian Sea. The Beaufort Sea’s ice extent as far back as July explains about 20% of the variance of the Barnett Severity Index, which is primarily a September metric. The Chukchi Sea is the only other region showing a significant association with the Barnett Severity Index, although only at a lead time of a month or two.

John E. Walsh et al.
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John E. Walsh et al.
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Short summary
The recent loss of Arctic sea ice complicates evaluations of statistical predictability of sea ice by inflating correlations between interannual variations of pan-Arctic and regional ice extent. We evaluate the contribution of the trend to the skill of monthly and seasonal ice forecasts on the pan-Arctic and regional scales, with a focus on a metric of Beaufort Sea navigability during the summer shipping season. Most of the skill of correlation-based forecasts is attributable to the trends.
The recent loss of Arctic sea ice complicates evaluations of statistical predictability of sea...
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