The Cryosphere | 2019
Benchmark seasonal prediction skill estimates based on regional indices
Abstract
Abstract. Basic statistical metrics such as autocorrelations and across-region lag\ncorrelations of sea ice variations provide benchmarks for the assessments of\nforecast skill achieved by other methods such as more sophisticated\nstatistical formulations, numerical models, and heuristic approaches. In this\nstudy we use observational data to evaluate the contribution of the trend to\nthe skill of persistence-based statistical forecasts of monthly and seasonal\nice extent on the pan-Arctic and regional scales. We focus on the Beaufort\nSea for which the Barnett Severity Index provides a metric of historical\nvariations in ice conditions over the summer shipping season. The variance\nabout the trend line differs little among various methods of detrending\n(piecewise linear, quadratic, cubic, exponential). Application of the\npiecewise linear trend calculation indicates an acceleration of the winter\nand summer trends during the 1990s. Persistence-based statistical forecasts\nof the Barnett Severity Index as well as September pan-Arctic ice extent show\nsignificant statistical skill out to several seasons when the data include\nthe trend. However, this apparent skill largely vanishes when the data are\ndetrended. In only a few regions does September ice extent correlate\nsignificantly with antecedent ice anomalies in the same region more than 2\nmonths earlier. The springtime “predictability barrier” in regional\nforecasts based on persistence of ice extent anomalies is not reduced by the\ninclusion of several decades of pre-satellite data. No region shows\nsignificant correlation with the detrended September pan-Arctic ice extent at\nlead times greater than a month or two; the concurrent correlations are\nstrongest with the East Siberian Sea. The Beaufort Sea s ice extent as far\nback as July explains about 20\u2009% of the variance of the Barnett Severity\nIndex, which is primarily a September metric. The Chukchi Sea is the only\nother region showing a significant association with the Barnett Severity\nIndex, although only at a lead time of a month or two.