Seasonal transition dates can reveal biases in Arctic sea ice simulations

Abstract

Abstract. Arctic sea ice experiences a dramatic annual cycle, and seasonal ice loss and growth can be characterized by various metrics: melt onset, break-up, opening, freeze onset, freeze-up and closing. By evaluating a range of seasonal sea ice metrics, CMIP6 sea ice simulations can be evaluated in more detail than by using traditional metrics alone, such as sea ice area. We show that models capture the observed asymmetry in seasonal sea ice transitions, with spring ice loss taking about 1.5–2 months longer than fall ice growth. The largest impacts of internal variability are seen in the inflow regions of melt and freeze onset dates, but all metrics show pan-Arctic model spreads exceeding the internal variability. Through climate model evaluation in the context of both observations and internal variability, we show that biases in seasonal transition dates can compensate for other unrealistic aspects of simulated sea ice. In some models, this leads to September sea ice areas in agreement with observations for the wrong reasons.

Arctic open-water periods are projected to lengthen dramatically by 2100

“…By contrast, overestimation is more common in the Gulf of St. Lawrence, Canadian Arctic Archipelago, Beaufort Sea, Chukchi Sea, Kara Sea, and Hudson Bay (34% of the study area altogether). This is consistent with Smith et al 34 , who reported mean open-water periods were overestimated for the area north of 66°N in a subset of CMIP6 models. Altogether, a good match exists between models and observations for the pan-Arctic mean, but this occurs in part because of compensating biases in different regions, highlighting the importance of regional analysis.…”

“…In general agreement with previous analysis 34 , bias resulting in excessively long open-water periods always occurs because of sea ice retreat occurring too early in the multi-model mean, and sometimes also advance occurring too late (Fig. 1d-e).…”

“…The only case of sea ice retreat occurring too late is in the Sea of Okhotsk. This partially compensates for the bias in other regions, so pan-Arctic retreat shows less consistent bias than the area poleward of 66°N described by Smith et al 34 No region exhibits a multi-model mean biased toward too early advance.…”

“…Open-water period calculation. As in several past studies 24,25,34 the open-water period for a given SIC threshold is defined as the continuous period between the last SIC observation above that threshold prior to the day of annual minimum SIC (hereafter "retreat day") and the first SIC observation above that threshold after the annual minimum (hereafter "advance day"). The annual minimum day is defined as the median of all days August-October that equal the minimum SIC for the year.…”

“…The only modification from the Stroeve et al 24 method is our definition of the sea ice year. Since Arctic sea ice reaches its maximum extent every March, most studies 24,25,34 of sea ice retreat and advance in the Arctic identify sea ice retreat for a given year (e.g., 2001) as occurring sometime after March 1 (e.g., after March 1, 2001) and sea ice advance as occurring before March 1 of the following year (e.g., before March 1, 2002). In this study, we define the sea ice year as starting on the median of all days January-April for which SIC equals the maximum SIC for that period.…”