Abstract
Abstract. Air temperature and winter precipitation changes over the last five decades have impacted the timing, duration, and thickness of the ice cover on Arctic lakes as shown by recent studies. In the case of shallow tundra lakes, many of which are less than 3 m deep, warmer climate conditions could result in thinner ice covers and consequently, in a smaller fraction of lakes freezing to their bed in winter. However, these changes have not yet been comprehensively documented. The analysis of a 20 yr time series of European remote sensing satellite ERS-1/2 synthetic aperture radar (SAR) data and a numerical lake ice model were employed to determine the response of ice cover (thickness, freezing to the bed, and phenology) on shallow lakes of the North Slope of Alaska (NSA) to climate conditions over the last six decades. Given the large area covered by these lakes, changes in the regional climate and weather are related to regime shifts in the ice cover of the lakes. Analysis of available SAR data from 1991 to 2011, from a sub-region of the NSA near Barrow, shows a reduction in the fraction of lakes that freeze to the bed in late winter. This finding is in good agreement with the decrease in ice thickness simulated with the Canadian Lake Ice Model (CLIMo), a lower fraction of lakes frozen to the bed corresponding to a thinner ice cover. Observed changes of the ice cover show a trend toward increasing floating ice fractions from 1991 to 2011, with the greatest change occurring in April, when the grounded ice fraction declined by 22% (α = 0.01). Model results indicate a trend toward thinner ice covers by 18–22 cm (no-snow and 53% snow depth scenarios, α = 0.01) during the 1991–2011 period and by 21–38 cm (α = 0.001) from 1950 to 2011. The longer trend analysis (1950–2011) also shows a decrease in the ice cover duration by ~24 days consequent to later freeze-up dates by 5.9 days (α = 0.1) and earlier break-up dates by 17.7–18.6 days (α = 0.001).
Highlights
Lake ice cover has been shown to be a robust indicator of climate variability and change
Assessment of grounded ice fractions during the winter seasons (1991–2011) with available ERS imagery indicates a gradual trend toward lower fractions of grounded ice in all months of observations in the image time series
Analysis of ice-thickness trends from CLIMo simulations during the 1950–2011 period indicates a trend toward thinner ice covers for the Alaskan lakes under study, a trend that is more evident with 53 % snow cover depth conditions and that indicates a decrease of a total of 38 cm in ice thickness, at a rate of 0.6 cm yr−1
Summary
Lake ice cover has been shown to be a robust indicator of climate variability and change. Persistent warmer air temperatures (Serreze et al, 2000; Trenberth et al, 2007) and increased snowfall observed in the Arctic over the last decades (Jones et al., 2011; Arp et al, 2012), associated with amplified reduction of sea-ice concentrations, thickness and extent (Serreze et al., 2007; Comiso et al, 2008; Walsh et al, 2011), have accelerated during recent years (Walsh et al, 2011) These changes in the Arctic climate system have likely had an impact on ice phenology of lakes in coastal regions adjacent to the Arctic. In order to complement these observations and be able to simulate freeze-up and break-up dates with a daily temporal resolution, CLIMo was employed
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