Lakes are prevalent in the Arctic and thus play a key role in regional hydrology. Since manyArctic lakes are shallow and ice grows thick (historically 2 m or greater), seasonal ice commonly freezes tothe lake bed (bedfast ice) by winter’s end. Bedfast ice fundamentally alters lake energy balance and meltoutprocesses compared to deeper lakes that exceed the maximum ice thickness (floating ice) and maintainperennial liquid water below floating ice. Our analysis of lakes in northern Alaska indicated that ice-out ofbedfast ice lakes occurred on average 17 days earlier (22 June) than ice-out on adjacent floating ice lakes (9July). Earlier ice-free conditions in bedfast ice lakes caused higher open-water evaporation, 28% on average,relative to floating ice lakes and this divergence increased in lakes closer to the coast and in coolersummers. Water isotopes (18O and 2H) indicated similar differences in evaporation between these laketypes. Our analysis suggests that ice regimes created by the combination of lake depth relative to ice thicknessand associated ice-out timing currently cause a strong hydrologic divergence among Arctic lakes.Thus, understanding the distribution and dynamics of lakes by ice regime is essential for predicting regionalhydrology. An observed regime shift in lakes to floating ice conditions due to thinner ice growth may initiallyoffset lake drying because of lower evaporative loss from this lake type. This potential negative feedbackcaused by winter processes occurs in spite of an overall projected increase in evapotranspiration asthe Arctic climate warms.1.
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