Tracer studies of pathways and rates of meltwater transport through Arctic summer sea ice

Abstract

At the Surface Heat Budget of the Arctic Ocean (SHEBA) program's field site in the northern Chukchi Sea, snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998. Pathways and rates of meltwater transport were derived from tracer studies (H218O, 7Be, and release of fluorescent dyes), complemented by in situ sea‐ice permeability measurements. It was shown that the balance between meltwater supply at the surface (averaging between 3.5 and 10.5 mm d−1) and ice permeability (between <10−11 and >10−9 m2) determines the retention and pooling of meltwater, which in turn controls ice albedo. We found that the seasonal evolution of first‐year and multiyear ice permeability and surface morphology determine four distinct stages of melt. At the start of the ablation season (stage 1), ponding is widespread and lateral melt flow dominates. Several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones. Significant formation of underwater ice, composed between <30 and >50% of meteoric water, formed at these drainage sites. Complete removal of snow cover, increase in ice permeability, and reductions in hydraulic gradients driving fluid flow mark stage 2, concurrent with a reduction in pond coverage and albedo. During stage 3, maximum permeabilities were measured, with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first‐year and thin multiyear ice. Enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration, concurrent with increases in pond salinities and 7Be. Advective heat flows of several tens of watts per square meter were derived, promoting widening of ponds and increases in pond coverage. Stage 4 corresponds to freeze‐up. Roughly 40% of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under‐ice ponds (with a total net retention of 15%). Based on this work, areas of improvement for fully prognostic simulations of ice albedo are identified, calling for parameterizations of sea‐ice permeability and the integration of ice topography and refined ablation schemes into atmosphere‐ice‐ocean models.

Seasonal evolution of melt ponds on Arctic sea ice

“…Melt Pond Class: Residual objects with means greater than (B) were classified as melt ponds (Figures 2c1-2c2). The image classification is complete at Step 6 for scenes exhibiting melt Stage 1 (pond formation) and melt Stage 2 (pond drainage) [Eicken et al, 2002;Polashenski et al, 2012]. Total computation time, including cloud and open water masking, is 15 min on an Intel i7 eight-core processor machine.…”

“…Especially evident on undeformed first-year ice, the lateral spread of meltwater can cause maximum pond fractions as high as 70% due to the low surface topography of snow drift bed forms [Scharien and Yackel, 2005;Petrich et al, 2012a;Polashenski et al, 2012]. On a broader scale, Webster et al [2014] have shown that snow depth distributions on both ice types in the western Arctic have decreased, and may have two effects on melt pond formation (1) more thinly snow-covered ice and bare ice may expose the ice surface to solar radiation earlier in the season and hasten melt, and (2) less freshwater may be available during early pond formation to form low-salinity ice surface layers [Eicken et al, 2002], which further motivates an analysis of seasonal melt pond evolution on drifting sea ice in more recent years.…”

Влияние Ложного Дна На Нелинейную Динамику Процесса Замерзания Воды

“…На основе дан-ных, полученных с сонара, в работе [10] сделан вывод о том, что образование подлёдных прудов должно быть широкораспространённым явлением в Арк-тике, а в работе [11] показано, что над подлёдными прудами часто находятся поверхностные пруды, что подтверждает механизм образования ложного дна, изображенный на рис. 1 по данным работ [8,12].…”

“…Ложное дно достаточно часто наблюдалось в полевых экспериментах SHEBA [12,13]. При этом было отмечено, что начальное образование ложно-го дна обычно занимало пару часов; большие кристаллы льда претерпевали боковой рост от стенок, образуя поверхность ложного дна.…”

“…Сравним теперь развиваемую модель с данными полевых наблюдений AIDJEX (детали эксперимента см., например, в работе [8]) и SHEBA (детали эксперимента см., например, в работах [8,12,13]). Поскольку доля твёрдой фазы ϕ b на нижней границе ложного дна близка к единице, в расчётах она принималась равной 0,99.…”