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Abstract. The melt of snow and sea ice during the Arctic summer is a significant source of relatively fresh meltwater. The fate of this freshwater, whether in surface melt ponds or thin layers underneath the ice and in leads, impacts atmosphere–ice–ocean interactions and their subsequent coupled evolution. Here, we combine analyses of datasets from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (June–July 2020) for a process study on the formation and fate of sea ice freshwater on ice floes in the Central Arctic. Our freshwater budget analyses suggest that a relatively high fraction (58 %) is derived from surface melt. Additionally, the contribution from stored precipitation (snowmelt) outweighs by 5 times the input from in situ summer precipitation (rain). The magnitude and rate of local meltwater production are remarkably similar to those observed on the prior Surface Heat Budget of the Arctic Ocean (SHEBA) campaign, where the cumulative summer freshwater production totaled around 1 m during both. A relatively small fraction (10 %) of freshwater from melt remains in ponds, which is higher on more deformed second-year ice (SYI) compared to first-year ice (FYI) later in the summer. Most meltwater drains laterally and vertically, with vertical drainage enabling storage of freshwater internally in the ice by freshening brine channels. In the upper ocean, freshwater can accumulate in transient meltwater layers on the order of 0.1 to 1 m thick in leads and under the ice. The presence of such layers substantially impacts the coupled system by reducing bottom melt and allowing false bottom growth; reducing heat, nutrient, and gas exchange; and influencing ecosystem productivity. Regardless, the majority fraction of freshwater from melt is inferred to be ultimately incorporated into the upper ocean (75 %) or stored internally in the ice (14 %). Terms such as the annual sea ice freshwater production and meltwater storage in ponds could be used in future work as diagnostics for global climate and process models. For example, the range of values from the CESM2 climate model roughly encapsulate the observed total freshwater production, while storage in melt ponds is underestimated by about 50 %, suggesting pond drainage terms as a key process for investigation.
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Meltwater sources and sinks for multiyear Arctic sea ice in summer
Abstract. On Arctic sea ice, the melt of snow and sea ice generate asummertime flux of fresh water to the upper ocean. The partitioning of thismeltwater to storage in melt ponds and deposition in the ocean hasconsequences for the surface heat budget, the sea ice mass balance, andprimary productivity. Synthesizing results from the 1997–1998 SHEBA fieldexperiment, we calculate the sources and sinks of meltwater produced on amultiyear floe during summer melt. The total meltwater input to the systemfrom snowmelt, ice melt, and precipitation from 1 June to 9 August wasequivalent to a layer of water 80 cm thick over the ice-covered and openocean. A total of 85 % of this meltwater was deposited in the ocean, and only 15 %of this meltwater was stored in ponds. The cumulative contributions ofmeltwater input to the ocean from drainage from the ice surface and bottommelting were roughly equal.
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- PAR ID:
- 10297231
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 15
- Issue:
- 9
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 4517 to 4525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. The melt of snow and sea ice during the Arctic summer is a significant source of relatively fresh meltwater in the central Arctic. The fate of this freshwater – whether in surface melt ponds, or thin layers underneath the ice and in leads – impacts atmosphere-ice-ocean interactions and their subsequent coupled evolution. Here, we combine analyses of datasets from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (June–July, 2020) to understand the key drivers of the sea ice freshwater budget in the Central Arctic and the fate of this water over time. Freshwater budget analyses suggest that a relatively high fraction (58 %) is derived from surface melt. Additionally, the contribution from stored precipitation (snowmelt) significantly outweighs by five times the input from in situ summer precipitation (rain). The magnitude and rate of local meltwater production are remarkably similar to that observed on the prior Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. A relatively small fraction (10 %) of freshwater from melt remains in ponds, which is higher on more deformed second-year ice compared to first-year ice later in the summer. Most meltwater drains via lateral and vertical drainage channels, with vertical drainage enabling storage of freshwater internally in the ice by freshening of brine channels. In the upper ocean, freshwater can accumulate in transient meltwater layers on the order of 10 cm to 1 m thick in leads and under the ice. The presence of such layers substantially impacts the coupled system by reducing bottom melt and allowing false bottom growth, reducing heat, nutrient and gas exchange, and influencing ecosystem productivity. Regardless, the majority fraction of freshwater from melt is inferred to be ultimately incorporated into upper ocean (75 %) or stored internally in the ice (14 %). Comparison of key source and sink terms with estimates from the CESM2 climate model suggest that simulated freshwater storage in melt ponds is dramatically underestimated. This suggests pond drainage terms should be investigated as a likely explanation.more » « less
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During the Arctic melt season, relatively fresh meltwater layers can accumulate under sea ice as a result of snow and ice melt, far from terrestrial freshwater inputs. Such under-ice meltwater layers, sometimes referred to as under-ice melt ponds, have been suggested to play a role in the summer sea ice mass balance both by isolating the sea ice from saltier water below, and by driving formation of ‘false bottoms’ below the sea ice. Such layers form at the interface of the fresher under-ice layer and the colder, saltier seawater below. During the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition in the Central Arctic, we observed the presence of under-ice meltwater layers and false bottoms throughout July 2020 at primarily first-year ice locations. Here, we examine the distribution, prevalence, and drivers of under-ice ponds and the resulting false bottoms during this period. The average thickness of observed false bottoms and freshwater equivalent of under-ice meltwater layers was 0.08 m, with false bottom ice comprised of 74–87% FYI melt and 13–26% snow melt. Additionally, we explore these results using a 1D model to understand the role of dynamic influences on decoupling the ice from the seawater below. The model comparison suggests that the ice-ocean friction velocity was likely exceptionally low, with implications for air-ice-ocean momentum transfer. Overall, the prevalence of false bottoms was similar to or higher than noted during other observational campaigns, indicating that these features may in fact be common in the Arctic during the melt season. These results have implications for the broader ice-ocean system, as under-ice meltwater layers and false bottoms provide a source of ice growth during the melt season, potentially reduce fluxes between the ice and the ocean, isolate sea ice primary producers from pelagic nutrient sources, and may alter light transmission to the ocean below.more » « less
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This metadata links to 16S and 18S rRNA amplicon data (raw sequence reads, NCBI Accession PRJNA895866) for seawater, sea ice, meltwater, and experimental samples from the Central Arctic Ocean collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in which the RV (Research Vessel) Polarstern was tethered to drifting sea ice from October 2019 to September 2020. Seawater samples were collected from the water column using a CTD (conductivity-temperature-depth) rosette or underway seawater tap during legs 1, 2, 3, 4, and 5 of the expedition. Sea ice samples were collected via coring (FYI (first-year ice), SYI (second-year ice)) or scooped with a saw and/or sieve (new ice formation) during legs 1, 3, 4, and 5 of the expedition. Summer meltwater was from surface layers within leads or melt ponds and was collected using pump systems during legs 4 and 5 of the expedition. Experimental samples were filtered and processed post nutrient addition, stable isotope, or elevated methane incubations to pair community structure with biogeochemical measurements. Original data published with the National Center for Biotechnology Information: https://www.ncbi.nlm.nih.gov/bioproject/895866 ; Please contact data creators before use.more » « less
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Abstract Sea‐ice melt ponds form in the depressions of pre‐melt surface topography, a process widely accepted yet lacking larger‐scale evaluation through explicit comparisons. During MOSAiC, we collected multi‐dimensional aerial data to examine the relationship between pre‐melt surface topography and melt pond evolution across an entire Arctic ice floe. Using hydrological models, we analyze the correlation between potential meltwater accumulation areas identified in winter and spring topographies, available meltwater, and observed pond coverage. Our findings demonstrate a strong connection, revealing a 72% accuracy in matching low areas to melt ponds, with 98% of basins deeper than 0.5 m transforming into ponds. Incorporating assumptions regarding meltwater availability improve predictions of melt pond fraction and highlight key factors driving extensive lateral runoff networks on the floe. No significant differences are observed between first‐ and second‐year ice. This study provides valuable ground truth for future modeling and measurements of pond formation.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/tc-15-4517-2021
