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Arctic Sea Ice Melt and Freeze Onset

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Arctic Sea Ice Melt and Freeze Onset Definitions Matter Arctic Sea Ice Melt and Freeze Onset ABIGAIL AHLERT AND ALEXANDRA JAHN UNIVERSITY OF COLORADO BOULDER INSTITUTE FOR ARCTIC AND ALPINE RESEARCH Feb 28, 2017 [email protected] Outline o Why do we care about melt onset and freeze onset ? o Definitions of melt onset / freeze onset / melt season length in climate models compared to satellite observations • CESM Large Ensemble (Kay et al, 2015) • Brightness temperature-derived melt and freeze onset dates (Stroeve et al, 2014) o Spatial variability and trends in melt onset / freeze onset / melt season length over the satellite era Photo: Karen Frey/Clark University Feb 28, 2017 [email protected] 2/15 Why do we care about melt season length? {Freeze onset date minus melt onset date} Phytoplankton Changes in surface albedo growth, polar impact ice loss during the bear access to summer months food Understanding Ecosystems changes in the Arctic Different definitions Implications for Climate make comparisons shipping, Societal model between satellite drilling, fishing, impact tourism assessment observations and models difficult Feb 28, 2017 [email protected] 3/15 Model melt onset definitions Definitions Average melt onset 75oN – 84.5oN • Using satellite observations: the first day that liquid water is continuously present in the snowpack • The first day that thermodynamic ice volume tendency passes below 0 8 days cm/day for 5 consecutive days • The first day that surface o temperature passes above -1 C for Days 5 consecutive days 32 days • The first day that snowmelt passes above 0.01 cm/day for 5 consecutive days Model year Feb 28, 2017 [email protected] 4/15 Average melt onset 1979-2014 CESM LE surface temperature CESM LE therm. volume tendency Stroeve et al, 2014 satellite data CESM LE snowmelt Earlier Later Feb 28, 2017 [email protected] 5/15 Trend in melt onset 1979-2014 CESM LE surface temperature CESM LE therm. volume tendency Stroeve et al, 2014 satellite data CESM LE snowmelt Earlier Later Feb 28, 2017 [email protected] 6/15 Model freeze onset definitions Definitions Average freeze onset 75oN – 84.5oN • Using satellite observations: when the average surface temperature is at the melting point and young ice grows in open water areas • The first day that thermodynamic ice volume tendency passes above 0 cm/day for 5 consecutive days Days • The first day that surface temperature passes o below -1.8 C for 20 consecutive days 17 days • The first day that congelation ice growth passes above 0 cm/day for 5 consecutive days • The first day that frazil ice growth passes above 0 cm/day for 5 consecutive days Model year Feb 28, 2017 [email protected] 7/15 Average freeze onset 1979-2014 CESM LE therm. volume tendency CESM LE congelation ice growth Stroeve et al, 2014 satellite data CESM LE frazil ice growth CESM LE surface temperature Earlier Later Feb 28, 2017 [email protected] 8/15 Trend in freeze onset 1979-2014 CESM LE therm. volume tendency CESM LE congelation ice growth Stroeve et al, 2014 satellite data CESM LE frazil ice growth CESM LE surface temperature Earlier Later Feb 28, 2017 [email protected] 9/15 Model melt season length definitions Definitions Average melt season length 75oN – 84.5oN • Freeze-up from satellite observations – melt onset from satellite observations • Freeze onset using thermodynamic volume tendency – Melt onset using thermodynamic volume tendency Days • Freeze onset using surface temperature – Melt onset using surface temperature • Freeze onset using congelation ice growth – Melt 30 days onset using snowmelt • Freeze onset using frazil ice growth – Melt onset using snowmelt Model year Feb 28, 2017 [email protected] 10/15 Average melt season length 1979-2014 CESM LE volume - volume CESM LE frazil- snowmelt Stroeve et al, 2014 satellite data CESM LE congelation - snowmelt CESM LE surface temp – surface temp Shorter Longer Feb 28, 2017 [email protected] 11/15 Trend in melt season length 1979-2014 CESM LE volume - volume CESM LE frazil- snowmelt Stroeve et al, 2014 satellite data CESM LE congelation - snowmelt CESM LE surface temp – surface temp Shorter Longer Feb 28, 2017 [email protected] 12/15 Conclusions •Different definitions of melt and freeze onset (and therefore melt season length) yield differences spatially and over time. •For Ensemble 35 melt season length, Congelation – Snowmelt performs most similarly to the satellite observations, but definition variations make it difficult to assess model bias •The observed and projected lengthening of the melt season is dominated by the freeze onset signal Feb 28, 2017 [email protected] 13/15 Next steps • Compare model definitions to another satellite dataset since recent work shows large differences in melt onset (Bliss and Anderson, 2014) • Assess influence of internal variability on trends • Study connection between melt season length and trends in ice extent and area • Build a satellite emulator to produce modeled brightness temperatures for more direct comparisons Feb 28, 2017 [email protected] 14/15 Thank you! This work is supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1144083. References Kay, J. E. et al. The community earth system model (CESM) large ensemble project : A community resource for studying climate change in the presence of internal climate variability. (2015). Bulletin of the American Meteorological Society. 96, 1333–1349. doi:10.1175/BAMS-D-13-00255.1 Markus, T. et al. Recent changes in Arctic sea ice melt onset, freezeup, and melt season length. (2014). Journal of Geophysical Research: Oceans. 114, 1-14. doi:10.1029/2009JC005436 Stroeve et al. Changes in Arctic melt season and implications for sea ice loss. (2014). Geophysical Research Letters, 41, 1216– 1225. Feb 28, 2017 [email protected] 15/15.
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