| Summary: | The declining summer Arctic sea ice is impacting cyclone-forcing of dynamic and thermodynamic processes in Arctic sea at different spatial and temporal scales throughout the annual cycle. A catalogue of daily synoptic weather types is generated for the southern Beaufort Sea, covering the period 1979 to 2011 using NCEP/NCAR reanalysis mean sea level pressure data, principle components and k-means cluster analyses. Synoptic type statistics are used to assess changes in atmospheric circulation characteristics, sea ice vorticity, and lead formation. Significant (p < 0.05) seasonal synoptic type frequency anomalies are revealed between 1979 – 1998 and 1999 – 2011, and indicate a stronger Beaufort high, and increased easterly wind forcing in autumn and winter. High rates of young ice production in November and December 2007 were linked to strong easterly wind forcing. A case study of the atmospheric boundary layer during a lead formation event (03 – 18 February 2008) revealed sensible heat fluxes between 0 – 80.14 W • m2 (13 February 2008). First-ever observations of a physical forcing mechanism between Arctic cyclones, the Arctic Ocean, and Arctic sea ice within the southern Beaufort Sea were observed on 06 September 2009. Large swells intruded into the multi-year pack ice, causing instantaneous widespread fracturing, and reduced the large (>1 km) parent ice floes to small (100 – 150 m diameter) floes. This process increased the ice floe perimeter exposed to the ocean by a factor of 4.5. Analysis of Radarsat-2 imagery showed that open water fractional area in the multi-year ice cover initially decreased from 3.7% to 2.7%, then increased to ~20% due to wind-forced divergence. 11.54 MJ • m-2 of additional energy was estimated for lateral melting as a result of the fracture event using radiation budgets prior to and following the event. Earlier occurrences of flexural fracture could hypothetically provide up to three times more additional energy for lateral melt. Furthermore, this process may increase the likelihood of storm-driven upwelling of ocean heat, thereby enhancing bottom melt in the ice cover. This process is therefore presented as a potentially powerful positive feedback process that may accelerate the loss of Arctic sea ice.
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