Regional atmospheric patterns and the delayed sea-ice freeze-up in the western Arctic

The western Arctic sea ice cover has dramatically changed since the late 1970s, particularly the timing of the autumn freeze-up. While atmospheric dynamic and thermodynamic processes associated with synoptic-scale weather patterns largely impact the onset of regional ice formation, linkages between the subseasonal occurrences of these patterns, across interannual to multidecadal time scales, and the freeze-up are not well understood. This manuscript takes a synoptic climatological atmospheric pattern (AP) classification approach to evaluate the role of warm season-dominant (i.e., May-October) mean sea-level pressure (MSLP) and 1000-500 hPa thickness APs, derived from daily NCEP/NCAR reanalysis fields, on the passive microwave-derived freeze-up dates for the marginal Beaufort/Chukchi Seas and western Arctic Ocean from 1979 to 2013. Analysis of the respective classifications' frequencies and their relationships to the freeze-up reveals that approximately one-third of freeze-up variance may be explained by early/middle warm season Beaufort Sea High surface pressure pattern frequency changes. A similar amount of freeze-up variance is explained by the occurrence of mid-warm season dominant thermal patterns, either earlier or later than their predominant season. Both results suggest that pattern changes may be associated with changing ocean-atmosphere heat exchanges affiliated with lengthened periods of melt conditions.