Climatological Analysis of Alaska Blocking Patterns, 1958-2014

Rapid changes to the climate and environment of greater Alaska are often physically interconnected and linked to oceanic and atmospheric processes that vary across disparate space and time scales.  Previous studies have also suggested that synoptic-scale circulation patterns over Alaska have especially profound impacts on the occurrence of temperature and moisture extremes, but there has yet to be a long-term assessment of mid-tropospheric circulation across the region, which may influence the temporal variability of such extreme events.  Here, an Alaska Blocking Index (ABI) for the 1958–2014 period is created and analyzed, representing the climatological, mid-tropospheric circulation field over Alaska.  This metric is developed over the domain (54–76N, 125–180W) by merging daily, gridded 500 hPa geopotential height fields derived from the ERA-40 (1958–1978) and ERA-Interim (1979–2014) reanalyses.  Climatological characteristics of the seasonal and annual ABI values are evaluated, and periods of prevalent blocking conditions are identified and subsequently analyzed with respect to a number of reanalysis-derived climate and environmental variables as well as the prominent modes of Pacific climate variability. 

The ABI has exhibited positive trends since 1979, especially during summer, autumn, and annually.  Many of the extreme high ABI values occur since 2000, including the highest annual values in 2013 and 2014.  Anomalous blocking patterns in winter and summer are associated with diminished terrestrial snow depth and sea-ice cover, positive near-surface air temperature anomalies, and poleward advection of heat and moisture across Alaska and its bordering seas.  Vector wind comparisons at the 500 hPa level between ABI, Pacific North American (PNA) pattern, and Pacific Decadal Oscillation (PDO) extremes reveal distinguishing dynamic characteristics as the ABI center of action and its associated anticyclonic wind field are shifted well north about central Alaska relative to the PNA and PDO.  Forthcoming analyses will look further into ABI relationships with regional Arctic change and potential downstream linkages between Alaska blocking and North American mid-latitude climate.