Assessing the climatic and environmental impacts of mid‐tropospheric anticyclones over Alaska. (27th June 2017) - McLeod, Jordan T.
ABSTRACT: Ongoing climatic and cryospheric changes observed throughout the greater Alaska region are interconnected and often linked to oceanic and atmospheric patterns and processes that operate on varying spatiotemporal scales. To evaluate the long‐term, mid‐tropospheric circulation field across Alaska, and possible connections to climate and environmental change in the Pacific sector of the Arctic, the Alaska Blocking Index (ABI) is developed over the domain (54°–76°N, 125°–180°W) using daily gridded 500 hPa geopotential height fields derived from the ERA‐40 (1958–1978) and ERA‐Interim (1979–2014) data sets, 1958–2014. Climatological characteristics of the seasonal and annual ABI conditions are evaluated and periods of prevalent blocking conditions are identified and subsequently linked to possible local and large‐scale forcing mechanisms. The ABI has exhibited positive trends during all seasons and annually since 1979. Many of the extreme high ABI values occurred since 2000, including the highest annual values in 2013 and 2014. Anomalous blocking patterns in all seasons are associated with diminished snow depth and sea‐ice cover, positive near‐surface air temperature anomalies, and anticyclonic flows of heat and moisture across the domain. The ABI is also shown to differ from the long‐term variability and atmospheric circulation responses associated with phases of the Pacific–North American pattern and Pacific Decadal Oscillation, revealing some notable spatial and temporal disconnects between the region‐centric, high‐latitude blocking flow and some of the predominant modes of sea surface temperature and middle tropospheric circulation variability in the Northern Hemisphere. Abstract : The Alaska Blocking Index (ABI) is developed and used to evaluate long‐term, mid‐tropospheric circulation patterns over Alaska. Extreme positive values of the ABI indicate a mid‐tropospheric blocking signature centred over Alaska, which is associated with diminished snow depth and sea‐ice cover, positive near‐surface air temperature anomalies, and anticyclonic flows of heat and moisture across the region. The ABI has exhibited positive trends during all seasons and annually since 1979, with many of the extreme high ABI values occurring since 2000.