Effects of recent temperature variability and warming on the Oulu-Hailuoto ice road season in the northern Baltic Sea

In cold climate regions, ice roads are engineered as temporary winter transportation routes on frozen lakes, rivers and seas. The ice road season start, end and duration principally depend upon ice thickness, which is controlled by surface air temperature (SAT) in terms of freezing and thawing degree-days (FDD and TDD, respectively). Both FDD and TDD are indicators of climate variability and change, and are naturally influenced by large-scale atmospheric circulation patterns (ACPs). This study examined the role of ACPs in interannual variations in the operating season of the Oulu-Hailuoto ice road in the Bay of Bothnia, northern Baltic Sea, during 1974-2009. Significant (p < .05) shortening in duration of the ice road season, mainly attributable to later start and earlier end days, was observed. In the Oulu-Hailuoto area, maximum ice thickness showed significant declines over time. This sea ice thinning was associated with SAT warming in cold months, manifested by statistically significant decreases in cumulative FDD during October-January within the water year (September-August). Significant increases in cumulative TDD during February-April, reflecting warmer SAT in mild months, resulted in earlier end day for the Oulu-Hailuoto ice road season. The Arctic Oscillation (AO) was the most influential ACP for variations in cumulative FDD (October-January), and accordingly for sea ice thickness and start day of the Oulu-Hailuoto ice road season. However, cumulative TDD (February-April) showed significant positive correlations with the East Atlantic (EA) pattern, which also controlled the end day of the Oulu-Hailuoto ice road season.