Long-term dynamics of lake ice phenology across the Northern Hemisphere

Freeze and breakup dates for lakes across the Northern Hemisphere show consistent and widespread changes in ice phenology, with most time series displaying long-term trends toward later freeze and earlier breakup consistent with global warming. We examine these trends as well as the relationship of shorter term dynamics with oscillations in large-scale drivers such as the Southern Oscillation (SOI), the Pacific Decadal Oscillation (PDO), and the North Atlantic Oscillation (NAO) for a 150 year period (the winters of 1855-1856 to 2004-2005) for lakes across the Northern Hemisphere (8 freeze and 17 breakup time series). Freeze date trends are 8.3 days/century later, and breakup date trends are 8.5 days/century earlier, though changes since ca. 1970 have been considerably faster. Fifty-six percent of the variation in the normalized mean anomaly of breakup date across study lakes was shared with the normalized mean anomaly of spring air temperature across the northern hemisphere, suggesting that a large percentage of the variation in ice breakup dates can be predicted from air temperature and that a large percentage of the variation in air temperature can be predicted from the ice breakup dates of a small number of lakes. In analyses using linear regression, the detrended breakup dates were related significantly to the winter NAO, with regional variation. Lakes in Finland and many United States lakes showed the same directional relationship with NAO, but Maine lakes were unrelated to the NAO.  In these initial analyses, the PDO and the SOI explained little variation.