Large-scale climatic forcing detected in historical observations of lake ice break-up

In lakes subject to freezing over, the timing of spring break-up is of critical importance for the initiation of the struggle between the physical processes of vertical mixing and stratification that play such a crucial role in determining the chemistry and biology of lacustrine systems. Break-up is the end result of a complex set of thawing processes driven by many meteorological forcing factors (e.g. GU & STEFAN 1990), of which air temperature appears to be the most important (RUOSTEENOJA 1986, VAVRUS et al. 1996). Break-up can be viewed as a temporally integrated response to meteorological forcing, and several authors have shown the timing of break-up to be closely related to local or regional air temperature integrated over a period of several weeks in spring (e.g. PALECKI & BARRY 1986, ROBERTSON et al. 1992, LIVINGSTONE 1997). Because climate in general, and air temperature in particular, depend to a great extent on forcing phenomena occurring on global, hemispheric, or planetary wave scales, we might expect to find some evidence of large-scale climatic forcing in historical observations of lake ice breakup. Evidence linking the timing of break-up to largescale climatic forcing would be of interest because such forcing is potentially important not just for individual lakes, but for entire lake regions. Such evidence would also increase the value of existing long (~150 years) series of observations of break-up date, which might then be able to provide information not only on local or regional air temperatures, but also on much larger scale climatological phenomena.