Freezing degree‐day thresholds and Lake ice‐out dates: Understanding the role of El Niño conditions.

In temperate lakes, the wintertime Accumulated Freezing Degree‐Days (AFDD) modulate the thickness and phenology of winter ice‐cover, which in turn influence lake ecosystem processes and functions across seasons. Empirical studies show that the El Niño‐Southern Oscillation (ENSO)‐winter AFDD relationship for North American regions depends on the location and amplitude of the winter ENSO‐related sea surface temperature (SST) warming/cooling in the tropical Pacific (TP), and consequently changes in the magnitude and frequency of different ENSO patterns engender shifts and transitions in the North American lake ice regimes. For eight lakes located across North America, we found quasi‐linear and non‐linear relationships between winter AFDDs and spring lake ice‐out dates, and in some cases, existence of thresholds above and below which regression slopes are materially different, thus illuminating differential sensitivities. Conditional quantile functions for winter AFDDs that incorporate ENSO indices as covariates were developed to estimate the relative risk of early/late lake ice‐out events for these lakes. For seven of the eight lakes, the canonical Eastern Tropical El Niño pattern increases likelihood of low winter AFDDs (associated with early ice‐out dates in these lakes) by 1.5–2.8 times to that of the climatology (1951–2010 average), while the typical Central Pacific El Niño pattern corresponds to a decrease or no significant change in the occurrence probability of early ice‐out dates in these Lakes. These results demonstrate that the conditional winter AFDD estimated based on a comprehensive characterization of ENSO allow for delineation of distinct local‐to‐regional patterns of elevated risk of early ice out and short lake ice‐cover season for North American region. [ABSTRACT FROM AUTHOR]