A regime shift in Lake Superior ice cover, evaporation, and water temperature following the warm El Niñ winter of 1997–1998

Significant trends in Lake Superior water temperature and ice cover have been observed in recent decades, and these trends have typically been analyzed using standard linear regression techniques. Although the linear trends are statistically significant and contribute to an understanding of environmental change, a careful examination of the trends shows important nonlinearities. We identify a pronounced step change that occurred in Lake Superior following the warm El Niño winter of 1997–1998, resulting in a “regime shift” in summer evaporation rate, water temperature, and numerous metrics of winter ice cover. This statistically significant step change accounts for most of the long‐term trends in ice cover, water temperature, and evaporation during the period 1973–2010, and it was preceded (and followed) by insignificant linear trends in nearly all of the metrics examined. The 1998 step change is associated with a decrease in winter ice duration of 39 d (a 34% decline), an increase of ∼ 2–3°C in mean surface water temperature (July–September averages), and a 91% increase in July–August evaporation rates, reflecting an earlier start to the summer evaporation season. Maximum wintertime ice extent decreased by nearly a factor of two, from an average of 69% of the lake surface area (before 1997–1998) to 36% after the step change. This reassessment of long‐term trends highlights the importance of nonlinear regime shifts such as the 1997–1998 break point—an event that may be related to a similar shift in the Pacific Decadal Oscillation that occurred around the same time. These pronounced changes in Lake Superior physical characteristics are likely to have important implications for the broader lake ecosystem.