Long-term changes in Lake Superior ice cover and summer water temperature: A regime shift following the 1997/98 El Niño
Abstract
Strong trends in surface water temperature and ice cover over Lake Superior have been observed in recent decades, and these trends have typically been analyzed using simple linear regression techniques. Although the linear trends are statistically significant and contribute to an understanding of lake change, a careful examination of the trends shows important nonlinearities. In this study, we identify a pronounced step change that occurred during the warm El Niño winter of 1997/98 and resulted a "regime shift" in summer water temperatures and various ice cover metrics (e.g., ice onset date, duration, and maximum ice extent). This statistically significant step change accounts for most of the long-term "trends" in water temperature and ice cover during the period 1973-2010, and it was preceded (and followed) by insignificant linear trends in both variables. The step change is associated with an increase of nearly 3°C in mean summer water temperature and a decrease of 38 days in winter ice duration. This marked drop in ice duration is the result of a 19-day later ice onset date and a 19-day earlier ice-off date. Maximum wintertime ice extent decreased from an average of 69% lake surface area (before 1997/98) to 36% (after the step change), while seasonal mean ice extent decreased from 34% to 19%. This reassessment of long-term trends in Lake Superior summer water temperature and wintertime ice cover highlights the importance of considering nonlinear regime shifts such as the 1997/98 event - a regime shift that may also have significant ecological implications for Lake Superior.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H51K1356V
- Keywords:
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- 1833 HYDROLOGY / Hydroclimatology;
- 1845 HYDROLOGY / Limnology;
- 1863 HYDROLOGY / Snow and ice;
- 1872 HYDROLOGY / Time series analysis