Recent climatic trends in nearshore water temperatures in the St. Lawrence Great Lakes

Abstract

In the Great Lakes region, the observational evidence for climatic change has been primarily limited to changes in lake-ice conditions, with no long-term trends identified in water temperatures. Seven nearshore water intake sites (Bay City, Michigan; Green Bay, Wisconsin; Sault Ste. Marie, Michigan; St Joseph, Michigan; Sandusky Bay, Ohio; Put-In-Bay, Ohio; and Erie, Pennsylvania) in the Great Lakes were chosen, and their data were examined for any climatic trends. Regression results on the annual mean temperatures showed varying support in favor of a warming trend at five of the seven sites. A new approach facilitated determination of the interannual variability in the timing of the 4°C temperature of maximum density. Two of the three sites with data records extending back to the early part of this century (Sault Ste. Made and Put-In-Bay, respectively) showed a 4- and a 6-h yr-1 rate of increase in the maximum potential duration of summer stratification (DSS). Over the time span of these two data sets, this equates to a 14- and 18-d increase in the potential DSS, respectively. The rate of increase in the duration data was skewed, with most of the increase due to an earlier transition to springlike conditions. Finally, the data do not extend far enough back in time to know if these climatic trends are part of an unresolvable natural cycle or forced by anthropogenic activity.