Cumulative degree-days (CDD) are a measure of heat accumulation over an entire season or year. Using the mean daily water temperature we calculated CDD for the surface water (1995-2013) and upper layer for the vertical water (2006-2012) temperature (see Water Temperature below).
Since 1973 ice cover has been interpolated from remotely sensed images. The daily percent ice cover (concentration) is available from 1973-2002 from the NOAA Great Lakes Ice Atlas, and since 2002 the NOAA Great Lakes Environmental Research Laboratory has continued to make observed days of ice available. We have summarized the data into monthly and annual ice cover concentration and an annual ice duration index (in days) for the years 1973-2013.
Spring rate of warming
We generated an index of spring warming to identify areas of potentially higher spring and early summer productivity. This index is the difference between June 1 and March 1 surface water temperatures (NOAA Great Lakes CoastWatch) divided by the number of days during that period to estimate the averaged change in temperature per day and was calculated for each grid cell for the years 1995-2014.
Stratification duration was derived from the NOAA Great Lakes CoastWatch surface temperature product (see Water Temperature below) for the years 1995-2013. Based on Fahnenstiel et al. (2010) recommending mid-year stratification occurring when the surface temperature is greater than or equal to 15 degrees Celsius. Using this threshold, the number of days a given pixel was at our above the threshold was tallied to calculate stratification duration in days for the years 1995-2013
The U.S. Environmental Protection Agency (EPA) Great Lakes National Program Office (GLNPO) samples stations on the Great Lakes multiple times each year since the early 1990s. Part of their sampling effort includes determining the location of the thermocline during the summer (August/September) sampling effort. We have extracted the depth to thermocline for each year by GLNPO sampling station.
Averaged surface water temperature annual, monthly, spring, and summer estimates of surface water temperature were calculated from remote sensing estimated daily data (NOAA Great Lakes CoastWatch) from 1995 to 2013. NOAA’s Great Lakes Coastal Forecasting System hydrodynamic model calculates vertical water temperature using a sigma coordinate system. We summarized these vertical values into 3 depth bins representing the epi- (0-20 m), meta- (20-40 m), and hypo-limnion (> 40m) and provided average temperatures annually, monthly, and for spring and summer for the years 2006-2012.