In this study, temporal and spatial variability of ice cover in the Great Lakes are investigated using historical satellite measurements from 1973 to 2010. The seasonal cycle of ice cover was constructed for all the lakes, including Lake St. Clair. A unique feature found in the seasonal cycle is that the standard deviations (i.e., variability) of ice cover are larger than the climatological means for each lake. This indicates that Great Lakes ice cover experiences large variability in response to predominant natural climate forcing and has poor predictability. Spectral analysis shows that lake ice has both quasi-decadal and interannual periodicities of ~8 and ~4 yr. There was a significant downward trend in ice coverage from 1973 to the present for all of the lakes, with Lake Ontario having the largest, and Lakes Erie and St. Clair having the smallest. The translated total loss in lake ice over the entire 38-yr record varies from 37% in Lake St. Clair (least) to 88% in Lake Ontario (most). The total loss for overall Great Lakes ice coverage is 71%, while Lake Superior places second with a 79% loss. An empirical orthogonal function analysis indicates that a major response of ice cover to atmospheric forcing is in phase in all six lakes, accounting for 80.8% of the total variance. The second mode shows an out-of-phase spatial variability between the upper and lower lakes, accounting for 10.7% of the total variance. The regression of the first EOF-mode time series to sea level pressure, surface air temperature, and surface wind shows that lake ice mainly responds to the combined Arctic Oscillation and El Niño–Southern Oscillation patterns.

[1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice models are intercompared over the period 1970 through 1999. Models' monthly mean outputs are laterally integrated over two subdomains (Amerasian and Eurasian basins), then examined as functions of depth and time. Differences in such fields as averaged temperature and salinity arise from models' differences in parameterizations and numerical methods and from different domain sizes, with anomalies that develop at lower latitudes carried into the Arctic. A systematic deficiency is seen as AOMIP models tend to produce thermally stratified upper layers rather than the ''cold halocline'', suggesting missing physics perhaps related to vertical mixing or to shelf-basin exchanges. Flow fields pose a challenge for intercomparison. We introduce topostrophy, the vertical component of VÂr r r rD where V is monthly mean velocity and r r r rD is the gradient of total depth, characterizing the tendency to follow topographic slopes. Positive topostrophy expresses a tendency for cyclonic ''rim currents''. Systematic differences of models' circulations are found to depend strongly upon assumed roles of unresolved eddies.

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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 Niñ o 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 , 2-3uC 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.