The Laurentian Great Lakes represent the largest freshwater basin on Earth, containing 21% of the world's surface fresh water by volume. Water level fluctuations are an on-going concern and have received considerable attention in the area. We present a trend analysis of meteorological (air temperature, cloud cover, and wind speed) and hydrological (precipitation, runoff, and evaporation) variables for Lake Michigan, Lake Huron, and Georgian Bay. Using the non-parametric Mann-Kendall test, our analysis identified significant upward trends in daily minimum air temperature, whereas daily maximum air temperature demonstrated weakly decreasing trends in space and time. Evaporation was found to be increasing from late spring until early fall and this pattern may be explained by the shortening of the ice/snow cover period, which results in faster warming of lake surface due to the induced variations in albedo feedback. Time-series analysis of the over-lake precipitation revealed mostly non-significant statistical trends. Recent temperature increases may have led to elevated winter runoff in the Great Lakes region, given that precipitation falls mainly as rain instead of snow. We also provide clear evidence of reduced cloud cover and wind speed. Our study offers critical insights into the patterns of within- and among-year variability of hydro-meteorological variables useful in elucidating the mechanisms that modulate water levels in the Great Lakes.

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