Modeling transitions in the hydrologic and thermal regimes of Earth's largest lake system
Abstract:Starting in the late 1990s, the seasonal hydrologic and thermal regimes of Earth's largest lake system have been characterized by very high surface water temperatures, below-average ice cover, persistent low water levels and extremely high over-lake evaporation rates. However, the harsh winter conditions of 2013-2014 led to very low surface water temperatures and an exceptionally broad and persistent areal extent of ice cover. The contrast between the extreme 2013-2014 winter conditions on the Great Lakes and the conditions from the preceding 15-year period raises compelling questions about the extent to which hydrometeorological conditions have changed in the Great Lakes region, how they might be expected to change in the future, and to what extent those changes are reflected in currrent regional research-oriented and operational forecasts.
Here, we analyze historical intra-seasonal relationships between late winter and subsequent late fall thermal regimes on the Great Lakes and find that, for some of the lakes, memory of seasonal heat content is strong, but can be significantly impacted by hydrometeorological conditions including wind speed and solar radiation. In fact, we find that the late 1990s, a period coinciding with one of the strongest El Ninos on record, represent a shift in the hydrologic and thermal regimes of the Great Lakes, and that projections for the fall of 2014 suggest that the regime might be offset by the recent cold winter. Our findings also provide evidence that the transition in the Great Lakes' altered thermal regime in the late 1990s was triggered by abrupt increases not only in air and water temperatures associated with the coincident El Nino, but also by a combination of reduced cloud cover and above-average summer solar radiation. The extent to which these changes are explicitly represented in regional forecasting systems is critically important to regional water resource management planning.