Thomas E. Croley

Regional Climate Change Projections for Chicago and the US Great Lakes

ABSTRACT Assessing regional impacts of climate change begins with development of climate projections at relevant temporal and spatial scales. Here, proven statistical downscaling methods are applied to relatively coarse-scale atmosphere—ocean general circulation model (AOGCM) output to improve the simulation and resolution of spatial and temporal variability in temperature and precipitation across the US Great Lakes region. The absolute magnitude of change expected over the coming century depends on the sensitivity of the climate system to human forcing and on the trajectory of anthropogenic greenhouse gas emissions. Annual temperatures in the region are projected to increase 1.4 ± 0.6 °C over the near-term (2010–2039), by 2.0 ± 0.7 °C under lower and 3 ± 1 °C under higher emissions by midcentury (2040–2069), and by 3 ± 1 °C under lower and 5.0 ± 1.2°C under higher emissions by end-of-century (2070–2099), relative to the historical reference period 1961–1990. Simulations also highlight seasonal and geographical differences in warming, consistent with recent trends. Increases in winter and spring precipitation of up to 20% under lower and 30% under higher emissions are projected by end-of-century, while projections for summer and fall remain inconsistent. Competing effects of shifting precipitation and warmer temperatures suggest little change in Great Lake levels over much of the century until the end of the century, when net decreases are expected under higher emissions. Overall, these projections suggest the potential for considerable changes to climate in the US Great Lakes region; changes that could be mitigated by reducing global emissions to follow a lower as opposed to a higher emissions trajectory over the coming century.

Recruitment Variability of Alewives in Lake Michigan

We used a long-term series of observations on alewife Alosa pseudoharengusabun- dance that was based on fall bottom-trawl catches to assess the importance of various abiotic and biotic factors on alewife recruitment in Lake Michigan during 1962-2002. We first fit a basic Ricker spawner-recruit model to the lakewide biomass estimates of age-3 recruits and the cor- responding spawning stock size; we then fit models for all possible combinations of the following four external variables added to the basic model: an index of salmonine predation on an alewife year-class, an index for the spring-summer water temperatures experienced by alewives during their first year in the lake, an index of the severity of the first winter experienced by alewives in the lake, and an index of lake productivity during an alewife year-classs second year in the lake. Based on an information criterion, the best model for alewife recruitment included indices of salmonine predation and spring-summer water temperatures as external variables. Our analysis corroborated the contention that a decline in alewife abundance during the 1970s and early 1980s in Lake Michigan was driven by salmonine predation. Furthermore, our findings indicated that the extraordinarily warm water temperatures during the spring and summer of 1998 probably led to a moderately high recruitment of age-3 alewives in 2001, despite abundant salmonines. A key problem in fisheries research is predicting recruitment from a given level of spawning stock size (Sissenwine et al. 1988; Myers et al. 2001; Kehler et al. 2002). Fish recruitment can be strong- ly influenced by many abiotic and biotic factors, including water temperature, water movements, predation, and spawning stock size (Sissenwine 1984; Hilborn and Walters 1992). Although im- portant factors affecting recruitment may vary across ecosystems (Madenjian et al. 1996), inter- esting patterns may emerge by comparing recruit- ment analyses for populations of a species across ecosystems (Myers 1998). An invasion of alewives Alosa pseudoharengus during the 1940s proved to be an important stressor to the Lake Michigan ecosystem (Wells and

Lake Erie hypoxia prompts Canada‐U.S. study

Because of its size and geometry, the central basin of Lake Erie, one of North Americas Great Lakes, is subject to periods in the late summer when dissolved oxygen concentrations are low (hypoxia). An apparent increase in the occurrence of these eutrophic conditions and ‘dead zones’ in recent years has led to increased public concern. The International Field Years for Lake Erie (IFYLE) project of the Great Lakes Environmental Research Laboratory (GLERL, a U.S. National Oceanic and Atmospheric Administration (NOAA) laboratory), was established in 2005 in response to this increase. This project is investigating the causes and consequences of hypoxia in the lake. As part of the effort, scientists from the United States and Canada conducted an extensive field study in 2005 to gather more information on the duration and extent of the hypoxic zone and its effects on the biota in the lake. This article gives a brief history and description of the problem and presents initial results from the field study.

Resolving Thiessen polygons

Abstract We have developed an automated computer forecast package for Lake Superior, North America, that uses near real-time meteorological data to produce operational outlooks of basin runoff for improving lake-level regulations. The data collection network changes frequently as stations are added or dropped or fail to report from time to time. Therefore, forecasts depend on semi-automatic updating of meteorological data, requiring efficient computations of Thiessen weights. Various methods of computing Thiessen weights either have large computational overheads or provide unacceptable approximations. A new algorithm is presented for quickly computing Thiessen weights for all stations in a collection network for each of several watersheds of interest. The algorithm determines weights by finding first the edges of the Thiessen polygons and then the intersections of the polygons with watershed areas. It makes use of the fact that Thiessen polygons are convex sets of points. Considerable computational savings result by defining polygons by their edges instead of by their areal extent. Although comparisons of methods depend on particular network configurations, an example application to 18 stations covering 22 watersheds, represented on a 760 × 516-km map at 1-km2 resolution (392,160 cells), requires only 10.0 CPU-seconds on a VAX® 11 780 minicomputer (2 CPU-seconds on a CDC® 750). This represents a 93% savings in computational time with no loss of accuracy when compared to conventional computer methods.

A one‐dimensional ice thermodynamics model for the Laurentian Great Lakes

Great Lakes hydrologic research requires the use of continuous-simulation daily ice cover models over long time periods in the absence of field observations. They must be physically based, rather than statistically based, for use under conditions different than those under which they were derived. But they also must match existing conditions for which data exist. A review discloses that existing ice dynamics models do not meet all of these criteria; a new one that does is based here on a prismatic ice pack heat balance, ice growth and temperature constraints, and thermodynamic flux terms from companion water heat balance and storage equations. The prismatic ice model is a good first step to understanding complex geometries and is supportable through the use of lake-averaged energy fluxes. The ice model is integrated into an existing lake thermodynamics and one-dimensional heat storage model, and the resulting combination is calibrated for Laurentian Great Lakes applications. Simulation experiments are used to analyze the models strengths and limitations and to explore its relevance. Comparisons between model output and existing data allow consideration of the ice climatology of the Great Lakes; the climatology description is extended through use of the new model. Promising potential model extensions include spatial extension, additional parameterizations for wind-ice movement, snow, and albedo, and inclusions of remotely sensed data.

Great Lakes Hydrology Under Transposed Climates

Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrological response of the Great Lakes to warmer climates. The Great Lakes Environmental Research Laboratory used their conceptual models for simulating moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and timing within the existing climate; this is not true for General Circulation Model-generated corrections applied to existing historical data in many other impact studies. The transposed climates lead to higher and more variable over-land evapotranspiration and lower soil moisture and runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident in the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spreads more throughout the annual cycle. The response of runoff to temperature and precipitation changes is coherent among the lakes and varies quasi-linearly over a wide range of temperature changes, some well beyond the range of current GCM predictions for doubled CO2 conditions.

Long‐term heat storage in the Great Lakes

Practical estimation of long-term daily Great Lakes evaporation requires one-dimensional (depth) models of heat storage and mixing. Conceptual models are preferable to physical models for small-computer simulations that are multiple, continuous, and long. This paper describes a new conceptual superposition model of heat storage to extend an existing evaporation model along the lake depth. The resulting daily model is recalibrated to remotely sensed surface water temperatures and is used to illustrate anew seasonal heating and cooling cycles, heat-temperature hysteresis, water column turnovers, and mixed-layer developments. It is used as well to compare the vertical distribution of temperatures with independent bathythermograph data. The time occurrence structure of evaporation on the Great Lakes is investigated, and the effects of summertime initial conditions on subsequent wintertime behavior of evaporation are simulated. Impacts of perceived large-lake thermodynamic behavior are analyzed, and suggestions are made for further research.

Dry Climate Disconnected the Laurentian Great Lakes

Recent studies have produced a new understanding of the hydrological history of North Americas Great Lakes, showing that water levels fell several meters below lake basin outlets during an early postglacial dry climate in the Holocene (younger than 10,000 radiocarbon years, or about 11,500 calibrated or calendar years before present (B.P.)). Water levels in the Huron basin, for example, fell more than 20 meters below the basin overflow outlet between about 7900 and 7500 radiocarbon (about 8770–8290 calibrated) years B.P. Outlet rivers, including the Niagara River, presently falling 99 meters from Lake Erie to Lake Ontario (and hence Niagara Falls), ran dry. This newly recognized phase of low lake levels in a dry climate provides a case study for evaluating the sensitivity of the Great Lakes to current and future climate change.

Great Lake basins (U.S.A.-Canada) runoff modeling

Abstract Large-scale watershed models are required in order to estimate basin runoff to the Great Lakes for use in routing determinations and operational hydrology studies. Data limitations, large-basin applicability and economic efficiency preclude the use of existing large-watershed models. This paper describes an interdependent tank-cascade model that uses a mass balance coupled with linear reservoir concepts. It is physically based and uses climatological considerations not possible for small watersheds; it employs analytical solutions to bypass numerical inaccuracies. Snowmelt and net-supply computations are separable from the mass-balance determinations and are based on a simple heat balance. Partial-area concepts are used to determine infiltration and surface runoff. Losses are determined from joint consideration of available energy for evapotranspiration and of available moisture in the soil horizons by using climatologic concepts. Also described are heuristic calibration procedures that give insight into the use of the model. The model is applied, for a 30-day computation interval, to the Genesee River Basin in New York State and compared with past 6-hr. computation interval applications of the Streamflow Synthesis and Reservoir Regulation (SSARR) and National Weather Service Hydrologic (NWSH) models to the same data set.

Warmer and Drier Climates that Make Terminal Great Lakes

ABSTRACT A recent empirical model of glacial-isostatic uplift showed that the Huron and Michigan lake level fell tens of meters below the lowest possible outlet about 7,900 14C years BP when the upper Great Lakes became dependent for water supply on precipitation alone, as at present. The upper Great Lakes thus appear to have been impacted by severe dry climate that may have also affected the lower Great Lakes. While continuing paleoclimate studies are corroborating and quantifying this impacting climate and other evidence of terminal lakes, the Great Lakes Environmental Research Laboratory applied their Advanced Hydrologic Prediction System, modified to use dynamic lake areas, to explore the deviations from present temperatures and precipitation that would force the Great Lakes to become terminal (closed), i.e., for water levels to fall below outlet sills. We modeled the present lakes with pre-development natural outlet and water flow conditions, but considered the upper and lower Great Lakes separately with no river connection, as in the early Holocene basin configuration. By using systematic shifts in precipitation, temperature, and humidity relative to the present base climate, we identified candidate climates that result in terminal lakes. The lakes would close in the order: Erie, Superior, Michigan-Huron, and Ontario for increasingly drier and warmer climates. For a temperature rise of T°C and a precipitation drop of P% relative to the present base climate, conditions for complete lake closure range from 4.7T + P > 51 for Erie to 3.5T + P > 71 for Ontario.