Great Lakes ice rhythms

Its hull a small black spot defiant against the surrounding field of white, the Coast Guard’s 140-foot Mobile Bay has cut through ice encumbering the waters of Green Bay each winter for three decades.

Icebreaking is such a massive task, the Coast Guard calls it Operation Taconite-keeping the way clear for commercial vessels in Lakes Michigan and Superior, the St. Mary’s River, and the Straits of Mackinac.

But aside from the Coast Guard-plus the occasional polar bear plunger or diehard ice fishermen-few humans pay close attention to the complex rhythms of lake ice building and receding that play out across the Great Lakes annually.

George Leshkevich, however, a scientist at the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory (GLERL) in Ann Arbor, Mich., has been studying lake ice for several decades.

According to Leshkevich, differences in the lakes’ depths and regional air temperatures affect their potential to develop ice cover. Lake Erie, for example, is the shallowest of the lakes and typically sees the most extensive ice cover each winter, relative to its surface area. Lake Superior, farthest north and subjected to colder temperatures, is next on the list, followed by Lakes Huron, Michigan, and Ontario.

The extent of ice cover on the Great Lakes can also vary greatly from year to year, Leshkevich said.

For example, between 1963 and 2001, the maximum extent of ice cover on Lake Michigan over the course of a single winter ranged from 13 percent (in 1963-64) to 96 percent (in 1978-79) of the lake’s surface, according to one study. The average over that same period was 40 percent.

Along with such year-to-year fluctuations, the Great Lakes have seen an overall downward trend in winter ice cover over the last 30 to 40 years, said Leshkevich. That decline has coincided with an overall increase in global air temperatures.

Both natural variability and human-induced climate change can influence Great Lakes ice cover, said Leshkevich and fellow GLERL scientist Jia Wang. However, the more dominant influence is natural variability, which drives the year-to-year fluctuations in ice cover, Wang said.

“Our region is very complex because it is affected by two [natural] climate patterns,” said Wang. One pattern comes from the Pacific and brings either warmer “El Niño” or colder “La Niña” temperatures to the Great Lakes region. A second pattern that comes from the Arctic likewise affects the region’s temperatures with its warmer “positive” and colder “negative” phases.

The Great Lakes generally have lower ice cover in years when both the Pacific and Arctic patterns are warm, and higher ice cover in years when both patterns are cold, Wang said. When the patterns differ, they either moderate each other’s effects or the stronger pattern dominates. Last winter, the patterns combined to bring to the Great Lakes one of the more extensive ice covers of the past decade, a period during which ice cover was generally low.

Regional & Local Effects

Lake ice affects both the environment and the economy of the Great Lakes. Ice is good at reflecting sunlight, so its presence decreases the ability of the lakes to absorb heat and keeps them colder, said Jay Austin, a scientist at the University of Minnesota-Duluth’s Large Lakes Observatory.

What’s more, said Austin, “the effect of ice is felt long after the ice is gone. The amount of ice during the winter can significantly affect lake temperatures throughout the summer.” His research shows that summer surface water temperatures of the northern Great Lakes-Michigan, Superior, and Huron-generally are warmer after winters with low ice cover, and colder after winters with high ice cover. He believes this is because ice cover delays the springtime stratification of the lake into layers of different densities, a phenomenon that promotes rapid warming of the surface layer in the summer.

Everything from algae to fish could be impacted by changes in lake temperatures and stratification that result from long-term changes in ice cover, according to Austin’s study.

Ice affects other aspects of the environment, too. In shallow areas, it can help protect the eggs of whitefish and other fall-spawning fish from currents and waves generated by strong winter winds. Ice cover also inhibits evaporation by shielding the lakes from dry, cold winter air. As a result, lake levels are often lower after winters with low ice cover, and higher after winters with high ice cover, said Wang.

Ice has implications for Great Lakes industries such as power production and shipping. Ice jams that form on the rivers connecting the lakes can constrict water flow and result in less water for downstream hydropower plants. Heavy ice can delay the opening of the shipping season and be a hazard to navigation, but it also can lead to higher water levels, allowing ships to carry more cargo.

Given ice’s environmental and economic importance to the Great Lakes region, scientists have good reason to continue studying it. In the future, Leshkevich plans to study ice cover thickness, and Austin is interested in developing numerical models that better explain ice’s connections to other phenomena. Wang is currently working on a model for forecasting ice cover.

Jennifer Yauck is a science writer at the UWM Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.

Forms of Great Lake Ice

• Pancake ice - circular, flat pieces of ice with turned-up edges that are shaped by wind- and wave-driven collisions with one another
• Brash ice - angular pieces of broken ice often piled on each other by wind and waves
• Lake/black ice - clear ice that looks dark when viewed from an aircraft or satellite
• Snow ice - milky white ice containing many bubbles, formed from water-soaked snow
• Frazil ice - fine spicules or plates of ice suspended in the water, formed during the first stage of freezing
• Source: George Leshkevich