Friday, October 20, 2017
 
What does “normal” mean anyway?

What does “normal” mean anyway?

A closer look at the changes in Lake Michigan’s surface water temperature, ice cover, and water levels

by Caroline Mosley, NOAA Research Communications 

In the Great Lakes region, memories of the brutal winter of 2013-2014 are still fresh in residents’ minds. That winter brought very cold surface water temperatures and high ice cover well into the 2014 spring. Coupled with a record-setting water level surge of nearly three feet between January 2013 and December 2014, people who live along the shore of Lake Michigan have been wondering whether this is the “new normal” for the lake. A new study by NOAA’s Office of Oceanic and Atmospheric Research Great Lakes Environmental Research Laboratory (GLERL) investigates whether the extreme cold and ice of the 2013-2014 winter signals a regime shift in Lake Michigan. 

The study, published in the Geophysical Research Letters in early May, examines how the 2013-2014 winter may signal a return to a period of cooler lake temperatures, more extensive ice cover, and higher water levels than anything seen in Lake Michigan for the past fifteen years.
Ice Cover on Lake Michigan

Ice Cover on Lake Michigan

Ice cover on Lake Michigan. March 10, 2014. (NOAA Great Lakes CoastWatch)

The Great Lakes are no stranger to water level fluctuations. Compared to marine coasts, the inland seas have a wider range of interannual water level changes. Lake Michigan has a historical annual average water level range of around six and half feet. These ebbs and flows are driven by shifts in climate, as well the amount of rain, run-off from the surrounding watershed, and evaporation over the surface of the lake.

But since the late 1990s, water levels have been especially low for reasons scientists don’t entirely understand. These low water levels have affected everything from commercial shipping and recreational boating to tourism and shoreline property. Lake carriers transporting iron ore, coal, grain, and other commodities lightened their loads while millions of dollars were spent dredging shipping canals and ports.

In 2013-2014, a wavy polar vortex allowed frigid Arctic winds into the central and eastern United States, and the Great Lakes region endured some of the coldest temperatures and highest ice cover in recent history. These frigid conditions coincided with a sudden rise in water levels, especially in Lakes Superior, Michigan, and Huron, releasing stress on the shipping industry and shoreline property owners.
Annual Water Levels, 1840-2015

Annual Water Levels, 1840-2015

Hydrograph for Lakes Michigan and Huron shows annual average water levels (black) from 1860 through 2014. Monthly levels for the beginning of 2015 (January through April) are shown in blue. (GLERL)
 

However, these sudden fluctuations spurred questions in minds of both Great Lakes communities and scientists alike. Higher water levels released some of the strain on the Great Lakes economy, but researchers at GLERL set out to investigate if the shifts seen in the Great Lakes were a signal of long-term change or just a short-term fluctuation. The study investigates interseasonal relationships in Lake Michigan’s water temperatures to better understand if the 2013-2014 winter is a signal that the period of low water levels is coming to an end.  

“We were bombarded by questions,” said Drew Gronewold, Ph.D., the lead author on the study. ”Everybody wanted to know if the cold winter and water level surge meant something in the long term.”

The issue is not just a matter of shipping, tourism, or aesthetics- understanding water levels is crucial to water resource management. The Great Lakes contain 18 percent of the world’s freshwater, supplying water to large cities such as Chicago and Detroit.

A challenge of working on the Great Lakes is getting enough measurements from the field to put into models. Gronewold and his colleagues focused on Lake Michigan because of a significant amount of data collected from buoys and field sampling. The field data was put into models in order to more closely examine interseasonal relationships in Lake Michigan’s thermal regime and shifts in surface water temperatures and water levels.

“We were bombarded by questions. Everybody wanted to know if the cold winter and water level surge meant something in the long term."

According to this study, in 1997-1998, a strong El Niño led to an abnormally mild winter in the Great Lakes region. That’s because increases in air and water temperatures from 1995 to 2001, usually associated with strong El Niño events, triggered a shift in Lake Michigan’s surface water temperatures. This shift was then sustained for over a decade through solar forcing, the amount of solar energy that is absorbed versus reflected back into space, a process influenced by greenhouse gases.

As the lake warmed, ice cover decreased and water levels dropped. Record-low water levels in Lake Michigan and Lake Huron were recorded in 2012 and 2013. However, by the end of 2014, water levels on Lake Michigan had finished a two-year, record-setting water level surge.

So does this mean that the winter of 2013-2014 has, indeed, set the stage for a "new normal" for the Great Lakes region?

The answer to that question, according to Gronewold, is “maybe.”

According to a main conclusion of the study, the researchers found “compelling evidence that one of Earth’s largest lakes was in an altered thermal regime for the past 15 years, marking a shift in thermal conditions that were relatively consistent before the late 1990s (and dating back to at least 1950).”

High waves near Chicago

High waves near Chicago

High waves in Lake Michigan along the Chicago shoreline. October 31, 2014. (L.S. Gerstner)

Gronewold goes on to explain how large-scale climate perturbations, like an El Niño and anomalous arctic polar vortex pattern, can act as driving forces behind regime shifts in the Great Lakes. These phenomena can cause the thermal (surface water temperatures) and hydrological (water levels) regimes in large freshwater bodies to swing back and forth.

Using field data and modelling, NOAA researchers can better understand possible scenarios of these regime swings. The lasting impacts of these patterns are often difficult to predict, and climate change only adds to the difficulty of anticipating possible future trends. Until these global circulation patterns can be better predicted, it is extremely difficult to determine a new “normal” for the Great Lakes region.

Gronewold believes that better understanding these large-scale climate patterns is the key to teasing apart short-term trends and long-term variability in the Great Lakes. Ultimately, a “normal” water level may very well not exist for Lake Michigan. But NOAA researchers will continue to study the thermal and hydrological regimes of the Great Lakes in order to better inform communities of the dynamic nature of the inland seas.

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