LAKE SUPERIOR CLIMATOLOGY
Sustained Assessment of the Great Lakes
Lake Superior Overview
Located along the U.S. Canadian border, Lake Superior is the northernmost of the five Great Lakes, connecting to Lake Huron through the St. Mary River. Lake Superior is the largest lake in the system by volume and surface area and is surrounded by several wildlife areas and national parks. Lake Superior is also the deepest of the Great Lakes, based on its bathymetry, and includes inflows from the Long Lac and Ogoki water diversions.
|Surface Area||Volume||Average Depth||Max Depth||Shoreline|
|82,097 sq. km||12,232 cu. km||149 m||406 m||4393 km|
|(31,700 sq. mi)||(2,935 cu. mi)||(489 ft)||(1,333 ft)||(2,730 mi)|
Variability of water levels in Lake Superior is observed on many different time spans, including seasonal, monthly, annual, and decadal (Figure 1). The Long Lac diversion was completed in 1941 and the Ogoki diversion was completed in 1943. Both divert water from the Albany River systems in Northern Ontario into Lake Superior for hydropower and logging purposes. Both diversions increased the water supply into the Great Lakes basin and permanently increased the water level of Lake Superior by 3 inches (USACE 2009).
Recent years have seen some of the most rapid fluctuations in the recorded history of Great Lakes water levels. Lake Superior had a sharp decline in water levels beginning in 1998. Following this decline, the lakes were characterized by an approximately 17 year span of warmer temperatures, low ice coverage, increased evaporation rates, and decreased runoff. Though this coincided with the 1998 El Niño, a causal relationship is not established. A rapid increase in Lake Superior water levels began in 2014, a year that coincided with a cold air outbreak, low temperatures, extensive ice cover, and high precipitation rates. These conditions continued through the end of the decade to reach record highs on several lakes in 2019.
Lake levels oscillate on annual cycles and multi-year periods of high and lows (Figure 2). The periods of sustained highs and lows are variable in length. The Great Lakes, including Lake Superior, have recently experienced a period of rising water levels, which follows a more than ten-year span of low levels. The past decade was the wettest on record and this record precipitation is the primary contributor to the recent rise in lake levels.
The variability of Lake Superior water levels is also observed on a decadal time scale (Figure 3). The current decade (2010s) of higher water levels occurred after a period of low lake levels (2000s).
Water levels fluctuate on an annual cycle, rising in the spring and summer due primarily to snowmelt runoff and low evaporation rates, and declining in the fall due to high evaporation rates from the temperature difference between the air (cold) and water (still warm from summer months). On average, the highest lake levels are observed between July and October (Figure 4).
The record lows on Lake Superior were set in the late 1920s, with the exception of August and September records that were broken in 2007 (Table 1). The record highs on Lake Superior were previously set in the 1980s, until 2019, when water level records were broken for the months of May to September.
Table 1: Mean, maximum, and minimum water levels in Lake Superior (Source: Army Corps of Engineers)
Net Basin Supply:
Water levels for an individual lake are driven by the net basin supply (NBS), which depends on the sum of over-lake precipitation and basin-runoff minus over-lake evaporation, as well as how much water flows from one lake to the next. Human-caused diversions, into and out of the lake, are also part of the NBS, but are much smaller than precipitation, runoff, and evaporation (see Lake Levels Overview page for more information).
Over-lake precipitation varies seasonally, with higher totals occurring in the late spring, summer, and early fall months (Figure 5). This seasonality aligns with the seasonal variability of lake levels, with the highest levels occurring at the same time as the greatest total precipitation. Precipitation totals vary annually as well as seasonally, and do not always follow the same pattern year to year.
Runoff rates follow the same general monthly pattern, but can vary in magnitude from year to year (Figure 6). Peak runoff occurs in late spring and early summer, primarily due to snowpack melt. Runoff amounts are also influenced by impervious surfaces such as concrete, that prevent water from entering the soil below, and agricultural practices within the basin.
Evaporation rates follow the same general monthly pattern from year to year. Maximum evaporation occurs in the late fall and winter months with minimum evaporation occurring in the late spring and early summer months (Figure 7). Because water has a higher heat capacity than air, the lake stays warmer in the fall as air temperature falls. This leads to increased evaporation due to the greater difference in temperature between the air and water. Evaporation is also closely related to high and low ice years. When evaporation rates are high in the fall, signifying more heat loss, it creates conditions for a year with high ice cover. Cold lake surface temperatures and high ice cover then reduce evaporation. The same is true for lower evaporation rates in the fall, signifying less heat loss, and low ice cover years. These conditions can then lead to increased evaporation in the following months (see Ice Cover Overview page for more information).
Precipitation, evaporation, and runoff are combined in total net basin supply for Lake Superior (Figure 8).
The variability of over-lake precipitation, evaporation, runoff, and total net basin supply can also be observed on a decadal time scale (Figures 9, 10, and 11). The most recent decade (2010s) had the most precipitation on record, which contributed to the increase in water levels that was seen throughout the Great Lakes, including Lake Superior. High runoff totals were also observed, primarily due to the high precipitation totals that have been recorded in the 2010s. NBS is the combination of precipitation, runoff, and evaporation, modeled by the equation: NBS = P + R – E (Figure 12).
There are multiple factors that lead to the differing levels of ice cover between the different lakes including depth, surface area, and latitude. Lake Superior is the northernmost of all the Great Lakes, exposing it to colder temperatures. However, it is also very deep, which allows a higher capacity for heat storage (Figure 13). The large surface area of the lake allows for loss of this stored heat, and therefore more favorable conditions for the formation of ice.
During the winter months, Lake Superior experiences varying levels of ice coverage. On average, higher ice coverage is observed closer to the coasts and channels due to more shallow water and protection from winds and currents (Figure 14). Below average ice cover was observed during most years in the past decade for most of the lake surface area. Though there were individual years with high ice cover, such as 2014 and 2019, the overall decadal average was low.
When considering annual maximum or average ice cover in the Great Lakes, it is common to treat each “ice year” as the period between December (of the previous year) and May (of the current year), as this is when freezing events occur in the region. Figure 15 demonstrates this, with peak ice cover between February and March almost no ice cover in the beginning of December and the middle of May.
There is variability in Lake Superior ice cover throughout the ice year as well as between different years. Natural modes of climate variability can influence ice coverage. Years with strong El Niño Southern Oscillation (ENSO) events contribute to lower maximum ice coverage, while years with polar vortex intrusions generally lead to higher maximum ice coverage (Figure 16).
Lake Surface Temperature
Surface temperatures of Lake Superior follow a monthly pattern with the warmest temperatures occurring in late summer and early fall and the coolest temperatures occurring in late winter and early spring (Figure 17). Overall, the surface temperature of Lake Superior has increased. Between 1979 and 2006, an increase of 4.5° fahrenheit was observed in summer surface temperature (would need reference for this if we keep it). This increase is twice that of the air temperature in the same period. 1
Basin Air Temperature
Despite year to year variability, an overall warming trend in air temperature has been observed in the Lake Superior basin from 1948 to 2014 (Figure 18).
Despite an overall warming trend, very warm or very cold years still occur due to annual variability (Figure 19). There are extreme high and low average temperatures throughout the period of record, but recent average temperatures are generally higher than in the past.