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NOAA Technical Memorandum GLERL-170

____________________________________________________________________________

Great Lakes Ice Climatology Update of 

Winters 2012-2017: Seasonal Cycle, 

Interannual Variability, Decadal 

Variability, and Trend for the period 

1973-2017

Jia Wang1, James Kessler2, Franky Hang3, Haoguo Hu2, Anne H. Clites1, and Philip Chu1

1 NOAA Great Lakes Environmental Research Laboratory

2 Cooperative Institute for Great Lake Lakes Research, University of Michigan

3 School of Engineering, University of Michigan

Great Lakes Environmental Research Laboratory

4840 S. State Road, Ann Arbor, Michigan

Tuesday, December 12, 2017

____________________________________________________________________________

UNITED STATES NATIONAL OCEANIC AND

DEPARTMENT OF COMMERCE ATMOSPHERIC ADMINISTRATION

Wilbur L. Ross, Jr. Benjamin Friedman

Secretary Acting Administrator
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NOTICE

Mention of a commercial company or product does not constitute an endorsement

by the NOAA. Use of information from this publication concerning proprietary

products or the tests of such products for publicity or advertising purposes is not

authorized. This is GLERL Contribution No. 1870 and CIGLR Contribution No. 1119.

This publication is available as a PDF file and can be downloaded from GLERL’s

web site: www.glerl.noaa.gov or by emailing GLERL, Information Services at

oar.pubs.glerl@noaa.gov.
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TABLE OF CONTENTS

INTRODUCTION ..................................................................................................................... 4

DISTRIBUTION OF ICE CHARTS BY WINTER SEASON .................................................. 4

ANALYSIS OF UPDATED GREAT LAKES ICE COVER: 1973-2017 ................................ 5

Season Cycle ............................................................................................................ 5

Seasonal Variations .................................................................................................. 7

Interannual Variability ............................................................................................. 7

Long-term Trends .................................................................................................... 11

Decadal Variability .................................................................................................. 13

CONCLUSION .......................................................................................................................... 13

ACKNOWLEDGEMENTS ....................................................................................................... 14

REFERENCES .......................................................................................................................... 14

LIST OF FIGURES

Figure 1. Long-term Mean Annual Maximum Ice Cover in the Great Lakes ........................... 5

Figure 2. Biweekly Climatology of Ice Cover in the Great Lakes (1973-2017 Winter Seasons)5

Figure 3. Climatological maps for monthly AMIC in all five Great Lakes for December, 

January, February, March, and April for the period 1973-2017 .................................... 6

Figure 4. Seasonal variation of Weekly Ice Cover .................................................................... 8

Figure 5. Time Series of Ice Cover in the Great Lakes, 1973-2017 .......................................... 9

Figure 6. Annual Average Ice Cover (AAIC) and Trend in the Great Lakes, 1973-2017.......... 10

Figure 7. Time series of AMIC during 1973-2017. The thick line is the 5-year running mean, 

indicating the decadal signal. The dashed red line is the climatological mean of 55%, 

and the blue dashed line indicates the long-term trend .................................................. 13

LIST OF TABLES

Table 1. Summary of Ice Chart Statistics .................................................................................. 4

Table 2. Statistics of Seasonal Cycle Dates (Week Number) .................................................... 7

Table 3. Trends in Great Lakes Annual Mean Ice Cover Averaged Over Dec. 1 to April 30 ... 11

Table 4. Statistical means and standard deviations of annual average ice cover in individual 

lakes and entire Great Lakes (basin), averaged from December 1 to April 30, for the 

period1973-2017 ............................................................................................................ 11

Table 5. Statistical means and standard deviations of annual maximum ice cover (AMIC) in 

individual lakes and entire Great Lakes (basin) at one day during December 1 to April 

30, for the period 1973-2017 ......................................................................................... 13

Table 6. Regression linear trend of AMIC for the period 1973-2015, x=a+bt in percentage (t in 

years starting with 0 at 1973, a is constant, and b is the trend/slope) ............................ 13

Table 7. The same as Table 6 (for the period 1973-2013), except excluding 2014 and 2015 ice 

seasons ........................................................................................................................... 13
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Great Lakes Ice Database Update of Winters 2012-2017:

Seasonal Cycle, Interannual Variability, Decadal Variability, and Trend for 

the period 1973-2017

INTRODUCTION

The 6-winter (2012-2017) digital ice cover data set includes 996 ice charts of total ice concentration, 

which were added to GLERL’s ice dataset. This report updates GLERL’s 45-winter ice climatology 

dataset, 1973-2017. The temporal distribution of the ice charts is also summarized. The original ice charts 

were produced by the U.S. National Ice Center (NIC) and downloaded from their website as ASCII files, 

http://www.natice.noaa.gov/products/great_lakes.html. The data format, quality control, processing using 

ArcGIS, and availability are the same as sections 2-6 of Wang et al. (2012a)

Since the last updated ice dataset (Wang et al. 2012a), there has been significant change in ice 

cover on the Great Lakes. This update of the ice dataset should be useful to the research 

community as well as others who rely on Great Lakes ice data for planning purposes. The

technical memorandum that updated the ice atlas for winters 2006-2011 (Assel et al., 2013; Wang et al. 

2012a) includes ArcGIS feature class files.

Lake ice cover is a sensitive indicator of regional climate and climate change (Assel et al (1995); Assel 

and Robertson (1995) ; Magnuson et al (1997,2000). Seasonal ice cover repeats each year with large 

interannual variability. For example, the maximum ice coverage over all of the Great Lakes was 95% in 

1979 and only 11% in 2002 (Bai et al. 2012). Recently, 2014 and 2015 winter ice cover in the Great 

Lakes reached 92% and 88%, respectively, while ice cover in winters 2016 and 2017 experienced very 

low ice cover, 34% and 19%, respectively. Possible contributors include interannual and interdecadal 

climate variability, and long-term trends (Assel 1990; Wang et al. 2017), some of which may be related to 

global climate warming. Even in response to the same climate forcing, Great Lakes ice cover may 

experience different spatial and temporal variability due to an individual lake’s orientation, depth (i.e., 

water heat storage, Assel et al. 2003), and turbidity (i.e., albedo due to sedimentation). 

DISTRIBUTION OF ICE CHARTS BY WINTER SEASON

The number of ice charts per winter season for the years 2012 to 2017 range from 136 to 195; the dates of 

first ice charts range from the middle of November to the beginning of December and the last ice charts 

range from the middle of April to the beginning of June. Since the winter season of 2011, the NIC has

been producing daily charts for each winter season allowing for daily ice chart analysis.

Table 1. Summary of Ice Chart Statistics

Winter Season Number of Ice Charts Date of First Ice 

Chart Date of Last Ice Chart

2012 136 November 29th April 13th

2013 177 November 29th May 28th

2014 192 November 24th June 5th

2015 195 November 14th May 29th

2016 160 November 27th May 5th

2017 136 December 11th April 26th
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ANALYSIS OF UPDATED GREAT LAKES ICE COVER: 1973-2017

Seasonal Cycle

The spatial, long-term mean annual maximum ice cover in the Great Lakes was constructed based on ice 

chart data from the winter seasons of 1973 to 2017 (Figure 1). The biweekly climatology of ice cover in 

the Great Lakes was also produced for all six individual lakes (Figure 2). Shallow lakes (St. Clair and 

Erie) are seen to have an earlier ice cycle; reaching maximum ice cover and ice break-up sooner than the 

deep lakes. The seasonal ice cycle in the Great Lakes consists of an ice onset, growing period in which 

the annual maximum ice cover area is reached and then an ice offset. The average growth of ice cover 

over time follows a similar pattern in all of the Great Lakes; ice onset begins in all of the lakes during 

early December and has a growing period that lasts anywhere between mid-February (Erie) to early 

March (Superior). In general, ice melts completely during late April to late May.

Figure 1. Long-term Mean Annual Maximum Ice Cover (AMIC) in the Great Lakes

Figure 2. Biweekly Climatology of Ice Cover in the Great Lakes (1973-2017 Winter Seasons)
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Figure 3. Climatological maps for monthly 

Annual Maximum Ice Cover (AMIC) in all five 

Great Lakes for December, January, February, 

March, and April for the period 1973-2017. 
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Figure 3 shows the spatial distribution of Great Lakes monthly AMIC for December to April average over 

the period 1973-2017. In November or December, ice starts to form along the shallow coast and 

embayments including western Lake Erie, and progresses into January, reaching the maximum ice cover 

in February in the southern lakes (Erie and Ontario) and in March in the northern Lakes (Superior and 

Huron). Ice gradually decays in April.

Seasonal Variations

To begin analyzing interannual variations in Great Lakes ice cover, the mean daily lake average ice cover 

for every measured date in the winter season is determined across the 45-winter period (Figure 4). On 

average, ice forms on Lake Superior in week 45, which is earlier than all the other lakes (Table 1). Iceonset occurs in week 46 on Lakes Michigan and Huron; and week 47 on Lakes Erie, Ontario, and St. 

Clair. The growing period of ice cover ranges from 11 to 16 weeks. Lakes Superior reaches the seasonal 

maximum area during week 9, Lakes Michigan, Huron, Ontario, and St. Clair during week 7 and Lake 

Erie during week 6. Ice cover begins breaking within a week or two after maximum ice cover is reached

and melts completely around week 20 for all lakes except Superior (week 21).

The corresponding weekly variations defined by standard deviations of annual ice cover indicates that 

Lakes St. Clair, Erie and Huron have the lowest ratio of standard deviations to the mean, and Lake 

Ontario has the highest ratio. The ratio is inversely proportional to predictability. This implies that ice 

cover in Lakes St. Clair, Erie and Huron have the highest predictability, while ice cover on Lake Ontario 

has the lowest predictability. The overall predictability in all of the Great Lakes is low since the

climatological standard deviations of lake ice cover are often of equal magnitude (and at times greater 

magnitude) than the corresponding climatological mean.

Based on Figure 4, Lake St. Clair is observed to have an apparent, local ice cover minimum during week 

5 and dual maxima during weeks 4 and 6. This large seasonal variability of lake ice may be a result of 

extreme weather events caused by natural, internal climate patterns (Wang et al., 2012a). 

Table 2. Statistics of Seasonal Cycle Dates (Week Number)

Superior Michigan Huron St. Clair Erie Ontario

Onset 45 46 46 47 47 47

Maximum 9 7 7 7 6 7

Break-up 10 8 10 8 7 9

Offset 21 20 20 20 20 20

Interannual Variability

Time series of weekly ice cover area for all six lakes have been constructed based on the ice concentration 

data for the winters of 1973 to 2017 (Fig. 5). All lakes are observed to have high interannual variability of 

ice cover. For shallow water lakes Erie and St. Clair, almost complete ice cover is achieved for all 45-

winter seasons (with the exception of 1983, 1991, 1998, 2002, 2006, 2012, 2013, 2016 and 2017 in Lake 

Erie and 1992, 2002, 2012, 2016 and 2017 in Lake St. Clair). This demonstrates that considering only 

lake ice cover may not be sufficient in detecting possible climate signals in shallow water lakes that are 

constrained by their bathymetry (Wang et al., 2012a).

Lake Erie experienced the least amount of ice cover during the 1983, 1991 and 1998 winter seasons 

(period of 7-8 years). Though this phenomenon is most apparent in Lake Erie, a similar result can be 

recognized across the other lakes as well. Furthermore, in recent years (starting in 1998), relatively low 

levels of ice cover have consistently occurred every 3-4 years. This might imply greater interannual 

variability of climate patterns in the Great Lakes within the past two decades compared to previous winter 

years. 
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Figure 4. Seasonal Variation of Weekly Ice Coverage

Time series plots for the ice cover annual mean in the Great Lakes are produced for each lake (Figure 6).

Each lake is observed to have large interannual variability over time. This may imply a uniform, major 

response of lake ice to atmospheric interannual variability of teleconnection patterns such as NAO and 

ENSO (Bai et al. 2012). 
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Figure 5. Time Series of Ice Cover in the Great Lakes, 1973-2017
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Figure 6. Annual Mean Ice Cover (AAIC) and Trend in the Great Lakes, 1973-2017.

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Table 5. Statistical means and standard deviations of annual maximum ice cover (AMIC) in individual 

lakes and entire Great Lakes (basin) at one day during December 1 to April 30, for the period 1973-2017.

AMIC Superior Michigan Huron Erie Ontario Basin

Mean 63 42 65 84 32 55

Std 30 22 23 25 21 24

Std/Mean 0.48 0.52 0.35 0.30 0.66 0.44

Table 6. Regression linear trend of AMIC for the period 1973-2015, x=a+bt in percentage (t in years 

starting with 0 at 1973, a is constant, and b is the trend/slope).

a b (annual trend) Decadal trend Total loss up 

to 2015

Superior 77.00 -0.90 -9.0 -38.70

Michigan 39.04 -0.39 -3.9 -16.77

Huron 66.87 -0.51 -5.1 -21.93

Erie 90.86 -0.48 -4.8 -20.64

Ontario 26.00 -0.36 -3.6 -15.48

Basin 62.00 -0.60 -6.0 -25.80

Table 7. The same as Table 6 (for the period 1973-2013), except excluding 2014 and 2015 ice seasons.

a b (annual trend) Decadal trend Total loss up to 

2013

Superior 79.73 -1.09 -10.9 -44.69

Michigan 50.57 -0.61 -6.1 -25.01

Huron 80.92 -0.66 -6.6 -27.06

Erie 97.82 -0.56 -5.6 -22.96

Ontario 43.80 -0.50 -5.0 -20.50

Basin 62.43 -0.79 -7.9 -32.39

Table 6 shows the regression analysis results of AMIC for five individual lakes and the entire basin. The 

trend was provided (column 4), and the total loss of lake ice at 2015, relative to 1973, is also provided 

(column 5). Lake Superior has the highest total loss among the lakes at -39%, partially due to the 

warming caused by the ice/water albedo feedback (Wang et al. 2005; Austin and Colman 2007). The 

basin-wide loss of ice cover from 1973 to 2015 is -26%.

Since the Great Lakes experienced extremely cold winters with severe ice cover in 2014 and 2015 (93% 

during 2013/14 (Clites et al. 2014) and 89% during 2014/15), it is obvious that these two winters will 

significantly change the trend. Table 7 shows the results, similar to Table 6, excluding these two winters. 

As expected, the trends of each lake for the period 1973-2013 (Table 7) are larger than those for the 

period 1973-2015 (Table 6).

Therefore, we must use trends cautiously when interpreting ice cover change. Again, the trend calculated 

within a specific period of time such as 1973-2015 (Table 6) can only be applicable to the same period, 

and cannot be extrapolated to the future and back to the past. It should not be interpolated to a period 

shorter than the time series of the data from which the trends are derived (e.g. Table 7), since there are 

decadal and multi-decadal changes in lake ice cover. 

Decadal Variability

With the last seven years (2011-2017) of data added to the previous time series (Wang et al. 2012a), 

decadal variability further stands out, as does the interannual variability (Fig. 7). The decadal variability 

in small lakes was investigated by previous studies (Magnuson et al. 2000; Ghanbari et al. 2009;

Weyhenmeyer et al. 2011). During this 45-year period, three high ice bands stand out: 1977-79, 1994, and 
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2014-15, with separation periods of 17 years and 21 years, respectively. Similarly three low ice bands 

also stand out: 1983, 1998-2002, and 2010-2013, separated by around 18 and 12 years, respectively. The 

longest low ice band started in 1998 and lasted to 2013. The 1997/98 El Niño was the largest El Niño 

event of the century (Van Cleave et al. 2014), which may be associated with a regime shift in lake ice 

cover and other environmental components. This is part of the remarkable downward trend observed 

during 1973-2013 (Table 7 vs. Table 6), which can be attributed to a combination of multi-decadal 

variability and human-caused secular climate warming. The 5-year running mean indicates that this multidecadal variability has a large peak in 1980 and a small peak in 1995.

Figure 7. Time series of AMIC during 1973-2017. The thick line is the 5-year running mean, indicating 

the decadal signal. The dashed red line is the climatological mean of 55%, and the blue dashed line 

indicates the long-term trend.

CONCLUSIONS

Growth of ice cover during the seasonal cycle in the Great Lakes is quite similar across all of the lakes. 

Some timing differences of ice onset and offset are observed between shallow and deep water lakes. 

Given the same atmospheric conditions, shallow lakes begin forming ice and reach maximum ice cover 

earlier than deep lakes. 

Weekly standard deviations of lake ice cover are recognized to have comparable magnitude (and 

sometimes greater magnitude) with their corresponding climatological means. This suggests overall poor 

predictability of long-term ice conditions due to the effects of large, interannual variability. In the lake ice 

cover time series for each of the six lakes, two significant periods of about 8 (before 1998) and 4 (after 

1998) years are observed. This strong interannual variability is possibly due to the effects of climate 

forcing on the Great Lakes, especially within more recent years (the past two decades). 

Lake ice cover shows a long-term, declining trend (winter seasons of 1973 to 2017). The greatest negative 

trends are in Lakes Superior, Huron, St. Clair and Erie. In total, a 69% loss in all Great Lakes annual 

average ice cover is observed over the entire 45-winter period. 

With the last six years of data (2012-2017) added to the analysis, in addition to large variability on

interannual time scales, decadal variability in Great Lakes ice cover stands out. Lake ice trends should be 

interpreted with caution, because the decadal variability in lake ice cover often contributes to the 

downward or upward trend.

Mean:

55%
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ACKNOWLEDGMENTS:

We appreciate support from the 2017 Summer Internship Project of CIGLR/GLERL. This is GLERL 

Contribution No. 1870 and CIGLR Contribution No. 1119.

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