Ice climate normals for the northern Canadian waters 1991 to 2020

Preface

The Canadian Ice Service (CIS) Regional Ice Chart database (CIS ice archive) encompasses over 50 years of ice information for:

  • Western Arctic (1968 to present)
  • Eastern Arctic (1968 to present)
  • Hudson Bay (1971 to present)
  • East Coast (1968 to present)
  • Great Lakes (1972 to present)

In these Canadian Ice Service ice climate normals, weekly regional charts, depicting ice concentration and stage of development, were used to compute a set of 30-year climatological standard normals for the period spanning January 1, 1991 through December 31, 2020.  This publication for Northern Canadian Waters (Western Arctic, Eastern Arctic and Hudson Bay) follows from two previous 30-year climate normals for the periods 1971 to 2000 and 1981 to 2010.

CIS 30-year climate normals are prepared in accordance with World Meteorological Organization (WMO) guidelines. The sea ice climate maps provide a week-by-week depiction of normal ice conditions throughout the year: its location, distribution, extent, thickness, and variability. They are used as the baseline for computing departure from normal ice concentration products and to provide guidance for planning safe operations in ice-infested Canadian waters.

Noteworthy in this edition

  • Chart data reliability: These 1991 to 2020 climate normals are the first 30-year CIS normals in the series where the majority (approximately 80%) of the chart data used was produced: a) digitally, using Geographic Information System (GIS) software that was introduced in 1995, and b) using primarily high-resolution (approximately 50m-100m) Synthetic Aperture Radar (SAR) satellite imagery with the launch of Radarsat-1 in 1995. Producing ice charts using GIS software has reduced the impact of errors associated with the digitization of paper charts, while using SAR imagery has significantly reduced analysis-related uncertainties (e.g. due to cloud contamination) and greatly improved the identification of old ice.
  • New products: These 1991 to 2020 climate normals contain three new products in addition to the regular suite of climate maps. Following a three year consultation process and to better serve northern communities, Frequency of Presence of Fast Ice charts and maps of Fast Ice Dates of Freeze-up and Breakup are included.

These normals are available on the Canadian Ice Service website.

Recommended citation: Canadian Ice Service. 2021. Sea ice climate normals for the northern Canadian waters 1991–2020.

1. Canadian Ice Service regional ice charts: production method and quality control

1.1 Chart preparation: satellite imagery and additional observations used

The data used in the production of these sea ice climate normals is derived from Canadian Ice Service (CIS) weekly regional ice charts (Figures 1.1 and 1.2), which are created using a combination of remote-sensing data and observations made by Ice Service Specialists (ISS) onboard dedicated aircraft and Canadian Coast Guard (CCG) ships.

Between 1991 and 1996 the proportion of Canadian waters covered by satellite data available for the production of regional ice charts increased from approximately 50% to 80% with the launch of the Canadian Space Agency’s (CSA) RADARSAT-1 satellite. Pre 1996, aerial surveillance (both visual and using Side-Looking Airborne Radar – SLAR) was the dominant data source along with optical imagery from the United States Geological Survey’s (USGS) Landsat-4, synthetic aperture radar (SAR) data from the European Space Agency’s (ESA) ERS-1, visible and infrared data from the National Oceanic and Atmospheric Administration’s (NOAA) AVHRR and passive microwave data from National Aeronautics and Space Administration’s (NASA) SSM/I sensors. From 1996 onward, high resolution SAR data available from the CSA’s RADARSAT missions quickly became the dominant source of satellite imagery used in ice chart preparation. The current normals are the first 30-year CIS climate normals to be produced from predominantly SAR-based charts (greater than or equal to 80%).

Since 1996, the main source of SAR imagery, typically 50 m to 100 m resolution, is from CSA’s RADARSAT-1 (1996 to 2013), RADARSAT-2 (2008 to 2020) and RADARSAT Constellation Mission (2020 to present).  These data have been supplemented with various sources of optical, passive microwave and additional SAR imagery over the years.  The primary sources of additional SAR imagery are the ESA ERS-1 and ERS-2 (1991 to 2010), Envisat (2002 to 2012) and Sentinel-1A and 1B (2015 to present) satellites. The primary sources of optical imagery used at CIS (visible, near-infrared and infrared), typically 250 m to 1 km resolution, are NOAA’s AVHRR (1981 to present), NASA’s MODIS (2000 to present), NOAA’s VIIRS (2012 to present) and most recently NOAA’s GOES-16 (2017 to present).  Passive microwave imagery is only used when other imagery is not available, resolution ranges from 12.5 km to 25 km, and data is available from NASA’s SSM and I-SSMIS (1987 to present) and AMSR-E (2002 to 2011), and the Japan Aerospace Exploration Agency’s (JAXA) AMSR2 (2012 to present).

The main advantage of SAR data is that it is independent of solar illumination and cloud cover. Under cold conditions, SAR imagery also provides a very clear distinction between seasonal ice and multi-year ice, where multi-year ice appears brighter due to a much larger concentration of air bubbles, lower salinity and generally rougher ice surface. These two factors alone eliminate large sources of uncertainty in the production of CIS ice charts.

SAR imagery does have some disadvantages, and in these instances optical imagery and sometimes passive microwave imagery can provide valuable information when producing ice charts. Compared to optical imagery, SAR imagery has a much smaller swath width (hundreds versus thousands of kilometers) and longer revisit frequency (days versus multiple times a day). In ice charting, it is difficult to distinguish between very smooth ice and calm open water in SAR imagery.  Optical imagery provides a very clear distinction between ice and water, making it useful for detecting and verifying open water polynyas and flaw leads in ice packs. During the summer months in the Arctic in the presence of wet snow and when melt water floods the surface of ice floes, there is a loss of definition between first year ice and old ice in SAR imagery, and sometimes between open water and the ice floes themselves.  Again, optical imagery does not suffer the same loss of detail and is a useful supplement to SAR data.

1.2 Chart preparation: methodology and quality assurance

1.2.1 Methodology

In 1996, regional chart production at CIS transitioned to an entirely digital workflow with the introduction of the Ice Service Integrated System (ISIS). It was built on Leica Geosystems’ Erdas Imagine and ESRI’s ArcGIS ArcInfo, which was still in use in August 2021. Prior to 1989, the personnel at CIS drew by hand the regional charts on paper. Between 1989 and 1995, source data was displayed digitally, on a common projection and scale, in the Ice Data Integration and Analysis System (IDIAS).  Although digital chart production was possible during this time, the system was slow and was primarily used as a tool to view data.  For climatological purposes, all hand-drawn charts were digitized in the late 1990’s and added to the digital archiveFootnote 1 . It should be noted that the original paper map scale of the Regional Ice Charts was 1:4,000,000.  The amount of detail and accuracy in the current GIS-produced analyses is comparable to the original maps. The 1991 to 2020 climate normals are the first normals based predominantly on digitally produced ice charts (greater than or equal to 80%) significantly reducing the error associated with digitizing paper charts and source data that is not displayed on a common scale or projection for chart production.

For chart production beginning in 1996, satellite imagery and daily ice charts are imported into GIS software (ISIS) and analyzed into the World Meteorological Organization’s (WMO) standardized egg code, which contains information for ice concentration, ice type, and floe size.  To identify ice type (an indication of age and thickness) in satellite imagery, various indicators such as brightness, fracture patterns and the shape of ice floes are used. The experience of the analyst plays an important role in chart production, meteorological and oceanographic factors such as air temperature, winds, water temperature, salinity, currents, waves, and tides, along with climatology to supplement information from satellite imagery and daily ice charts are considered.   In addition, ground-truth data provided by ISS’s on CCG ships and aircraft are incorporated when available. A notable difference between the Great Lakes, East Coast and Arctic regional ice charts is the reliance on daily ice charts during production.   The Great Lakes and East Coast regional charts are generated primarily from daily ice charts whereas the Arctic regional charts are generated primarily from satellite imagery with the daily ice charts used as guidance.

1.2.2 Quality assurance

Quality Assurance steps taken during the preparation of regional ice charts are both automated and manual.  Automated checks are from GIS scripts that scan for: a) errors when coding partial concentrations and floe sizes, and b) inconsistencies in overlap areas between charts (Figure 1.1) for adjacent regions.  Manual checks during production ensure that the analysis is consistent with: a) the climatology for a given region and time of year, and b) with the expected ice thickness associated with the accumulated freezing degree days (FDDs) during the freeze-up and winter seasons.  After production, the team leader or another dedicated person in operations manually inspects each ice chart for errors and inconsistencies before it is disseminated, archived and published online.  Chart errors noticed internally in operations at CIS after a chart has already been issued are corrected and a chart labeled “amendment” or “correction” is disseminated, archived and published as soon as possible.

A map of the extents of the regional ice analysis charts from the Canadian Ice Service.
Figure 1.1. Canadian Ice Service regional ice chart extents. The climate normals for northern Canadian waters is derived from charts for the Western Arctic, Eastern Arctic and Hudson Bay. Where northern charts overlap, although consistency between charts is ensured during the chart production stage, the data from the Eastern Arctic chart only was used. Note that with the exception of the Great Lakes, all regional chart extents changed in 1997 with the switch from hand-drawn to GIS charts.
A map of the Western Arctic depicting coloured areas of ice concentration and descriptions of ice type present on June 21, 2010.
Figure 1.2. Example weekly regional ice chart (Western Arctic).

1.3 Regional chart archive: quality control and homogenization

1.3.1 Chart dates: original chart dates vs Historical Dates

1.3.1.1 Original dates

Regional Ice Charts are currently produced weekly throughout the entire year for the Western Arctic, Eastern Arctic and Hudson Bay regions (see section 1.3.1.4 for details regarding northern waters chart production prior to 2012). 

Although the date specified on the charts is the Monday of each week, they represent a rough approximation of ice conditions for a seven-day period centred on this day and are finalized and issued on the Wednesday of that week. Satellite imagery up to 2 to 3 days on either side of the Monday may be consulted during the production of a regional chart but preference is given to images 2 to 3 days before to 1 day after the Monday. 

1.3.1.2 Historical Dates

Since the Monday of any given week does not fall on the same date each year, the charts in the database are assigned a "Historical Date" for consistency between years. The Historical Dates (HDs), one per week, are every 7 days starting January 1. The chart date is assigned the closest Historical Date.

There are two exceptions to this 7-day progression in HDs:

  1. the 8-day period between February 26 and March 5 that occurs in leap-years
  2. the 8-day period between November 26 and December 4 that accounts for the fact that there are 365 days in a year, not 364.

The week of November 26 is chosen to have 8 days because it is a week after the shipping season in northern waters has ended and before the ice forming in southern waters has an impact on shipping.

1.3.1.3 October 1

In many areas of the Arctic, sea ice does not completely melt away by the end of the summer. At the end of the melt season, on October 1, any first year ice (FYI) remaining from the previous winter is reclassified as second year ice (SYI), while any old ice (OI) already present is reclassified as multi-year ice (MYI). Note that SYI and MYI are both sub-categories of OI, and these are tracked separately during the months of October - December.  Regional charts assigned to the October 01 HD maybe had had original charts dates of September 28 to 30, extra care was taken to ensure that all FYI and OI areas for this HD were correctly reclassified.  

1.3.1.4 Northern waters: frequency of winter charts

Regional Ice Charts for the Arctic and Hudson Bay regions have always been produced weekly during the shipping season, for the weeks or HDs of June 04 (and June 11) through November 26. Winter charts were not produced prior to 1980 and then only produced once per month from 1980 through the winter of 2005-06 for the following HDs: December 04, January 01, January 29, February 26, April 02, April 30, and May 14.  The rational for only one chart per month was the lack of shipping and relatively stable ice conditions throughout the winter season. In preparation for the International Polar Year in 2007-2008, CIS began producing winter regional ice charts for the northern regions every two weeks from the winter of 2006-07 through the winter of 2010-11. From the winter of 2011-12 onwards, CIS has been producing winter regional ice charts for the northern regions on a weekly basis.

Because of the once-per-month winter analysis practices for northern waters prior to 2007, only one map per month is generated for the months of December to April, and only two maps are generated for the month of May.  To be consistent with previous CIS climate normals, Historical Dates were not necessarily used, dates used in the normals for this period are:  December 04, January 01, February 01, March 01, April 01, May 01, and May 15.  With the exception of December 04 and January 01, the first of the month and May 15 maps are computed using charts for the nearest Historical Date (e.g. February 01 is actually HD January 29).

1.3.2 Missing charts

The World Meteorological Organization (WMO) recommends that:

  • climate normals be computed when data is available for at least 80 per cent of the years in a 30-year averaging period
  • where data is missing, it should not be missing for more than three consecutive years (WMO, 2011: Guide to Climatological Practices (WMO-No.100), Geneva).

Apart from Arctic analysis practices prior to 2012, where charts were intentionally produced on a monthly or biweekly basis during the winter (described in section 1.3.1.4 above), CIS weekly regional ice charts are occasionally missing in the archive for other reasons. A chart may be missing if:

  1. A paper chart from the hand-drawn era was not digitized
  2. A chart’s data was not sent to the archive
  3. It was not produced.

For the current 30-year period, all missing charts related to the first two cases have been located and re-digitized, and re-integrated into the database. Missing charts related to the third case were replaced using analogue charts in winter and interpolation during summer, as explained below. 

During the winter months, when charts were found to be missing for key winter dates prior to 2012 (described in section 1.3.1.4 above), analogue chart replacements were deemed more reliable than interpolation because the time span between charts is more than one week. In these situations, winter ice conditions for the year in question were examined in detail and analogue charts for missing dates were chosen from years with similar seasonal ice characteristics and old ice distributions. During the summer months when regional charts were produced weekly, interpolation was used to fill in missing chart data.  The only HD with missing charts is June 4, for this HD, 4 of the 30 weeks were missing.  In these cases, ice concentration and stage of development data was linearly interpolated from adjacent weeks within the same ice season. This differs from the previous climate normals, missing chart data was interpolated from adjacent Historical Dates after the 30-year statistics were computed.

1.3.3 Chart coding and polygon errors

Occasionally, in spite of the Quality Assurance steps taken during the production of the Regional Ice Charts, errors in the coding of the ice do occur (e.g. thick first year ice "4." might erroneously be coded as young grey ice "4").  In addition, during the digitization process of the pre 1996-97 paper charts, mistakes were made when transcribing the ice codes and in the interpretation of polygon boundaries. Polygon boundaries were sometimes incorrectly interpreted or assigned in areas where multiple ice codes were used within single large polygons (something that used to be allowed prior to the GIS period, and which was rectified during the digitization process to match current practices).   

Beginning with the production of the 1971 to 2000 climate normals, a continued effort has been made over the years to manually inspect and correct as many archived charts as possible. Additional errors identified using automated scripts (e.g. coding errors related to iceberg concentrations and fast ice areas) were corrected during the production of these 1991 to 2020 normals.  While the majority of such errors in the Regional Ice Chart digital archive have now been corrected, some errors likely remain – especially in charts for the early years.

1.3.4 Temporal homogeneity

In addition to changes in data sources and chart preparation technology over the years (e.g. changes from optical to SAR satellite imagery, changes from hand-drawn to GIS based chart production), some changes in ice analysis practices have also occurred. For example, in 1982, the code used to describe the ice in each polygon changed from a "ratio" code to an "egg" code (for a description of the egg code, see chapter 3 in the Manual of Ice (MANICE). Around the same time, the encoding of first year sea ice changed from a single category to three sub-categories (thin, medium and thick).

Changes in ice analysis practices that impact the current normals period are outlined below, as well as the steps taken to address these discontinuities in the chart data to mitigate their impacts on the climate normal products.

  • 1996 to 1998: With the adoption of GIS software, the practice of allowing multiple ice eggs within a single large polygon was changed to allow only a single ice egg per polygon. To compensate for the earlier practice, polygons with multiple ice codes were divided into separate polygons during the digitization of the hand-drawn charts.
  • 2004-2005: Polygons containing fast ice can be encoded using either an ice egg or a point symbol.  Prior to 2005, fast-ice point symbols were generic, only indicating that the ice had a concentration of 10/10 and was fast.  As of 2005, however, fast ice point symbols were affixed with the stage of development (SOD) of the predominant ice type.  Because this change in practice falls in the middle of the 30-year data period of the 1991 to 2020 sea ice climate normals, steps were taken to address this discontinuity (as done for the 1981 to 2010 climate normals). Pre 2005 generic point fast ice values were assigned SODs based on accumulated Freezing Degree Days and available climatology of shore-based ice thickness observations. Note that although this method has verified well where seasonal ice is the predominant fast ice type, it can lead to occasional errors in the fiords of far northern Ellesmere Island where the fast ice is predominantly multi-year ice (i.e. where fast ice persists in fiords from one year to the next).  For this reason (and others – see the Eastern Arctic bullet in section 1.3.5 below) extreme high Arctic areas have been excluded from the climate normals.
  • 2014-2015: Prior to 2014-2015, fast ice was almost always considered to have a concentration of 10/10 (i.e. to be "consolidated”). Similarly, non-fast or mobile ice packs, with the exception of vast or giant ice floes, was almost always assumed to contain fractures and to therefore have a concentration of less than 10/10. Improvements in satellite image resolution (both optical and SAR), has now allowed for the definitions of fast and consolidated ice to be separated on a more frequent basis. As such, fast ice is now often analyzed as "fractured" during the melt season and non-fast ice is sometimes analyzed as "consolidated" during the winter where mobile ice has temporarily been compressed into a solid pack. This recent change in practice has an impact on the interpretation of the median ice concentration products in these climate normals.  Prior to this change in practice, areas of 10/10 median concentration served as a proxy for coastal fast ice extents throughout the year, something of great interest for community and marine navigation planning. With the new practice, climatological information regarding fast ice extents is no longer implicit within the median ice concentration maps. To address this discontinuity and to be consistent with past climate normals, all fast ice was assigned a concentration of 10/10 and all mobile, non-fast ice, was assigned a concentration of less than 10/10 prior to computing median concentrations. Almost all changes were made to charts after 2013 (less than 25% of charts). To further address this issue moving into the future, new "fast ice" specific map products not based on median concentrations ("Frequency of Presence of Fast Ice" and "Dates of Fast Ice Freeze-up and Breakup") have been developed for these climate normals.

1.3.5 Spatial homogeneity

In addition to the technological and analysis related changes described above, changes in chart extents have also taken place over the years.  Steps taken to address the impact of changes in chart extents on the climate normals products include:

  • Western Arctic: Hand drawn paper charts produced prior to the changeover in 1997 to GIS-prepared charts did not include the northern part of the Beaufort Sea (Figure 1.1). Although this area technically has enough data for the computation of 30-year statistical normals (greater than or equal to 80%), the data cannot be brought to 100% using interpolation or analogue charts because it contains more than three consecutive years of missing data (1991 to 1996). For this reason, as was done in previous normals, this area is masked as "no data" in the weekly and monthly maps of ice statistics.
  • Eastern Arctic: Hand drawn paper charts produced prior to the changeover in 1997 to GIS-prepared charts did not include the extreme northern part of Ellesmere Island and Nares Strait (north of approximately 81.5°N, Figure 1.1). Because this area contains more than three consecutive years of missing data (1991 to 1996), as was done in previous normals, it is masked as "no data" in the weekly and monthly maps of ice statistics. Note that this area was also excluded from statistical calculations due to the lack of consistent ice shelf and multi-year fast ice differentiation along the north coast of Ellesmere Island prior to approximately 2008.
  • Hudson Bay: Hand drawn paper charts produced prior to the changeover in 1997 to GIS prepared charts did not include the extreme southern part of James Bay (south of approximately 52.5°N, Figure 1.1). Since more than three consecutive years of data are missing in this area, in principle it should be excluded from computations of 30-year statistical normals and masked as "no data" in the weekly and monthly maps (as was done in previous normals). For the current 1991 to 2020 climate normals, however, it was decided to include the southern tip of James Bay in the statistical computations because the area affected is so small and because the ice in that area has been consistently and reliably analyzed since 1997.

Changes in CIS ice chart coastlines have also taken place over the years, due to:

  1. A change in datum from NAD27 to WGS84
  2. Improved geographic detail along the Greenland coast as a result of higher resolution satellite imagery
  3. The fracture and retreat of ice shelves and glaciers along Canadian Arctic and Greenland coasts and in their fiords, exposing the true shoreline in their wake.

Because such coastal variations are small compared to the scale of analysis in the regional ice charts, to compensate for spatio-temporal changes in coastline details and extents when producing the climate normals maps, a "climate normals coastline" comprising of the union of all coastline extents through time was used.  This decision has the potential to impact the climate normals in very small bays and inlets, but given the scale of the maps and the existing exclusion of northern Ellesmere Island from the statistical calculations (the only location where ice shelves are found), these minor small scale issues were deemed insignificant and acceptable.

2. Description of climate normals statistics based on CIS ice charts

All statistics used in the climate normals are described below. Mathematical definitions of each statistic can be found in Appendix 1 (coming soon), illustrations in terms of Ice Eggs can be found in Appendix 2 (coming soon).

2.1 Statistics by historical date

As with previous CIS climate normals, two key statistics have been selected to describe the 30-year climatological ice conditions in Canadian waters: medians and frequencies. These statistics are computed for various ice chart elements for each week (or Historical Date) in a given region’s ice season or year. For a complete understanding of the climatological ice conditions in any given location and time of year, median and frequency climate normals products should be used together.

Typical ice conditions

  • For information about "normal or average ice conditions" use the "Median of Ice Concentration" and "Median of the Predominant Ice Type**" products as a pair
  • For information about "normal or average old ice (includes multi-year ice and second-year ice) conditions" use the "Median of Old Ice Concentration" product
  • For information about the "normal or average extent of fast ice" including the predominant locations of ice arches use the "Median of Ice Concentration product"where an ice concentration equal to 10/10 is a proxy for fast ice.

Range of ice conditions

For information describing the range of ice conditions that could be encountered use the frequency of presence products.

  • For information about the "range of ice extent" (ice concentration greater than 1/10) including the "maximum and minimum ice extent" use the "Frequency of Presence of Sea Ice" product.
  • For information about "expected ice conditions considering only those years where ice is present"use the "Frequency of Presence of Sea Ice"product paired with the "Median of Ice Concentration When Ice Is Present" and the "Predominant Ice Type When Ice is Present" products together.
  • For information about the "range of old ice extent" use the "Frequency of Presence of Old Ice" (old ice concentration greater than 1/10) and the "Frequency of Presence of Old Ice 4/10 to 10/10 "products.
  • For information about the "range of fast ice extent" use the "Frequency of Presence of Fast Ice" product.

Typical timing of break-up and freeze-up

  • For information about the "typical timing of spring break-up and fall freeze-up when the ice concentration shrinks below 1/10 or exceeds 1/10" use the "Date of Fist Ice" and "Date of Last Ice" products.
  • For information about the "typical timing of spring break-up and fall freeze-up when ice concentration shrinks below 4/10 or exceeds 4/10" use the "Break-Up Date" and "Freeze-Up Date" products
  • For information about the "typical timing of the formation of fast ice in the fall and the break-up of fast ice" in the spring use the "Fast Ice Break-Up Date" and "Fast-Ice Freeze-Up Date" products.

2.1.1 Medians

Medians are used rather than averages because of the often abrupt day-to-day changes observed in sea ice concentrations (e.g. as witnessed during the fracture and break-up of fast ice areas during the melt season, or when wind-driven ice compression events are followed by abrupt ice dispersals when a weather system passes and winds change direction). 

To illustrate, consider a five week sample of ice concentrations in a given area during the break-up period with the following values: 10/10, 10/10, 10/10, 0/10, 0/10.  The average ice concentration for this period is (10 + 10 + 10 + 0 + 0)/5 equals 6/10. However, open drift (4/10 to 6/10) ice concentrations were never actually observed during this period. Although the ice may have briefly experienced open drift concentrations during its sudden fracture and dispersal, as a climatological normal it is not a useful measure. On the other hand, the median ice concentration for this period (determined by ordering the concentrations from lowest to highest and choosing the middle value) is 10/10, a realistic "normal" situation. The median is thus considered to be a more appropriate representation or measure of the true "normal" ice conditions over the 30-year climate normals period. Note that for even sets of values, as occurs with 30-year periods, CIS uses the higher, not the average, of the two middle values when computing medians. This is done to avoid fractional values and to err on the side of heavier ice conditions for marine safety.

2.1.1.1 Median of ice concentration

These products depict the 30-year median of the "total ice" concentration at any map location for each week of the ice season or year.  In these climate normals, areas of 10/10 median ice concentration can be used as proxies for fast ice extents on any given Historical Date within the 30-year period (see section 1.3.4 for caveats). This proxy should be used in conjunction with the new Frequency of Presence of Fast Ice products (described in section 2.1.2.3 below) for a clearer overall picture of normal fast ice conditions for specific weeks of the year.

2.1.1.2 Median of old ice concentration

These products depict the 30-year median of the total concentration of "old ice only" at any map location for each week of the ice season or year. Note that old ice comprises both second year ice and multi-year ice.

2.1.1.3 Median of predominant ice type

These products depict the predominant ice type that is normally encountered at any map location for   each week of the ice season or year. Open water is considered an ice type in the calculation of the median predominant ice type.  This is a new product for the 1991 to 2020 climate normals.

"Predominant" is defined as the ice type with the greatest partial concentration except in cases where:

  1. Total sums of all old ice concentrations are greater than or equal to 4/10.

    Old ice is always considered to be the predominant ice type whenever its concentrations are ≥ 4/10 (as opposed to ≥ 5/10) because sea ice polygons are generally analyzed from satellite imagery in terms of concentration "categories" (e.g., very open drift = 1/10 to 3/10, open drift = 4/10 to 6/10, close pack = 7/10 to 8/10, and very close pack = 9/10 to 9+/10) before determining the actual concentration value, which is somewhat subjective and can vary between analysts. To account for uncertainty related to this subjectivity, and for safety reasons pertaining to ship navigation in ice-infested waters, any open drift concentrations of old ice (4/10 to 6/10), when present, are considered to be the predominant ice type.

  2. Partial ice concentrations are equivalent.

    If three ice types are reported and they all have equal partial concentrations, then the thickest ice type is deemed to be the predominant ice type (for marine safety reasons). If only two ice types are reported and they have equivalent concentrations, then again the thicker ice type is deemed to be the predominant ice type. Similarly, if more than two ice types are reported and the two thickest ice types have equal concentrations, then the thicker of the two is deemed to be the predominant ice type.

Once the predominant ice type at a given location has been determined for each year for a given Historical Date, the "Median of the Predominant Ice Type" is then computed by ordering the predominant ice types from thinnest to thickest and selecting their middle value (see Appendices 1 and 2).

2.1.1.4 Median of ice concentration when ice is present

These products depict the median of the total ice concentration but at any given point on the map, only use years when ice was present with a concentration greater than or equal to 1/10 in the median calculations.  If in any one of the years only trace amounts or no ice was present at a certain location for the week (or Historical Date) in question, that year’s concentration of less than 1/10 is omitted in the calculation of the median for that location and week. If in every one of the years no ice with a concentration greater than or equal to 1/10 was ever present at that location and week, a median value of 0/10 is assigned to that location.

Interpretation: The most appropriate way to interpret these charts is to view them in conjunction with the 30-year frequency of presence of sea ice charts. To illustrate, suppose that for a given Historical Date a particular point has a  frequency of presence of sea ice with a range of 34% to 50% and a median of ice concentration when ice is present of 9/10 to 9+/10. This means that, for this Historical Date, this location has a 34% to 50% chance of experiencing sea ice and, when ice is present, it is "normally" 9/10 to 9+/10 concentration.

Important:  Unlike in the "Median of Ice Concentration" products, areas of 10/10 median ice concentration in the "Median of Ice Concentration when Ice is Present" products do not serve as a reliable proxy for fast ice extents, particularly in the vicinity of fast ice margins during the spring to summer breakup period.  Because of the nature of the calculation, at these times and in these locations areas of 10/10 median ice concentration "when ice is present" are prone to appearing discontinuous and fractured, not a realistic "normal" situation. 

2.1.1.5 Median of predominant ice type when ice is present

These products depict the predominant ice type that is normally encountered when ice is present at any map location for a given week of the ice season or year.

"Predominant" is defined in section 2.1.1.3.

Once the predominant ice type at a given location has been determined for each year for a given Historical Date, the "Median of the Predominant Ice Type when Ice is Present" is then computed by ordering the predominant ice types from thinnest to thickest and selecting their middle value (see Appendices 1 and 2).

Interpretation:  As with the "Median of Ice Concentration when Ice is Present" map products, the most appropriate way to interpret these charts is to view them in conjunction with the frequency of presence of sea ice charts. For example if, at a particular point, the 30-year frequency of presence of sea ice is in the range of 34% to 50% and the median of predominant ice type when ice is present is thin first-year ice, then at this point there is a 34% to 50% chance of encountering sea ice, and when ice is present, it is "normally" thin first-year ice.

2.1.2 Frequencies

Frequencies (expressed as percentages) are determined by summing the number of observations of an occurrence or event (e.g., the presence of sea ice during a specific week of the year) over the 1991 to 2020 climate normals period and then dividing by thirty, the total number of years in the period (see Appendices 1 and 2). 

These products can be interpreted as the probability of sea ice, or sea ice of a specific type and concentration, being present at a given location on a given Historical Date, based on the normal for the 1991 to 2020 period. The charts can also be used as proxies for maximum and minimum ice extents. The 0% line on the charts represents the maximum ice extent, beyond which no ice of the specified type and concentration was reported during the 30-year period for that Historical Date. The 100% line represents the minimum ice extent, within which there has always been ice of that type and concentration reported during the 30-year period for that Historical Date. In between these two lines, areas with frequencies of 1% to 33% represent above normal extents. Areas with frequencies of 34% to 66% represent near normal extents. Areas with frequencies of 67% to 99% represent below normal extents and can also be interpreted as those regions where the specified ice type and concentration is consistently found even during years with below normal ice extents.

2.1.2.1 Frequency of presence of sea ice (%) 

These products depict the likelihood of encountering total ice concentrations greater than or equal to 1/10 at any particular location on any Historical Date during the 30-year period. For example if, at a particular point on the chart for a given Historical Date, the 30-year frequency of presence of sea ice falls within the category of 34% to 50%, then for this week of year and at this location there is a 34% to 50% chance of encountering sea ice with a concentration greater than or equal to 1/10. Furthermore, for this Historical Date, any ice encountered at this location lies near the normal limit of ice with total concentrations greater than or equal to 1/10.

2.1.2.2 Frequency of presence of old ice (%) greater than or equal to 1/10 and  4/10

These products depict the likelihood of encountering old ice concentrations greater than or equal to 1/10 (or greater than or equal to 4/10) at any particular location on any Historical Date during the 30-year period. For example if, at a particular point on a given Historical Date, the 30-year frequency of presence of old ice greater than or equal to 4/10 is in the range of 16% to 33%, then at this time of year and at this location there is a 16% to 33% chance of encountering old ice with a concentration of greater than or equal to 4/10. Furthermore, for this Historical Date, any old ice encountered at this location lies beyond the normal limit of old ice with total concentrations greater than or equal to 4/10.

As with the median of predominant ice type, old ice concentrations greater than or equal to 4/10 (as opposed to greater than or equal to 5/10) are considered significant when computing frequencies of presence, because sea ice concentrations are generally analyzed in terms of "categories" (e.g. very open drift equals 1/10 to 3/10, open drift equals 4/10 to 6/10, close pack equals 7/10 to 8/10, and very close pack = 9/10 to 9+/10) and the actual concentration value chosen within a given category is somewhat subjective (depending on the analyst). To account for uncertainty related to this subjectivity and for marine safety reasons, the frequency of presence of open drift concentrations of old ice (4/10 to 6/10) are computed (rather than greater than or equal to 5/10).

2.1.2.3 Frequency of presence of fast ice

These products are a new addition to the 1991 to 2020 climate normals. They depict the likelihood (on a weekly basis) of the ice being "fast ice" at any given point during the ice season or year.  For example if, at a particular point on a given Historical Date, the 30-year frequency of presence of fast ice is in the range of 67% to 84%, then at this time of year and at this location there is a 67% to 84% chance of encountering ice that is "fast". Furthermore, for this Historical Date, any ice encountered at this location lies well inside the normal 30-year fast ice limit.

2.2 Date of freeze-up and break-up

2.2.1 Dates of freeze-up and break-up – all ice

The "Dates of First Ice", Dates of Last Ice", "Freeze-up Dates" and "Break-up Dates" maps (derived from the median of ice concentration products) summarize the evolution of the ice extent on a biweekly basis during the freeze-up and break-up periods. 

To create these summary products, the median of ice concentration for every second Historical Date within the freeze-up period spanning September 10 to December 4 and the breakup period spanning June 4 to September 10 is consulted. Outside of the permanent Arctic ice pack, the date of first ice and the date of  freeze-up at any location is defined as the first occurrence of a median of ice concentration of 1/10 or greater and 4/10 or greater respectively within the above set of freeze-up dates. The date of last ice at any location is defined as the last occurrence of a median of 1/10 or greater, within the above set up break-up dates. The date of break-up at any location is defined as the first occurrence of a median ice concentration less than 4/10, within the above set of break-up dates. 

Note that in the far north, in the Arctic Ocean and in the fiords of northern Ellesmere Island, sea ice can form for the winter season prior to the week of September 10 in extreme years.  These areas are outside the chart extents of the climate normals products and have no impact on the freeze-up dates.

2.2.2 Dates of freeze-up and break-up – fast ice

The "Fast Ice Freeze-up Dates" and "Fast Ice Break-up Dates" maps summarize the changes in the fast ice extent on a biweekly basis during the freeze-up and break-up seasons. Unlike the freeze-up/break-up dates computed for all ice, this calculation is based on the frequency of presence of fast ice and not the median ice concentration (because median ice concentrations of 10/10 are only a proxy for fast ice extents – see section 2.1.1.1). Fast ice freeze-up dates are defined as the first week the frequency of fast ice at a given location is greater than or equal to 50%, while breakup dates are defined as the first week the frequency of fast ice is less than 50%.

3. Regional ice regimes and influences

3.1 Regional notes

A map of the Canadian Arctic with water currents and bathymetry.
Figure 3.1 Arctic bathymetry and water currents
A map of Hudson Bay and the Labrador Sea with water currents and bathymetry.
Figure 3.2 Hudson Bay bathymetry and water currents

Beaufort Sea and Amundsen Gulf

Median shipping season

South of the main Arctic pack: end July to mid-October; Amundsen Gulf: mid-July to late October; Elsewhere: not applicable.

Old ice

Present year round, primarily within the main ice pack in the Beaufort Sea, up to 4.5m thick, continuously circulates with currents and winds.

Special ice features

The main Arctic ice pack (a mix of old ice and first year ice) generally reaches to within 200km north of mainland coast. South of this pack is a zone of predominantly first year ice in winter (fast along the coast and mobile offshore) and predominantly open water in late summer. Breakup or melt in this zone is aided in early summer by southerly winds, which create an initial flaw lead between the coastal fast ice and the mobile pack ice by mid-July. In Amundsen Gulf, in winter, the position of the leading western edge of the consolidated ice can vary considerably from year to year. In extreme years, it can be located as far west as the southern tip of Banks Island or as far east as the entrance to Dolphin and Union Strait. During easterly wind events, a polynya typically develops between this fast ice edge and the mobile Beaufort pack.

Canadian Arctic Archipelago - Parry Channel and northwards

Median shipping season

Lancaster Sound: late June to start October; Barrow Strait and Jones Sound: early to mid-August to late September; Eastern Norwegian Bay and Eureka Sound: early September only; Elsewhere: not applicable.

Old ice

Present year round, drifting into Sverdrup Basin, western Parry Channel and Nares Strait with the currents from the Arctic Ocean; also forming locally in Sverdrup Basin and northwards.

Special ice features

Shallow water depths, narrow channels and extreme cold mean that winter ice becomes consolidated (shore-to-shore fast ice) everywhere. Exceptions are Lancaster and Jones Sounds where water is deeper and where the fast ice extent can be highly variable from year to year. Strong currents leading through Hell Gate from Sverdrup Basin to Jones Sound result in a large persistent polynya in this strait. Iceberg and ice island intrusions from Nares Strait and northern Baffin Bay into Lancaster Sound are common. Ice Islands and icebergs from northern Ellesmere Island occasionally drift into Sverdrup Basin.

Canadian Arctic Archipelago - south of Parry Channel

Median shipping season

Amundsen to Queen Maud Gulfs: end July to mid-October; Larsen Sound, Peel Sound, and Gulf of Boothia: late August to end September; Elsewhere: not applicable.

Old ice

Present year round, mainly in straits and sounds leading directly southwards from Parry Channel and locally in the Gulf of Boothia. Traces elsewhere.

Special ice features

Shallow water depths, narrow channels and extreme cold mean that ice becomes consolidated everywhere in winter (shore to shore fast ice), except in central Gulf of Boothia where the ice remains mobile most years.  In summer, old ice drifting into Larsen Sound and straits south of it (mainly from M'Clintock Channel) can create a choke point, hindering navigation through the main southern Northwest Passage routes.

Baffin Bay and Davis Strait (north of 60N)

Median shipping season

Davis Strait and Baffin Bay (east of 60°W): end June to start December; Western Davis: late July to late November; Western Baffin Bay: late July to late October

Old ice

Old ice from the Arctic Ocean drifts into Baffin Bay and Davis Strait via Nares Strait, mainly during the early winter and early summer months (the influx is blocked in winter when ice in Nares Strait is consolidated). Old ice may become the predominant ice type in the early summer as the thinner first year ice melts first.   

Special ice features

Icebergs circulate the region with the currents, drifting northwards along the Greenland coast and southwards along the Baffin coast; in winter, in northern Baffin Bay, north winds and currents create a large polynya south of the Nares Strait fast ice edge. Large ocean salinity anomalies periodically propagating into Davis Strait and the Labrador Sea have been correlated with significantly increased winter ice extents.

Hudson Bay

Median shipping season

Early July to mid-November (northwest), to early December (southeast)

Old ice

Occasional low concentrations are observed in the northeast part of the Bay, drifting in from Foxe Basin.

Special ice features

Freeze-up progresses from northwest to southeast, while melt progresses from shore to centre. During freeze-up, prevailing northwesterlies maintain a flaw lead in the northwest part of the Bay.  The presence of old ice and the late clearing of sea ice around Salisbury and Nottingham Islands create a choke point along the shipping route into Hudson Bay.

Hudson Strait and Ungava Bay

Median shipping season

Start July to end November

Old ice

Low concentration intrusions from Davis Strait and Foxe Basin starting in late winter. Icebergs mixed into Davis intrusions.

Special ice features

Due to strong currents and frequent gales, ridging, rafting, hummocking and ice congestion often affect Ungava Bay and the south side of Hudson Strait. Conversely, a shore or flaw lead is frequently present on the north side of the Strait.

Foxe Basin

Median shipping season

Late August to end October

Old ice

Present in low concentrations, especially in the northwest part of the Basin where it drifts in from the Gulf of Boothia.

Special ice features

Shallow water combined with large tidal ranges and strong winds keep a large amount of bottom sediments in suspension. Thus the ice is generally very rough, much of it in small floes and often muddy in appearance.

Labrador Coast (south of 60N)

Median shipping season

Late June to end December

Old ice

Low concentrations from Davis Strait January to August.

Special ice features

Winter coastal fast ice forms locally, but most offshore pack ice drifts in from the north and this can be up to 1.5m thick with many embedded icebergs. Strong, persistent easterly winds can compress the mobile pack against the coast resulting in very large ridges. Westerly wind events can create a flaw lead along the shore and spread the pack ice up to 500km seaward.

James Bay

Median shipping season

Early July to mid-November

Old ice

Not present

Special ice features

James Bay ice is noted for its discoloration, caused by freezing of shallow muddy water, or by run-off concentrating sediments on the surface of the ice.  Thinner ice and occasional open water areas south of Akimiski Island are often observed during winter and are caused by currents.

3.2 Factors influencing sea ice in Canadian northern waters

Solar energy and albedo

In spring or early summer, increasing solar radiation warms low-albedo ocean surfaces along the ice pack margins and in flaw leads and polynyas. Increasing solar energy also triggers the formation and growth of low-albedo melt ponds on the ice surface (which are much more effective at absorbing incoming solar radiation than ice and snow). The ice (and snow) melt initiated by this warming is amplified by the positive ice-albedo feedback, whereby low-albedo water areas warmed by the sun serve to melt adjacent sea ice, further increasing the size of the low-albedo water areas.

Air temperature

During freeze-up, sea ice formation is mainly determined by air temperature. Weather systems bring frigid air masses from continental areas, Greenland and the Canadian Arctic Archipelago (CAA), cooling the ocean surface to the freezing point where ice then forms and spreads. Ice thickening is proportional to the temperature difference between the atmosphere and the ocean: rapid at first then slower as the thickness of the insulating ice barrier between the cold air and the warmer sea increases. In spring and early summer, southern continental areas warm more quickly than the sea. River runoff will also be warmer than the sea during this time. As a result, ice areas close to land melt earlier than areas in the centre of the offshore ice pack as is seen in Hudson Bay and Baffin Bay, where the general progression of ice melt is from shore to centre (as opposed to the main progression of freeze-up which is from the northwest to the southeast parts of these bays).

Water depth, water temperature, and upwelling

In general, surface waters in shallow, undisturbed coastal areas and within the shallow channels of the CAA cool more quickly during freeze-up than those in deeper offshore areas, due to limited vertical mixing with deeper warmer waters. Winter ice therefore forms in these areas first, later spreading to offshore areas with winds and currents. Similarly, the eastward extent of the winter pack ice drifting southwards from Davis Strait along the Labrador coast is generally limited to the continental shelf; beyond this area mixing of surface waters with deeper warmer waters serves to melt the ice. In places, the upwelling of deeper warmer waters (rising to replace surface waters pushed away by winds and currents), can result in semi-permanent areas of open water or polynyas within ice packs.

Water salinity

Because fresh water has a warmer freezing point (0°C) than sea water (-1.8°C), large pools of fresher-than-normal sea water propagating around the North Atlantic sub-polar gyre are correlated with increased ice extents in the Davis Strait and Labrador Sea. On a smaller scale, greater than normal amounts of freshwater runoff from rivers into coastal bays and fiords (e.g. Cumberland Sound) during freeze-up can cause “flash-freezes” or the sudden wide-spread formation of new ice. During early freeze-up, extensive flash freezes have also been observed in central and northwestern Baffin Bay during calm, cold weather. These are possibly related to a combination of propagating surface salinity anomalies and river runoff.

Currents

Currents (density-driven, wind-driven and sometimes tidal) play an important role in determining the overall long-term or mean motion of mobile ice packs.  The clockwise Beaufort Gyre is responsible for the southward drift and re-circulation of old ice from the Arctic Ocean into and within the Beaufort Sea. In Baffin Bay, a counter-clockwise circulation exists with a warm, north-flowing current along the Greenland coast and a cold, south-flowing current along the Baffin Island coast. As a result, the thickest winter ice is found in the north and west of Baffin Bay and icebergs from Greenland glaciers first drift northwards along the Greenland coast before turning southwards along the Baffin Island coast towards Davis Strait.  During the winter along the Labrador coast, thick first year ice with embedded old ice floes and icebergs from Baffin Bay and Davis Strait continuously drifts southwards with the Labrador Current (occasionally entering and circulating around Hudson Strait first). During the summer months, currents leading from the Arctic Ocean into the Sverdrup Basin bring large concentrations of old ice and occasional ice islands or icebergs into the Sverdrup Basin and Nansen Sound, although in some years this influx is blocked by the presence of fast ice “plugs” (persistent sections of consolidated ice at the mouths of the northern channels, left over as the fast ice in the area breaks up). Such ice plugs can cause a temporary depletion of old ice concentrations in the Sverdrup Basin.  Prior to freeze-up and consolidation, currents also bring old ice from the Arctic Ocean into northern Baffin Bay via Nares Strait. Along the Labrador coast, throughout the winter and spring (even after ice formation has stopped), currents continue to drift the existing ice towards warmer waters, where it eventually melts.  During winter, strong tidal currents can also create polynyas in very narrow straits. These polynyas are similar to current-related open water areas found in the centres of frozen rivers in the south and are generally much smaller than the wind-generated polynyas. Examples of polynyas caused by strong water currents in the Canadian Arctic are found in Hell Gate (leading from Sverdrup Basin into Jones sound), in Penny Strait (near the northwest tip of Devon Island) and in Bellot Strait (leading from Larsen Sound to Prince Regent Inlet).

Winds

Winds play an important role in the shorter-term variability of the extent, concentration and drift of mobile ice packs. Storm winds can result in the compaction or dispersal of the ice pack. Strong and persistent onshore winds (e.g. along the Labrador coast) can compress the ice pack towards the coast, resulting in a narrow band of >9/10 concentration and causing the ice to pile up into large ridges. Strong and persistent offshore winds, on the other hand, can spread the pack far seawards, decreasing ice concentrations and opening flaw leads near the shore. In winter, such flaw leads are commonly observed between the coastal fast ice and the pack ice along the north shore of Hudson Strait, and also in the northwest part of Hudson Bay.  At the start of break-up, in the Western Arctic, southerly winds commonly create a flaw lead between the mainland fast ice and the offshore pack.  In the summer, a wind-driven flaw lead occasionally develops between the northwest coast of Ellesmere Island and Arctic Ocean ice pack.

In winter, periods of persistent counter-clockwise winds associated with anomalous quasi-stationary lows in the Beaufort Sea can temporarily slow or stop the general clockwise drift of the entire mobile Beaufort ice pack. In some cases, it can lead to a strong compression of the ice towards the Canadian Arctic Archipelago, creating a large temporary area of consolidated ice – which is then released via large abrupt fracture events back into individual mobile floes when normal wind and drift conditions resume.

In fiords and at the mouths of channels in the CAA, winter winds enhanced by topographic funnelling can push the mobile pack away from fast ice edges (seaward) creating areas of open water and thin new ice (polynyas). Such wind-generated polynyas can be permanent where caused by prevailing winds aided by strong currents, as is found in the northern part of Baffin Bay (south of the consolidated ice in Nares Strait), or they can be transient, opening and closing depending on the direction of wind (e.g. at the mouths of Jones and Lancaster Sounds, in M’Clure Strait, in Amundsen Sound, in Frobisher Bay and Cumberland Sound).

Waves and tides (vertical displacement)

In protected coastal bays and inlets and in the narrow channels of the CAA, where the water is undisturbed by large waves or tides, winter ice generally becomes consolidated (10/10 concentration) and land-fast from shore to shore. The fast ice edges at the mouths of channels leading into the CAA are often arched, with the curves of the arches eating inwards into the consolidated ice (notably in Amundsen Gulf, M’Clure Strait, Lancaster and Jones Sounds and in Nares Strait). This is related to increased stresses from the prevailing winds and currents on the ice edges in the centres of the channels (away from the shore). Elsewhere (offshore), constant vertical displacement by wind-generated waves and tides plays a role in breaking areas of ice into individual floes, with floe sizes decreasing towards the edges of the mobile ice pack where wave damping is least. In these locations (e.g. the eastern and southern ice margins in Davis Strait and the Labrador Sea), small floes (usually <500m) are often organized into strips and patches. Areas with large tidal ranges (e.g. Foxes Basin) will also generally have smaller floe sizes. Vast (2-10km) and sometimes giant (>10km) floes are observed in the Arctic Ocean where ice concentrations and thicknesses are greatest and where maximum wave-damping occurs.  During freeze-up, waves generated by storm winds can hinder the production of sea ice by destroying thin newly formed ice.

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