Sea Ice Climatic Atlas for the East Coast 1981-2011: chapter 1
Regional Ice Charts and the Atlas Products
Data used in Regional Analysis
Canadian Ice charts used in the production of the atlas are produced using imagery from RADARSAT-1 (since 1996) and RADARSAT-2 (since 2008). Other remote sensing data sources include Envisat, NOAA AVHRR and Modis imagery. Where possible, the interpretation of the satellite data is verified using observations from Ice Service Specialists onboard dedicated aircraft and CCG ships. In addition, the United States has exchanged ice data with us for many years: the National Ice Center at Suitland, MD (Arctic ice information); and the International Ice Patrol (IIP), under the jurisdiction of the United States Coast Guard (East Coast of Canada - sea ice and iceberg information). In addition, the Danish Meteorological Institute in Copenhagen has shared ice information covering the waters west of Greenland.
The Regional Ice Charts are not always done on the same dates each year, so a seven-day period centered on the Historical Dates has been selected for this climatological atlas. The climate data represents information from charts within three days on either side of the date.
It should be noted that the original scale of the Regional Ice Chart was 1:4,000,000 and plotted on paper maps. Although the current analyses are prepared using GIS computer applications the amount of detail and accuracy is still comparable to the original maps.
From 1980 to 1995 the Regional charts were drawn on paper. These charts were digitized in the late 1990’s for use in the climatology. Since 1995 computer technology was used to generate a digital version of the charts. The Regional Ice Chart collection now encompasses over 40 years of sea ice information spanning from 1968 to the present.
The dates of analysis varied through time as do the first and last charts of any given season. In early years the regional ice charts started and ended depending upon the resources available. To provide more complete information for this climatology an experienced ice forecaster reviewed the start and end of the season to ensure that any instances of first or last ice were included. If necessary, analogue charts representing the appropriate pattern of ice formation were used to ensure a complete set of charts for each historical date. Occasional missing charts were handled in a similar manner.
In the previous atlas, Lake Melville was manually re-analysed using analogue years to reflect realistic ice conditions. In this atlas, the technique was further refined and provided a more systematic and robust methodology for dealing with such instances. For the Bay of Fundy, there has not been enough information to provide statistics, and as a result, the area has been masked out for data processing. In other areas minor corrections have been made, after-the-fact, in order to maintain consistency between historical dates and also between products.
Areas of fast ice in this atlas now contain a stage of development. Since 2004, this information has been added at the time of the chart production; however, for charts before that time, a senior forecaster provided stages of development for areas of fast ice based primarily on Freezing Degree Days (FDDs), amongst other meteorological parameters for the period.
The data itself is analyzed with GIS software and using well-established customized scripts to produce the various statistical outputs. Once the original vector data is assigned a historical date, it is then converted to a raster data format at 1 km resolution. Various algorithms then perform operations to statistically summarize the individual ice charts and output the climatological products seen in the atlas.
In preparing an ice atlas, medians rather than averages are used. If one considers a single data point near the edge of the fast ice in late spring, the ice conditions can be ten tenths when fast ice is present or open water after the ice breaks up. Rarely will the four to six tenths range of ice concentrations occur, which is the inevitable result if one averages between no ice and ten tenths. A median on the other hand will be either zero or ten tenths depending on the relative frequency of break-up before or after the given date. This is more appropriate for an atlas describing ice conditions. With a thirty-year time period, an even number of values are used for each particular grid point and the higher of the two middle values is chosen as the median, a policy that has been adopted since the production of the Hudson Bay and Approaches atlas in the early 1980s.
Definition of Sea Ice Climatic Charts
The ice charts contained within this atlas are derived climatological products representing a 30-year “normal” of various ice parameters. Two key statistical terms have been used to derive and describe the charts: median and frequency. The “median” is a statistical technique used to examine a dataset and is calculated by ordering all the values of the dataset from smallest to largest and selecting the middle value of an odd-numbered dataset or the average of the two middle values in an even-numbered dataset. For this atlas, the middle value of the even-numbered dataset was considered to be the upper observation, thus avoiding the averaging situation for an even-numbered dataset. The median is employed with ice statistics due to the ordinal nature of the ice attributes. For example, 9+/10 ice concentration is greater than 9/10 concentration and first-year ice is greater (thicker) than grey-white ice.
The median is more appropriate than the average or mean when considering ice attributes. The example cited in the Methodology section of a fast ice edge where, during the break-up season, concentration values at a single point over a number of years are either 10/10 or less than 1/10 may be used to illustrate why the median is more appropriate. Consider the following dataset of 5 observations of ice concentration in tenths: (10, 10, 10, 0, 0). The average value would be (10 + 10 + 10 + 0 + 0)/5 = 6/10 which would not be a “real” ice situation.
The “frequency” is another statistical technique used to examine a dataset and is calculated by summing the number of observations of an occurrence or event (e.g. the presence of sea ice or old ice) and dividing by the total number of observations for the dataset and expressed as a percent of the total number of observations.
Dates of Freeze-up and Break-up
The "Dates of Freeze-up and Break-up" depicts the extent of ice on a bi-weekly basis during the freeze-up and break-up seasons. They provide a pictorial representation of the evolution of ice during those periods.
Median of Ice Concentration
The “Median of Ice Concentration” charts consider total concentration of ice on a weekly period from November 12 to August 27. The charts do not represent any real ice season but rather a statistical composite for the period.
The charts represent the statistical “normal” ice concentration for the appropriate date.
Median of Ice Concentration When Ice is Present
The “Median of Ice Concentration When Ice is Present” charts consider total concentration of ice on a weekly period from November 12 to August 27.
The charts are a new addition to the atlas and are meant to assist in interpreting the complementary “Median of Predominant Ice Type When Ice Is Present” charts. The most appropriate way to interpret the charts is to view the median of ice concentration when ice is present in conjunction with the frequency of presence of sea ice charts. For example, at a particular point, the frequency of presence of sea ice might be in the range of 34-50% and the median of ice concentration when ice is present might be 9/10 to 9+/10. Thus, at this location, there is a 34-50% chance of encountering sea ice, and when ice is present, it is “normally” 9/10 to 9+/10 concentration. Additional insights may be provided by examining the “Predominant Ice Type When Ice Is Present” charts.
The charts represent the statistical “normal” ice concentration when ice is present for the appropriate date.
Median of Predominant Ice Type When Ice Is Present
The “Median of Predominant Ice Type When Ice Is Present” charts consider the predominant ice type (ice type of the greatest concentration in a given area) on a weekly period from November 12 to August 27.
The most appropriate way to interpret the charts is to view the median of predominant ice type in conjunction with the frequency of presence of sea ice charts. For example, at a particular point, the frequency of presence of sea ice might be in the range of 34-50% and the median of predominant ice type when ice is present might be first-year ice. Thus, at the point, there is a 34-50% chance of encountering sea ice, and when ice is present, it is “normally” first-year ice. Additional insights may be provided by examining the ice concentration when ice is present charts.
The charts represent the statistical “normal” predominant ice type when ice is present for the appropriate date.
Frequency of Presence of Sea Ice (%)
The “Frequency of Presence of Sea Ice (%)” charts consider the likelihood of total concentration of ice greater than or equal to 1/10 on a weekly basis period from November 12 to August 27 and are anticipated to give the reader an idea of the likelihood that ice will occur at a particular location for the appropriate date.
The charts can be interpreted as the “probability of sea ice being present at that time of year for the period.” The charts depict above normal extent (1 to 33%), near normal extent (34 to 66%) and below normal extent (67 to 99%). The 0% line represents the maximum extent of sea ice, beyond it no ice was reported in the period; the 100% line represents the minimum extent of sea ice, within it there has always been ice reported in the period.
Frequency of Presence of Old Ice (%)
The “Frequency of Presence of Old Ice: 1 to 10/10 (%)” charts consider the likelihood of old ice greater than or equal to 1/10 on a weekly basis from November 12 to August 27 and can provide an idea of the likelihood that old ice will occur at a particular location for the appropriate date. Please note that the previous atlas included traces of old ice whereas in this atlas it was deemed appropriate to omit traces of old ice.
The charts can be interpreted as the “probability of old ice being present at that time of year for the period.” The charts depict above normal extent (1 to 33%), near normal extent (34 to 66%) and below normal extent (67 to 99%). The 0% line represents the maximum extent of old ice, beyond it no old ice was reported in the period; the 100% line represents the minimum extent of old ice, within it there has always been old ice reported in the period.
Total Accumulated Coverage (TAC)
In this atlas, we have introduced a new parameter that depicts an entire ice season as a single value known as “Total Accumulated Coverage (TAC)”. In the years since the last publication, this has proven to be a more robust parameter than the individual observations themselves and permits comparisons of one season to another.
In order to calculate the TAC, each polygon area is multiplied by the associated total ice concentration, summed up for the entire chart, and then normalized by the total area to arrive at a single value for each chart. This value is then summed for the entire season and finally normalized by the number of weeks in a season to arrive at the single value for the season.
Weather has a direct bearing on the planning and execution of winter navigation because temperatures control the extent and thickness of ice that forms, and the surface winds modify its location, form and distribution. Winds also play a major role in the extent of the ice cover especially at the beginning of the season; strong winds can cause ice destruction when the ice is relatively thin and temporarily suppress ice development. During winter, cold air from the Canadian Arctic can be carried seaward across Eastern Canada, resulting in temperatures far below the freezing point, causing superstructure icing and rapidly increasing the volume and extent of the sea ice. On the other hand, migratory low pressure centres from the Southeastern United States may result in mild air sweeping northward and creating melting conditions that last anywhere from a few hours to several weeks. The winter seasons vary considerably in severity depending upon the relative frequency and the paths of these migratory storm systems.
In considering ice formation, ice growth and ice deterioration, the amount of heat exchange between ice, water and air is of basic importance. However, due to the complexity of these processes and their measurement, air temperature is often used to quantify the effect of freezing and melting conditions. More specifically, when the mean air temperature for a day is below 0°C, the numerical value can be expressed as the number of Freezing Degree-Days (FDD) and, when above 0°C, expressed as Melting Degree-Days (MDD).
The main oceanographic factors influencing the ice regime are bathymetry, currents, and tides.
The bathymetry of these areas is reasonably well known. The Gulf of St. Lawrence has a deep trench, known as the Laurentian Channel, running from Cabot Strait to the Saguenay River with depths of 500 m decreasing to 200 m above Rivière du Loup. The Saguenay River itself has water depths of 90 to 275 m.
There is an extension of this deep trench into Jacques Cartier Passage and the Northeast Arm of the Gulf with water depths of 175 to 275 m. The southwestern part of the Gulf averages less than 75 m in depth and the limiting water depth in the Strait of Belle Isle is 50 m. Northumberland Strait also has shallow water depths running between 17 and 65 m with the deepest waters located at each end of the strait. The fishing banks south and east of Nova Scotia are relatively shallow with water depths mostly between 50 and 90 m.
The Grand Banks to the east-southeast of Newfoundland are very well known and have average depths of about 75 m. To the northeast between Fogo Island and the Strait of Belle Isle, depths are somewhat greater averaging over 300 m but there are a few small banks with depths less than 200 m.
Along the Labrador coastline, 50-100 km from shore there is a “marginal trough” with depths ranging from 200-800 m. Farther offshore there are a series of broad banks with minimum depths in the 100-200 m range. The continental shelf extends 150-175 km from shore.
The general water motion over these areas is relatively simple but the details are complicated. In the Gulf of St. Lawrence the current is generally counter-clockwise. In the Estuary of the St. Lawrence River there is a net eastward current but superimposed on it are tidal streams that alternately accelerate and decelerate the motion. The current is strongest at 2 to 12 nm offshore of the Gaspé Peninsula and has a mean speed of 6 to 10 nm per day. Once into the main portion of the Gulf, the water spreads over the Madeleine Shallows and drifts generally towards Cabot Strait but some portions also follow the deep Laurentian Channel directly across the Gulf. After reaching the vicinity of Cape Breton Island, the current, known as the Cape Breton Current, pours around Cape North at speeds of 5 to 7 nm per day, sweeps through Sydney, and dissipates on the Scotian Shelf off Scatarie Island. Typical rates of motion over the Madeleine Shallows (area between Prince Edward Island and Iles de la Madeleine) of 3 to 5 nm per day. There is a northeastward-flowing current, having a mean speed of 2 to 4 nm per day, flowing along the west coast of Newfoundland past Bay of Islands and Daniel’s Harbour.
The general water motion in the offshore areas of Southern Labrador and East Newfoundland is dominated by the cold Labrador Current. Off the Labrador coast, the southward motion is mainly confined to the continental shelf and the water is coldest in the upper layers near shore. After passing Hamilton Inlet, just as the continental shelf widens, so does the breadth of the current. As a result, it decelerates and floods eastward over the Grand Banks while portions of the current continue southwestward from Cape Race towards Nova Scotia. In the Belle Isle-Newfoundland area, surface currents are usually less than on the Labrador coast, and the drift westward from Cape Race towards Nova Scotia waters is even slower. Through the Strait of Belle Isle is a variable tidal stream which complicates the water motion, but overall there is a significant current flowing into the Gulf of St. Lawrence with a mean speed of 6 to 8 nm per day.
Along the northern Labrador Coast, rates of motion are in the range of 8 to 10 nm per day but these speeds can vary from one season to the next or one year to the next.
The tidal ranges on the Labrador and Newfoundland coasts are fairly small but consistent, for, at most locations the mean range is from 0.8 to 1.6 m. In the Gulf of St. Lawrence the situation is somewhat more complicated because the tidal surge enters from both Cabot Strait and the Strait of Belle Isle. The main tidal surge progresses in a counter-clockwise manner around the Gulf after entering at Cabot Strait and mean ranges vary from 0.8 to 1.1 m at Cape North and Cape Ray to 1.2 to 1.5 m on the west coast of Newfoundland and along the north shore of the Gulf. In the Estuary, ranges increase progressively towards the southwest from 2.5 m inthe Pointe-des-Monts to Mont-Joli area to about 4.1 m near Quebec City. In Chaleur Bay the tidal range is from 1.3 to 2.0 m but in the Iles de la Madeleine only 0.7 m. Northumberland Strait has a complicated tidal pattern. In the west end there is essentially one tide per day while in the eastern section there are the normal two with ranges of 1.2 to 1.8 m. The Strait of Belle Isle has tides in the 0.8 to 0.9 m range.
The major effect on the ice from these tidal forces and tidal streams is that the ice moves back and forth as the tides rise and fall. It is most apparent in the upper Estuary but is also apparent in Chaleur Bay and its approaches Fast ice in these areas tends to be limited due to the constant motion.
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