Sea Ice Index

July 1, 2013: The Sea Ice Index team has replaced the 22-year base period with a 30-year period.
Read more.

June 1, 2013: New June weather mask released: Removing Residual Weather Effects from the NRTSI Product

The Sea Ice Index provides a quick look at Arctic- and Antarctic-wide changes in sea ice. It is a source for consistent, up-to-date sea ice extent and concentration images and data values from November 1978 to the present.

Table of Contents

  1. Summary
  2. Overview of Sea Ice Index Processing
  3. Detailed Data Description
  4. Coverage and Resolution
  5. Additional Information on Accessing and Using Images and Data
  6. Additional Information on Source Data, Processing, Algorithms, and Accuracy
  7. References and Related Data Resources
  8. Contacts, Product History, and Acknowledgments
  9. Document Information

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Citing These Data

Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated daily. Sea Ice Index. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Download as PDF

Sea Ice Index Documentation (PDF 1.0 MB)

Overview

Parameters

Sea Ice Concentration
Used to make spatial images of ice extent and concentration, data files of hemisphere-wide ice extent and ice-covered area over time, and times-series graphs of sea ice extent.

Spatial Coverage & Resolution

Spatial Coverage: Sea Ice Index images and data are based on gridded concentrations that come from a product with a spatial coverage that is north of 30.98° N for the northern hemisphere and south of 39.23° S for the southern hemisphere.

Spatial Resolution: The gridded concentration data used by the Sea Ice Index have a nominal grid cell size of 25 km. However, this is not the same as the resolution of the satellite instrument sensor channels used to create those data: that resolution ranges from about 28 to 69 km, depending on frequency. For more information, see the Spatial Coverage and Resolution section of this document.

Temporal Coverage & Resolution

November 1978 to present; monthly and daily products.

Projection/Grid Description Sea Ice Index images and alphanumeric data are based on gridded concentration data in a polar stereographic projection. For more information, see the Polar Stereographic Projections and Grids Web page.

Data Format

Monthly and Daily Images: Portable Network Graphics (.png)
Data Files: ASCII text (.txt) and comma delimited text files (.csv)
GIS Compatible Files: Shapefiles (.shp)
Google Earth™ Compatible Files: Zipped keyhole markup language files (.kmz)

File naming convention

See Table 6 for a description of the naming convention variables.

Monthly Image and Data Files:
Images: h_yyyymm_type.png
Graphs: h_mm_plot.png
Data Files: h_mm_area.txt
Google Earth: NSIDC_SeaIceExtent.kmz

GIS Compatible Shapefiles:
extent_h_yyyymm_polyline.zip
extent_h_yyyymm_polygon.zip
median_h_mm_yyyy_yyyy_polyline.zip

Daily Images and Data Files:
Images: Not archived
Graphs: Not archived
Data Files: hh_seaice_extent_nrt.csv and hh_seaice_extent_final.csv
Climatology: hh_seaice_extent_climatology_1981-2010.csv

Metadata Access

View Metadata Record

Data Access

Monthly data and images are available via FTP.
Daily images are not archived but data are. However, the most recent images are available from the Sea Ice Index Web site.
For information on tools for viewing and animating the Sea Ice Index data, see the Data Access and Tools section.

1. Summary

Sea Ice Index images depict ice cover and trends in ice cover in the Arctic and Antarctic oceans. Sea Ice Index data files tabulate ice extent in numbers. The images and data are produced in a consistent way that makes the Index time-series appropriate for use when looking at long-term trends in sea ice cover. Both monthly and daily products are available. However, monthly products are better to use for long-term trend analysis because errors in the daily product tend to be averaged out in the monthly product and because day-to-day variations are often the result of short-term weather.

Sea Ice Index products are derived from two data sets: the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081) and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051). These satellite passive microwave-derived data sets are used to generate the daily and monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present. Information on the accuracy and precision of passive microwave-derived sea ice concentration products can be found in this documentation as well as in the documentation for the NRTSI and GSFC products.

Monthly images show sea ice extent with an outline of the median extent for that month for comparison. Other monthly images show sea ice concentration as well as trends in concentration for that month. Monthly extent products are also available as geographic information systems (GIS) compatible shapefiles. Plain ASCII text data files contain monthly mean extent and area, in millions of square kilometers, by year. These monthly extent numbers are used for graphs of extent anomalies with trend lines and significance intervals. Anomalies and median extent are calculated using a 30-year reference period of 1981 through 2010.

Daily images show sea ice extent, with an outline of the median extent for that day for comparison, and sea ice concentration. Plain ASCII text data files contain daily extent, in millions of square kilometers, for almost every day from 1978 onward. These daily extent numbers are used for graphs of daily extent over the last four months.

The daily images of sea ice extent and concentration, along with a time-series graph of extent for the most recent four months, can be downloaded from the Sea Ice Index product site in Portable Network Graphics (PNG) format but are not archived. Daily extent data values in ASCII comma separated value text files are also available and are archived on the NSIDC FTP server.

The daily products are good for tracking the seasonal growth and retreat of ice, but there are more accurate views of sea ice on any given day. For example, the Multisensor Analyzed Sea Ice Extent (MASIE) products show daily extent at 4 km resolution and are distributed in partnership with the operational National Ice Center (NIC). For more information about current conditions and their significance, see Arctic Sea Ice News and Analysis.

2. Overview of Sea Ice Index Processing

2.1 Introduction

Records of sea ice extent and concentration from satellite passive microwave brightness temperature data are available beginning in October 1978 with the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR); and, since July 1987, from a series of Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) instruments on Defense Meteorological Satellite Program (DMSP) satellites. Sea ice concentration can be estimated from brightness temperature data because sea ice and water have differing passive microwave brightness temperature signatures. For example, water has a highly polarized signature within a certain frequency band (that is, its brightness temperature in the vertical channel is higher than that in the horizontal), while sea ice does not. Most algorithms use some form of a polarization difference or ratio and a linear mixing formula with brightness temperature tie points to estimate the concentration of sea ice within the field of view (FOV) of the sensor. Sea Ice Index processing converts gridded ice concentration estimates from the NASA Team algorithm to images and numerical data files.

2.2 Mean Concentration Fields and Median Ice Edge Position

On monthly extent images, ice ends and water begins where the concentration estimates of grid cells in the gridded average, or mean, concentration field for that month drop below 15 percent. To compare this edge position with what is typical for the month, based on the January 1981 to December 2010 portion of the data set, we computed a median edge for the month using those grid cells for which there is a 50 percent probability of ice occurring at 15 percent concentration or greater. This appears as a pink line in the monthly extent images. Alternatively, we could have computed a climatological edge by averaging the 30 mean concentration fields for that month between 1981 and 2010, and using the 15 percent cutoff in the average concentration image as the climatological edge. However, this method results in an average or mean edge that is unlikely to resemble any typical ice edge, because the location of the edge varies considerably from year to year. The median is a more meaningful representation.

On daily extent images, the same method was used to get the median ice edge position for each day of the year, using the same base period. This appears as an orange line in the daily extent images.

2.3 Data Sources

All Sea Ice Index images and data are derived from daily gridded sea ice concentration that come from two sources: The Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product) data created at and distributed from NSIDC with data set ID NSIDC-0081 and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product) created at the Goddard Space Flight Center (GSFC) and distributed from NSIDC with data set ID NSIDC-0051.

These source data sets are created using the NASA Team algorithm (Cavalieri et al. 1997) which converts brightness temperatures to gridded ice concentration estimates. For more information, see the NASA Team Sea Ice Algorithm document on the NSIDC Web site.

The documentation for the GSFC product and the NRTSI product have more information on differences in the processing of the GSFC and NRTSI products, along with information on instruments, data acquisition methods, and derivation techniques used. See the following documents:

The GSFC product is more tightly quality controlled; and for this reason, NSIDC considers it the final authoritative SMMR, SSM/I, and SSMIS passive microwave sea ice concentration record. However, NSIDC does not receive this product from GSFC until roughly a year to a year and a half after the data are acquired. We use the NRTSI product to fill the gap until the final GSFC product data are available. NRTSI data are processed at NSIDC like GSFC data are processed at Goddard, but the brightness temperature data source for the two products is different along with other small differences. For more information, see the Consistency of the Data Record section of this document.

Sea Ice Index code runs every day to make the daily products and once a month, a few days after the end of each month, for the monthly products. Index processing code uses the NRTSI product. When GSFC data become available, we manually run the Index processing code to reprocess the NRTSI version with the GSFC version. The data_type column in the monthly extent and area data files indicates the source of that row's extent and area. Daily data files from the GSFC product (hh_seaice_extent_final.csv) end about a year and a half ago and daily data files from the NRTSI product (hh_seaice_extent_nrt.csv) take up where the GSFC daily files end. Data from Goddard are delivered to NSIDC about every 18 months.

2.4 Instrument Description

The GSFC and NRTSI product data come from the Scanning Multi-channel Microwave Radiometer (SMMR) instrument on the Nimbus-7 platform and from a series of Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) instruments on the Defense Meteorological Satellite Program (DMSP) satellites. Table 1 lists the satellite platform and instrument along with the time period over which brightness temperatures from that instrument were used in the processing of the input GSFC product and the NRTSI product.

Table 1. Usage Period for Each Instrument
Platform and Instrument Time Period
Nimbus-7 SMMR 26 October 1978 - 08 July 1987
DMSP-F8 SSM/I 09 July 1987 - 02 December 1991
DMSP-F11 SSM/I 03 December 1991 - 04 May 1995
DMSP-F13 SSM/I 05 May 1995 - 31 December 2007
DMSP-F17 SSMIS 01 January 2008 - present

For a complete description of each sensor, see the following links:

For more information on the DMSP suite of satellites see the Defense Meteorological Satellite Program (DMSP) Satellite F17 Web page.

2.5 Overview of Processing Steps

The Sea Ice Index Daily Product Processing flow chart (Figure 1) and the Monthly Product Processing flow charts (Figures 2, 3, and 4) illustrate the full processing sequence. Green blocks are Sea Ice Index data files that can be downloaded, although not all are archived permanently. The data set IDs NSIDC-0001, NISDC-0080, NSIDC-0051 and NSIDC-0081, correspond to the following NSIDC data sets: DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures, Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures, Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product) and Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product), respectively. The equivalent Sea Ice Index NSIDC data set ID number is G02135. For in-depth processing details for each product, see Section 3: Detailed Data Description.

Sea Ice Index Daily Processing Flow Chart
Figure 1. Sea Ice Index Daily Product Processing Flow Chart. Click image for high-res version.
Sea Ice Index monthly processing using NRTSI product flow chart
Figure 2. Sea Ice Index Monthly Product Processing Using NRTSI Product. Click image for high-res version.
Sea Ice Index monthly processing using GSFC product flow chart
Figure 3. Sea Ice Index Monthly Product Processing Using GSFC Product. Click image for high-res version.
Sea Ice Index processing for 1979-2000 median and means flow chart
Figure 4. Sea Ice Index Processing for 1981-2010 Median and Means. Click image for high-res version.

3. Detailed Data Description

This section describes each image and data file in detail including a file description, file format, file naming convention, processing steps that NSIDC takes to create the file, and directions on how to access the file.

Use the following list to navigate this section:

3.1 Monthly Image and Data Files

These images and data files present ice extent and area averaged over a month. As each month concludes, NSIDC runs a processing script that first creates a monthly average gridded concentration field from the daily gridded NRTSI field. That, in turn, is used to make the sea ice concentration and extent images and to arrive at that month's ice extent and ice area numbers.

Each image contains a data type label on the left hand side of the image. The two data types are near-real time and final. If the image says near-real time data, then the image was created from the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081). If the image says final data, then the image was created from the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051).

All monthly images are provided in PNG format (.png), and data files are in ASCII text format (.txt). See each section below for specifics about that file.

3.1.1 Monthly Sea Ice Concentration Images

The concentration images, like those in Figures 5a and 5b, show a particular month's ice concentration with each 25 km data cell color-coded in shades of blue to white, where dark blue is zero percent ice (ocean) and white is 100 percent ice. The area around the North Pole that is not imaged by the satellite is often called the pole hole. There is no way to know what the actual concentration is within this area, so the area is left out of the images (dark grey circle). The SMMR pole hole appears on SMMR images, and the SSM/I pole hole appears on SSM/I and SSMIS images. The SSMIS pole hole is slightly smaller; but to simplify the NRTSI product processing, the SSM/I pole hole is used.

File Naming Convention

h_yyyymm_conc.png
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain gridded input data.
    Concentration images are constructed from two sources: the daily portion of the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product); and the daily Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product). The GSFC product approximately covers all but the last 18 months of data, and the NRTSI product is used to extend the record to present. The Data Sources section of this document explains why the two sources are needed. For specific information on how these sea ice concentration data sets are produced, refer to each data set's documentation by visiting the links provided above.

  2. Derive gridded monthly mean concentrations for near-real-time data.
    NSIDC constructs a month's mean from the daily gridded data mentioned in step 1. To do this, the record of daily concentration at each grid cell is averaged to produce an intermediate Sea Ice Index version of the monthly gridded ice concentrations. When the NRTSI data is used, an intermediate NRT SII monthly product is created (blue box in Figure 2), and when the GSFC data is used, an intermediate final SII monthly product is created (blue box in Figure 3).

    Occasionally, data from one or more days within the month are missing for a given 25 km grid cell. Monthly averages are computed from the daily product data for that cell only if there are at least 20 days of data for that cell. Otherwise, the cell is labeled as missing (colored dark grey in the images). Note: The first instrument that provided data in this series, SMMR, only operated every other day, resulting in half as much data as for SSM/I and SSMIS instruments. For this reason, GSFC product processing only requires 10 days of data each month for computing the mean.

    A threshold value of 15 percent is used as a cut off for the lowest ice concentration: values less than 15 percent are too uncertain to use. Values less than 15 percent are considered to be 0 percent ice and are colored dark blue in the images. Values greater than 100 percent are considered to be 100 percent ice and are colored white in the images.

    Note that there may be small differences in the monthly mean concentration grids from the NRTSI product and those from the GSFC product that later replace them. These result from differences in processing discussed in the Additional Information on Source Data, Processing, Algorithms, and Accuracy section of this document.

    Be aware that in constructing monthly mean ice concentrations, a temporal and spatial average are conflated. That is, a mean concentration of 50 percent for a particular grid cell in September could be the result of 100 percent concentration at that location for 15 days and 0 percent concentration for 15 days, or 50 percent concentration for 30 days, or some other combination. The meaning of average concentration can therefore be ambiguous, especially near the ice edge, where wind can move ice rapidly into and out of the area covered by a grid cell
  3. Compute sea ice area.
    The area in the bottom margin of the ice concentration image is the sum of the area covered by ice. For a description of how this is computed, see processing Step 3: Compute the sea ice area under Monthly Sea Ice Extent and Area Data Files.

    Note: The area value is always less than the extent value because extent includes the entire expanse within the ice edge and includes the area under the pole hole, while area takes ice concentration within that edge into account and excludes the area under the pole hole. For a complete discussion on the difference between extent and area, see the Frequently Asked Questions on Arctic Sea Ice Web page: What is the difference between sea ice area and extent?
  4. Create image from data.
    Sea Ice Index processing maps the monthly concentration fields from the NRTSI and GSFC products as a color-coded image, with a color bar in shades of blue to indicate concentration. The color bar is in 5 percent increments, even though concentration of less than 15 percent is mapped as water, not ice, and accuracy may not be as good as 5 percent. Subtle changes in shade are probably not significant. Missing sea ice concentration data, whether for a region, an individual cell, or for the pole hole, appear as dark grey dots or areas in the image.
Access Files

To access the concentration images, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Images
April 2010 Northern Hemisphere Sea Ice Concentration April 2010 Southern Hemisphere Sea Ice Concentration
Figure 5a. Monthly Sea Ice Concentration Image for the Northern Hemisphere for April 2010. Note the dark gray circle in the center is the SSM/I pole hole.(N_201004_conc.png) Figure 5b. Monthly Sea Ice Concentration Image for the Southern Hemisphere for April 2010 (S_201004_conc.png)

3.1.2 Monthly Sea Ice Extent Images

These images, like those in Figures 6a and 6b, show the expanse covered by ice at greater than 15 percent monthly mean concentration for both the Arctic and Antarctic. The median line (in pink) shows a typical ice extent for that month, based on a 30-year climotology from 1981 to 2010. We assume that the area around the North Pole that the satellite does not image, called the pole hole, is covered by ice at greater than 15 percent concentration; so it is shown as ice-covered in extent images. Note: Monthly extent is a better measure of conditions from year to year than is daily ice extent; thus, when doing climatological studies, use the monthly data.

File Naming Convention

h_yyyymm_extn.png
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain gridded input data.
    Obtain the monthly gridded concentration data for the month just completed. See the Monthly Sea Ice Concentration Images section of this document for a description of how these gridded concentrations are created.
  2. Determine extent from concentration.
    The extent is mapped by taking any cell with a concentration of 15 percent or greater and labeling that cell as ice (colored white in the images).
  3. Compute extent.
    For a description of how the extent number in the bottom margin of the image is computed, see the Monthly Sea Ice Extent and Area Data Files section of this document.
  4. Overlay the month's median ice edge position.
    The monthly median ice edge (pink line) is where median concentration, based on the January 1981 to December 2010 portion of the data set, drops below 15 percent. One way to think of this is that, within the pink line, there is a greater than 50 percent probability of ice being present for that month; while outside the line, the probability is less than 50 percent, based on conditions from 1981 through 2010.

    This line is provided as a quick visual comparison with the most recent month's mean ice extent. For more information about this ice edge, see the Mean Concentration Fields and Median Ice Edge Position section of this document.
  5. Create image from data.
    Areas labeled as ice are colored white, ocean is dark blue, and land is grey. The median ice edge is shown in pink.
Access Files

To access the extent images, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Images
April 2010 Sea Ice Extent for the Northern Hemisphere April 2010 Sea Ice Extent for the Southern Hemisphere
Figure 6a. Monthly Sea Ice Extent Image for the Northern Hemisphere for April 2010 (N_201004_extn.png) Figure 6b. Monthly Sea Ice Extent Image for the Southern Hemisphere for April 2010 (S_201004_extn.png)

3.1.3 Monthly Sea Ice Concentration Anomaly Images

These images of anomalies in ice concentration, like those in Figures 7a and 7b, show, in percent, how much the ice concentration for the month differs from the mean calculated for that month over the 1981 to 2010 time range. The total anomalous area of sea ice for that month is also shown in the bottom margin of the image. The SMMR pole hole appears on these Arctic images (the large white circle with no anomalies seen in Figure 7a), because we derive the average concentration over a period that includes SMMR data.

Note: These images should be used with caution because the accuracy of passive microwave derived sea ice concentration is not good when taken over the relatively small area of a grid cell (Fetterer 2002). What appear to be large anomalies near the ice edge are often simply artifacts of the movement of the ice edge.

File Naming Convention

h_yyyymm_anom.png
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain gridded input data.
    The input data for these images are monthly mean concentration fields and monthly climatological fields from 1981 to 2010. The monthly concentration data are described in the Monthly Sea Ice Concentration Images section of this document, and the monthly climatological fields are described below.
  2. Derive monthly climatology gridded fields.
    These are created by averaging the monthly concentrations for a given month from 1981 to 2010. This is done for all twelve months.
  3. Derive concentration anomalies.
    To produce the concentration anomaly images, the monthly climatology gridded fields for the month in question are subtracted from the monthly sea ice concentration data . The result is a gridded field of concentration anomalies.
  4. Create image from data.
    The gridded concentration anomalies are mapped to shades of red (for positive concentration anomalies) or blue (for negative concentration anomalies).If the computed trend is not significant to within a 95 percent confidence interval, the grid cell is shown as white. Note: An area may have a positive anomaly for a given month while at the same time showing a negative trend in concentration over time.
Access Files

To access the concentration anomaly images, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Images
April 2010 Northern Hemisphere Sea Ice Concentration Anomalies April 2010 Southern Hemisphere Sea Ice Concentration Anomalies
Figure 7a. Monthly Sea Ice Concentration Anomaly Image for the Northern Hemisphere for June 2010 (N_201006_anom.png) Figure 7b. Monthly Sea Ice Concentration Anomaly Image for the Southern Hemisphere for June 2010 (S_201006_anom.png)

3.1.4 Monthly Sea Ice Concentration Trend Images

These images of trends in sea ice concentration, like those in Figures 8a and 8b, show if the trend in sea ice concentration at a particular grid cell location is positive or negative and give some indication of its magnitude. The SMMR pole hole appears on these Arctic images (the large white circle of no trend seen in Figure 8a), because we derive the trend in concentration over a period that includes SMMR data.

Note: While the images of concentration trends can be interesting, these images should be used with caution for two reasons. First, the accuracy of passive microwave derived sea ice concentration is not good when taken over the relatively small area of a grid cell (Fetterer 2002). Second, what appear to be large trends near the ice edge are often simply artifacts of the movement of the ice edge.

File Naming Convention

h_mm_trnd.png
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain gridded input data.
    The input data for the sea ice concentration trends are the monthly gridded sea ice concentration data files for a given month, beginning in November 1978. For a description of how these are generated, see the Monthly Sea Ice Concentration Images section of this document.
  2. Derive concentration trends.
    Least squares regression is used to calculate the trend in ice concentration for that month at each grid cell. Cells that have zero concentration are left out of the time series when calculating trends. The slope of the linear fit gives the trend in concentration at a given grid cell. This number is converted to percent difference in concentration per decade.
  3. Create image from data.
    Significant trends in concentration are mapped to shades of red (for positive trends) or blue (for negative trends). If the computed trend is not significant to within a 95 percent confidence interval, the grid cell is shown as white.
Access Files

To access the concentration trend images, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Images
April 2010 Northern Hemisphere Sea Ice Concentration Trends April 2010 Southern Hemisphere Sea Ice Concentration Trends
Figure 8a. Monthly Sea Ice Concentration Trends Image for the Northern Hemisphere for June 2012. Note the white SMMR pole hole in the center of the image (N_06_trnd.png). Figure 8b. Monthly Sea Ice Concentration Trends Image for the Southern Hemisphere for June 2012 (S_06_trnd.png).

3.1.5 Monthly Sea Ice Extent Anomaly Graphs

These graphs, like those in Figures 9a and 9b, show monthly ice extent anomalies plotted as a time series of percent difference between the extent for the month in question and the mean for that month, where the mean is based on the January 1981 to December 2010 portion of the data set.

File Naming Convention

h_mm_plot.png
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain input data.
    The input data are those in the extent column of the Monthly Sea Ice Extent and Area Data File. For a description of how the monthly extent number is computed, see the Compute the sea ice extent step, under the Monthly Extent Data Files section of this document.
  2. Compute ice extent anomaly.
    To compute the anomaly, the mean extent value for the month in question, using the period 1981 through 2010 for the mean extent, is subtracted from the numbers in the extent column of the Monthly Sea Ice Extent and Area Data Files. The anomaly in ice extent is converted to percent difference by dividing it by the 1981-2010 average and then multiplying by 100.

    Example using June 2012:
    The June mean extent, based on 1981 to 2010 data, is 11.89 million square km. This is the average of all the values in the extent column of the June Monthly Extent file (Figure 10) from the 1981 to 2010 rows. The June 2012 anomaly is the difference between 10.92 million square km (the extent value for June 2012 in Figure 10) and the 1981-2010 June mean extent: 10.92 M sq km - 11.89 M sq km = -0.97 M sq km. The percent difference is then computed: (-0.92 / 11.89) X 100 = -8.16%.

    This is continued for every June for the entire time series and then the points are plotted against time to produce the June extent anomaly graph (the black plus signs in Figure 9a). A similar procedure is applied to all other months to create an anomaly time series graph for each of them.
  3. Compute the trend line.
    Once the extent anomalies are plotted, the trend (dashed grey line in Figures 9a and 9b) is obtained by applying a simple linear regression (the slope) to the extent anomaly time series and then multiplying by 10 to give the result in percent per decade. The 95 percent confidence interval for the trend is given at the bottom of the graph. For information on the limits and applicability of linear regression as it applies to the Sea Ice Index, see the Sea Ice Index: Interpretation Resources for Sea Ice Trends and Anomalies document (Fetterer 2002).
  4. Plot data and trend line.
    The extent anomaly data points are plotted as plus signs and the trend line is plotted with a dashed grey line (Figures 9a and 9b).
Access Files

To access the extent anomaly graphs, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Images
 
Northern Hemisphere Monthly Extent Anomalies Graph Southern Hemisphere Monthly Extent Anomalies Graph
  Figure 9a. Monthly Sea Ice Extent Anomalies Graph for the Northern Hemisphere through June 2012 (N_06_plot.png)   Figure 9b. Monthly Sea Ice Extent Anomalies Graph for the Southern Hemisphere through June 2012 (S_06_plot.png)

3.1.6 Monthly Sea Ice Extent and Area Data Files

These data files are in ASCII text format and tabulate extent and area, in millions of square kilometers, by year for a given month for the entire time series. There are 12 files (one for each month) for each hemisphere for a total of 24 files. The files contain six columns of data that are described in Table 2. An example of the file is shown in Figure 10.

Table 2. Monthly Data Files Column Description
Column Description
Year 4-digit year
mo 1- or 2-digit month
data_type Input data set
Goddard: Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data
NRTSI-G: Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations
region Hemisphere that this data covers
N: Northern
S: Southern
extent Sea ice extent in millions of square km
area Sea ice area in millions of square km
File Naming Convention

h_mm_area.txt
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain the input data.
    Monthly sea ice extent and area are calculated from two intermediate versions of monthly gridded ice concentration fields: Sea Ice Index near-real-time monthly concentration fields (created from the daily NRTSI product) and Sea Ice Index final monthly concentration fields (created from the daily GSFC product). For information on these input data, see the Monthly Sea Ice Concentration Images section.
  2. Compute the sea ice extent.
    Each month's extent is obtained by summing the area covered by all grid cells that have 15 percent or greater ice concentration. We assume that the area not imaged by the sensor at the North Pole, the pole hole, is entirely covered by grid cells showing more than 15 percent concentration for that month, so the area is considered to be ice covered.

    The area of each grid cell is obtained from static reference files that are noted in the NRTSI product and GSFC product documentation. These files, psn25area_v3.dat and pss25area_v3.dat, are used when calculating the extent. Each grid cell is nominally 625 km2 (25 km by 25 km), but the area of each cell is slightly different according to the polar stereographic projection that the source data are in. The area is given by multiplying the nominal grid cell size (625 km2) by the square of the map scale at the center of the grid cell. Grid cell areas range from 382 km2 to 664 km2 for the Northern Hemisphere grid domain and 443 km2 to 664 km2 for the Southern Hemisphere grid domain. For information on this projection, see NSIDC's Polar Stereographic Projections and Grids Web page.

    Note: The extent values are useful in a temporal series, but use caution when citing the numbers apart from the time series or when comparing with values derived from other studies. Ice concentrations are sensitive to the algorithm used, and the resulting numbers for extent depend not only on algorithms but on other processing steps as well. The extent values have uncertain significance when taken individually. For example, the 15 percent concentration cutoff for extent is somewhat arbitrary. Using a 20 percent or 30 percent cutoff gives different numbers, although similar trends, for extent. For examples, see Parkinson et al (1999).
  3. Compute the sea ice area.
    The values for ice area are obtained by summing the concentration of ice within each grid cell over the entire expanse of ice. For example, if a grid cell 's area is 600 km2 and its ice concentration is 75 percent, then the ice area for that pixel would be 450 km2 (600 X 75%).

    The area is always less than extent because extent is the entire expanse within the ice edge and includes the pole hole, while area takes ice concentration within that edge into account and excludes the pole hole. For a complete discussion on the difference between extent and area, see the Frequently Asked Questions on the Arctic Sea Ice Web page: What is the difference between sea ice area and extent?

    Note: Unlike ice extent, the Arctic values for ice area do not include the area near the pole not imaged by the sensor, the pole hole. This area is 1.19 million square kilometers for SMMR (from the beginning of the series through June 1987) and 0.31 million square kilometers for SSM/I (from July 1987 to present). Therefore, there is a discontinuity in the area data values in this file at the June/July 1987 boundary. There is no discontinuity at the SSM/I and SSMIS boundary, because the SSM/I pole hole is used for SSMIS in order to simplify nsidc-0081 (NRTSI product) processing.
  4. Append values to appropriate monthly file.
    Each monthly file is updated once a year as that month concludes, unless we receive a new section of the GSFC product final sea ice concentrations, in which case, all files, both image as well as the text data files, are regenerated using the GSFC product final data. Note: When insufficient satellite data are available to process the area and extent values, -9999 is substituted for that months area and extent value.
Access Files

To access the monthly sea ice extent and area data files, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation and look for files named like those in the convention described above.

Sample Data File
Screenshot of monthly extent data file for June
Figure 10. Sample monthly extent data file for June (N_06_area.txt).

3.2 Monthly Shapefiles

The shapefiles (.shp) are geospatial vector data for use in geographic information systems (GIS) software. There are two types: extent and median edge outline. Extent shapefiles are available for Northern and Southern hemispheres as both polygons and polylines. Median shapefiles are available for Northern and Southern hemispheres as polylines only. See each section below for more information:

3.2.1 Extent Shapefiles

Sea ice extent shapefiles are available for Northern and Southern hemispheres as both polygons and polylines, like those in Figures 11a, b, c, and d, for every month in the times series beginning in November 1978. The polygon files provide a GIS compatible extent shape and the polyline files provide the outline of the extent for a particular month and year.

File Naming Convention

extent_h_yyyymm_polyline.zip
extent_h_yyyymm_polygon.zip

Each zipped file contains four files with the same naming convention as above and the following extensions: .shp, .dbf, .prj, and .shx.
See Table 6 for a description of the naming convention variables and file extensions.

Access Files

To access the extent shapefiles, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation, then choose the shp_extent directory, and look for files named like those in the convention described above.

Sample Images
Northern Hemisphere Sea Ice Extent Polygon Shapefile for April 2010 Southern Hemisphere Sea Ice Extent Polygon Shapefile for April 2010
Figure 11a. Northern Hemisphere Sea Ice Extent Polygon Shapefile for April 2010 (extent_N_201004_polygon.zip) Figure 11b. Southern Hemisphere Sea Ice Extent Polygon Shapefile for April 2010 (extent_S_201004_polygon.zip)
Northern Hemisphere Sea Ice Extent Polyline Shapefile for April 2010 Southern Hemisphere Sea Ice Extent Polyline Shapefile for April 2010
Figure 11c. Northern Hemisphere Sea Ice Extent Polyline Shapefile for April 2010 (extent_N_201004_polyline.zip) Figure 11d. Southern Hemisphere Sea Ice Extent Polyline Shapefile for April 2010 (extent_S_201004_polyline.zip)

 

3.2.2 Median Shapefiles

The polyline median shapefile, like those in Figures 12a and 12b, provides a GIS compatible ice edge position line that is typical for a month, based on median extent from the period 1981 through 2010. There are a total of 24 files: 12 (one for each month) for each hemisphere. The polylines are the same as the pink lines overlaid on the extent images. They are described more fully in the Monthly Sea Ice Extent Images section.

File Naming Convention

median_h_mm_1981_2010_polyline.zip
Each zipped file contains four files with the same naming convention as above and the following extensions: .shp, .dbf, .prj, and .shx.
See Table 6 for a description of the naming convention variables and file extensions.

Access Files

To access the median shapefiles, go to the FTP directory. Choose a month from the list of directories labeled by a 3-character month abbreviation, then choose the shp_median directory, and look for files named according the convention described above.

Sample Images
Northern Hemisphere Median Shapefile Southern Hemisphere Median Shapefile
Figure 12a. Northern Hemisphere Median Shapefile for month of April, 1981 - 2010 (median_N_04_1981_2010_polyline.zip) Figure 12b. Southern Hemisphere Median Shapefile for month of April, 1981 - 2010 (median_S_04_1981_2010_polyline.zip)

3.3 Daily Image and Data Files

All daily images and graphs are provided in PNG (.png) format; all data are provided in comma delimited ASCII text files (.csv). See each section below for specifics about that file.

3.3.1 Daily Sea Ice Concentration Images

These images, like those in Figures 13a and 13b, show the percentage of ice cover for each roughly 25-kilometer-square data cell that is more than 15 percent covered by ice on a given day. The daily images may be missing data (missing data appears as dark grey dots or areas). The area around the North Pole that is not imaged by the satellite is often called the pole hole is excluded from the area because there is no way to know what the concentration is within this area.

Caution: There is greater uncertainty in daily fields than in the monthly average fields and greater uncertainty in the concentration fields than the extent fields. Areas that appear to have low sea ice concentration often reflect only atmospheric and surface changes, including clouds and water vapor, melt on the ice surface, and changes in the character of the snow and ice surface. Day to day changes seen in the concentration images should be viewed in this light. See the section on Accuracy and Precision for more information.

Processing Steps
  1. Obtain input data.
    The input data for the most recent day's concentration image comes from the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product). Refer to that data set's documentation for specific information on how gridded sea ice concentration is produced.
  2. Create daily concentration images.
  3. Sea Ice Index processing writes the daily concentration fields from the NRTSI product out as a color-coded image, with a color bar in shades of blue to indicate concentration. The color bar is in 5 percent increments, even though concentration of less than 15 percent is mapped as water, not ice, and accuracy may not be as good as 5 percent. Missing sea ice concentration data, whether for a region, an individual cell, or the pole hole, appear as dark grey dots or areas in the image.
Access Data

These images are not archived and only the previous day can be obtained from the Sea Ice Index Web site. However, you can access similar images, NRTSI product browse images, from the NRTSI product's FTP site (click on the browse directory).

Sample Images
 
Daily Northern Hemisphere Sea Ice Concentration for May 5, 2010 Daily Southern Hemisphere Sea Ice Concentration for May 5, 2010
Figure 13a. Daily Sea Ice Concentration Image for the Northern Hemisphere for 05 May 2010. Figure 13b. Daily Sea Ice Concentration Image for the Southern Hemisphere for 05 May 2010.

3.3.2 Daily Sea Ice Extent Images

The daily extent images, like those in Figures 14a and 14b, show the extent of ocean covered by ice at any concentration greater than 15 percent for a given day. This snapshot of current ice conditions may be missing data (missing data appears as gray dots or areas). The median line (orange in images) shows a typical ice extent for that day based on data from 1981 through 2010. We assume that the area around the North Pole that the satellite does not image is covered by ice at more than 15 percent concentration.

Caution: The location of the ice edge is not accurate enough for operational purposes.

Processing Steps
  1. Obtain input data.
    The input data come from the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product). Refer to that data set's documentation for specific information on how gridded sea ice concentration is produced.
  2. Create daily extent images.
    Sea Ice Index processing takes any cell with concentration 15 percent or greater and flags that cell as ice. The extent image is then created with ice colored white, ocean as dark blue, and land as grey. Missing data, whether for a region or an individual cell, appear as grey dots or areas in the image. The median ice edge for that day, shown in orange, is added to the image.
Access Data

These images are not archived and only the previous day can be obtained from the Sea Ice Index Web site.

Sample Images
 
Daily Northern Hemisphere Sea Ice Extent for May 5, 2010 Daily Southern Hemisphere Sea Ice Extent for May 5, 2010
Figure 14a. Daily Sea Ice Extent Image for the Northern Hemisphere for 05 May 2010. Figure 14b. Daily Sea Ice Extent Image for the Southern Hemisphere for 05 May 2010.

3.3.3 Daily Sea Ice Extent Time Series Graphs

The daily extent graphs, like those in Figures 15a and 15b, provide a snapshot of changes in ice extent for the last four months. The graphs show a time series for both the Northern and Southern hemispheres (solid blue line). Both graphs also include a comparison line for the 1981 to 2010 average (solid grey line) that is plotted for the previous four months as well as the upcoming month. The light gray area around the 1981 to 2010 average line shows the two standard deviation range of the data. This serves as an estimate of the expected range of natural variability.

The graphs include lines for a selected earlier year, for comparison. These are described as follows:

Northern Hemisphere

Along with current and median extent, the daily extent graph also includes the extent during 2012 (dashed green line). As of July 2012, this was the year with the record low minimum extent. Note that during the transition across the calendar year, data from 2011 or 2013 are included for continuity of the plot with the 2012 record year. This comparison shows the substantial recent change observed in Northern Hemisphere sea ice.

Southern Hemisphere

The daily extent graph for the Southern Hemisphere shows the previous year as a comparison (dashed blue line). During the transition across the calendar, data from the year before or the year after the comparison year are included.

Processing Steps
  1. Obtain input data.
    The input data for these graphs are the last four months of data from the daily extent data files and the climatology data file for mean extent values over the last four months plus one month into the future. For a description of how the daily extent file is created, see the Daily Sea Ice Extent Data File section of this document. For a description of the climatology data, see the Daily Climatology Data File section.
  2. Reduce erroneous artifacts.
    On daily scales, these extent values can have fairly large variations, both due to real changes in ice extent from growth, melt, or from motion of the ice edge, and due to ephemeral weather and surface effects. To reduce erroneous variations, a 5-day trailing mean is used in the plot. This is calculated by averaging the extent value from a given day with the extent value from the previous four days to produce a 5-day average, so the value plotted for a day is the average of that day and the four previous days.

    Note: With a 5-day trailing average, the average value on the last day will be higher than the true daily value when ice is decreasing (during the melt season) and lower than the true daily value when ice is increasing (during freeze-up). Computing the average may change the value that is plotted, but the value in the data file stays the same. However, this may change slightly if missing data are filled. For more information, see processing Step 2: Compute Extent under the Daily Extent Sea Ice Data files section. For the 1978 to 1987 SMMR period where extent values are only available every other day, we interpolate between days to create daily time series for the plot.
  3. Plot data.
    The most recent four months of extent data are plotted in blue. The climatology is plotted in grey with its standard deviation (the light grey swath around the climatology line). The comparison line is also drawn: dashed green for Northern Hemisphere and dashed blue for Southern Hemisphere.
Access Data

These graphs are not archived, however, the daily extent value data file and daily climatology file used to make them are. To access the most recent daily extent time series graph, see the Sea Ice Index Web site. To plot different years of daily extents, see the ChArctic Interactice Sea Ice Graphing Tool.

Sample Images
Daily Northern Hemisphere Sea Ice Extent Graph for May 5, 2010 Daily Southern Hemisphere Sea Ice Extent Graph for May 5, 2010
Figure 15a. Daily Sea Ice Extent Time series. Graph for the Northern Hemisphere for 24 June 2013. Figure 15b. Daily Sea Ice Extent Time series. Graph for the Southern Hemisphere for 24 June 2013.

3.3.4 Daily Sea Ice Extent Data Files

These files provide the Arctic- or Antarctic-wide sea ice extent for each day for the entire period of record. There are two data files for each hemisphere: one provides extent computed from the GSFC product's daily gridded concentration files and the other provides extent computed from the NRTSI product's daily gridded concentration files. Daily extent from the NRTSI product begins where daily extent from the GSFC product ends. See the Temporal Coverage and Resolution section for the specific dates.

The data files are in comma-delimited ASCII text format (.csv). The first two lines of the files are header rows, and they contain six columns of data that are described in Table 3. An example of the data file is shown in Figure 16.

Table 3. Daily Data Files Column Description
Column Description
Year
YYYY
4-digit year
Month
MM
1- or 2-digit month
Day
DD
1- or 2- digit day of month
Extent Sea ice extent in millions of square km
Missing Total of missing data, in millions of square km, in regions that are not masked. Note: These missing values may include regions not covered by sea ice.
Source Data The FTP address of the input data file used to create the extent data value.
File Naming Convention

hh_seaice_extent_nrt.csv
hh_seaice_extent_final.csv
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain input data.
    The final data files (hh_seaice_extent_final.csv) have extent numbers derived from the daily portion of the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product). In these daily fields, there are often several scattered grid cells with no data due to the limitations of the sensor. These missing regions are filled in by interpolation of values from the day before and the day after during the production of the gridded concentration fields. Refer to the Filling Data Gaps section in the GSFC product documentation for more information.

    The near-real-time data files (hh_seaice_extent_nrt.csv) have extent numbers derived from the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product). These daily fields do not have missing data filled by interpolation; grid cells missing data are simply flagged with a value for missing. Before we compute extent from these daily fields, the Sea Ice Index processing code fills missing data in a way similar to that used for the GSFC product — by interpolating data from the day before and day after. The exception is at the beginning and end of the time series, when only the day after or the day before, respectively, are available.

    The way missing data are handled in the input data is noted in the Consistency of the Data Record section of this document.
  2. Compute extent.
    The final data files (hh_seaice_extent_final.csv) have values for ice extent obtained by summing the area covered by all grid cells in the GSFC product that have 15 percent or greater ice concentration. The near-real-time data files (hh_seaice_extent_nrt.csv) have values for ice extent obtained by summing the area covered by all grid cells in the NRTSI product that have 15 percent or greater ice concentration, after the Sea Ice Index processing has filled grid cells that are missing data.

    In some cases, even after interpolation, some missing grid cells remain. The area of these cells is summed and provided in the column labeled Missing. The area in this column may include ice-free areas, so the missing area cannot simply be added to the extent to obtain corrected extent. Sea ice extent values should be used with caution if there are missing grid cells.

    The most recent sea ice extent value (last line) in the near-real-time data files may change slightly the next day (day n+1), when both a day before (day n-1) and a day after (day n+1) become available in the NRTSI product gridded concentration time series, allowing interpolation to fill any missing data in day n's gridded concentration field. When missing data cells have been filled, the extent value for day n is recalculated. This may change day n's extent value slightly if the gridded concentration on day n was missing a considerable amount of data. Note: Because of this, if you download the near-real-time file (hh_seaice_extent_nrt.csv) one day and then download it again the next day, the last value in the file from the first day you downloaded it (day n), is the second to last value on the second day's download (day n+1), and this value may have changed slightly.

    We assume that the area not imaged by the sensor near the North Pole is entirely covered by ice at more than 15 percent concentration, so the pole hole area contributes to the ice extent number.

    The area of each grid cell is obtained from static reference files that are noted in the NRTSI product and GSFC product documentation. These files, psn25area_v3.dat and pss25area_v3.dat, are used when calculating the extent. Each grid cell is nominally 625 km2 (25 km by 25 km), but the area of each cell is slightly different according to the polar stereographic projection that the source data are in. The area is given by multiplying the nominal grid cell size (625 km2) by the square of the map scale at the center of the grid cell. Grid cell areas range from 382 km2 to 664 km2 for the Northern Hemisphere grid domain and 443 km2 to 664 km2 for the Southern Hemisphere grid domain. For information on this projection, see NSIDC's Polar Stereographic Projections and Grids Web page.

    The extent values are useful in a temporal series, but caution should be used citing the numbers apart from the time series or comparing with values derived from other studies. Ice concentrations are sensitive to the algorithm used, and the resulting numbers for extent depend not only on algorithms but on other processing steps as well. The extent values have uncertain significance when taken individually. For example, the 15 percent concentration cutoff for extent is somewhat arbitrary. Using a 20 percent or 30 percent cutoff gives different numbers, although similar trends, for extent. For examples, see Parkinson et al (1999).
  3. Append values to appropriate monthly file.
    The near-real-time file is updated daily. The final file is updated when new GSFC data become available, approximately every 18 months. The final data files have an extent value only every other day for the SMMR portion of the record (26 October 1978 through 20 August 1987). The 14 September 1984 ice concentration field contains bad data, so the extent value is in error. The value is provided for completeness, but should not be used in analysis. At a future date, reprocessing of Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product) will correct this date. There are no data from 3 December 1987 to 13 January 1988 due to satellite problems.
Access Files

To access the daily data click on the links below.

Sample Data File
Screenshot of Northern Hemisphere daily extent data file
Figure 16. First 16 lines of the Final Northern Hemisphere Daily Sea Ice Extent Data File (NH_seaice_extent_final.csv).

3.3.5 Daily Climatology Data Files

These files contain the daily average extent data for 1981 to 2010 and its standard deviation. These are plotted as the solid grey line and the light grey swath, respectively, in the Daily Sea Ice Extent Time Series Graphs. The data files are in comma delimited ASCII text format (.csv). The files contain five columns of data that are described in Table 4. A sample of the data file is shown in Figure 17.

Table 4. Daily Data Files Column Description
Column Description
DOY 3-digit day of year
Avg Ext Average sea ice extent in millions of square km from 1981 to 2010.
Std Deviation Standard deviation of the average extent from 1981 to 2010 in millions of square km.
File Naming Convention

hh_seaice_extent_climatology_1981-2010.csv
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain input data.
    The input for the NH and SH climatology files is the final Daily Sea Ice Extent Data File (hh_seaice_extent_final.csv) for that hemisphere. Values from 1981 through 2010 are used. See the Daily Sea Ice Extent Data file section for information on how this is processed.
  2. Fill missing extent values
    For years that are missing an extent value on a given date, the value is filled by the value from the day after, if it is available, or if not, by the extent value from the day before. This happens frequently for the data from 1979 to 1987 because the SMMR sensor only collected brightness temperature data every other day. Since sea ice extent values are highly correlated from one day to the next, this is a reasonable thing to do.

    If there are no values on the day before or day after, that day is omitted from the calculation of average and standard deviation. The climatological extent value for that day has one less data point contributing to it for the 30 years from 1981 through 2010. That is, 29, rather than 30, extent values go into the calculation. This happens for 3 December 1987 to 13 January 1988 when there are about six weeks of no data.
  3. Compute mean and standard deviation.
    Each day-of-year's average extent and standard deviation in extent are calculated from the daily extent values for that day-of-year (DOY) over the year range 1981 through 2010. Note: Leap years (there are six from 1981 through 2010) introduce a complication: DOY 60 corresponds to February 29 in a leap year, but to March 1 in other years. DOY 366 is always 366 days from the first of the year, and for most years that corresponds to January 1st of the following year, but for leap years DOY 366 is December 31.
Access Files

To access the climatology data click on the links below.

Sample Data File
Sample of Northern Hemisphere daily climatology file
Figure 17. Sample lines from the Northern Hemisphere Daily Climatology Data File. Shows the two header rows and then the first ten rows of data.

3.4 Google Earth Files

Minimum and Maximum Sea Ice Extent Google Earth Data Files

These files provide views of the September minimum sea ice extent and March maximum extent on a virtual globe such as Google Earth™. September and March mean monthly extent for each year since 1979 are provided as separate layers.

File Naming Convention

NSIDC_SeaIceExtent.kmz
See Table 6 for a description of the naming convention variables.

Processing Steps
  1. Obtain input data.
    The input for these files are the Monthly Sea Ice Extent Images.
  2. Create Google Earth files.
    Load each extent image into a .kml file and zip the file into a .kmz file.
Access Files

To access the Google Earth files, click on the links below.

Sample Image
Sea Ice Index in Google Earth
Figure 18. Sea Ice Index in Google Earth

4. Coverage and Resolution

4.1 Temporal Coverage and Resolution

The Sea Ice Index time series begins in November 1978 and extends through the present. Over this long record, different instruments have been used to capture the data. Table 1, in the Instrument Description section, lists the instruments used by the Sea Ice Index's source data products and gives the temporal coverage of the them. The temporal coverage of the source data products (as of October 2013) is:

  • GSFC Product data: 26 October 1978 through 31 December 2012. There are no data from 3 December 1987 to 13 January 1988 due to satellite problems.
  • NRTSI Product data: 01 January 2013 to present.

The temporal resolution of the Sea Ice Index is daily and monthly. Only the monthly data and images are archived. Daily data (extent values) are archived as well. Daily ice extent image files are available for the previous day only and are not archived. Daily ice concentration browse images from the GSFC product and NRTSI product are archived, however. See the NRTSI product FTP site and the GSFC product FTP site and click on the browse directory.

Note: For the SSMR portion of the record, a given polar region was, depending on latitude, only sensed every other day or less. For the SSM/I and later portions of the record, a given polar region was sensed every day.

4.2 Spatial Coverage and Resolution

Coverage, Projection, and Grid Cell Size

The GSFC and NRTSI source data products are in a polar stereographic projection and grid that specifies the spatial coverage and grid cell size. The spatial coverage of the Sea Ice Index images is simply a convenient pictorial representation of those southern and northern hemisphere grids.

In the polar stereographic projection, each grid cell is nominally 625 km2 (25 km by 25 km), but the area actually varies with latitude according to the polar stereographic projection that the source data are in. Grid cell areas range from 382 km2 to 664 km2 for the Northern Hemisphere grid domain and 443 km2 to 664 km2 for the Southern Hemisphere grid domain. For more information on the GSFC and NRTSI source data projection and grid combination, see NSIDC's Polar Stereographic Projections and Grids Web page and the documentation for the NRTSI and GSFC products.

The area of each grid cell is obtained from static reference files (psn25area_v3.dat and pss25area_v3.dat) that are part of the GSFC and NRTSI data sets. The area of each grid cell is used when summing up ice area from ice concentration for a grid cell and when summing up the the area covered by cells with concentration greater than 15 percent. The summations give hemisphere-wide area and extent values, respectively.

Source Data Resolution

Source data grid cells are nominally 25 km by 25 km, but this is not the same as the resolution of the satellite instrument sensor channels used to create source data ice concentration products. That resolution, or field of view (FOV), is an ellipsoid with a long axis that ranges from about 28 to 69 km, depending on channel frequency and instrument series. Table 5 provides the FOV for the SMMR, SSM/I, and SSMIS channels used by the NASA Team algorithm.

Table 5. FOV for SMMR, SSM/I, and SSMIS
Instrument Frequency (GHz) FOV (km)
SMMR
18.0 55 x 41 (1)
37.0 27 x 18 (1)
SSM/I 19.35 69 x 43 (2)
37.0 37 x 28 (2)
SSMIS 19.35 74 x 45 (3)
37.0 45 x 28 (3)

1.Gloersen and Barath 1977, 2. Hollinger et al 1990, 3. Northrop Grumman 2002

Spatial Coverage: The Pole Hole

The satellite-borne instruments that collect brightness temperatures do not image a circular area over the poles, due to orbit inclination. This area is referred to as the pole hole. The SSM/I pole hole is 0.31 x 106 km2 and the SMMR pole hole is 1.19 x 106 km2. In calculating northern hemisphere ice extent, we assume that the entire area of the pole hole is covered by ice at greater than 15 percent concentration. In calculating northern hemisphere ice area, however, the area is left out. Because of this, there is a discontinuity in the time series of northern hemisphere ice area recorded in the monthly extent and area data file. This discontinuity, or apparent jump in ice area, occurs in 1987 when the data source changes from SSMR to SSM/I. There is no jump in area when the data source changes from SSM/I to SSMIS, even though the SSMIS pole hole is smaller, because the SSM/I pole hole area is used for the entire series after 1987.

The northern hemisphere ice concentration anomaly and trend images (Figures 3a and 4a, respectively) have no data for the area covered by the larger SMMR hole even when they are for years after 1987, because the time series upon which these derived values are based includes the SMMR instrument.

5. Additional Information on Accessing and Using Images and Data

5.1 File Naming Convention

The specific naming convention for each file type is given in Section 3: Detailed Data Description. Table 6 describes all of the naming convention variables used.

Table 6. File Naming Convention for Sea Ice Index Files
Variable Description
h Hemisphere: N (north) and S (south)
hh Hemisphere: NH (north) and SH (south)
yyyy 4-digit year
mm 2-digit month
anom Sea ice concentration anomalies
conc Sea ice concentration
extn Sea ice extent (used in image files)
extent Sea ice extent (used in shapefiles)
final Final data (used in daily data files)
nrt Near-real-time data (used in daily data files)
plot Monthly extent anomaly plot.
trnd Sea ice concentration trends
area Sea ice area and extent (used in monthly data file)
polyline Type of shapefile. File provides an ice extent contour line.
polygon Type of shapefile. File provides ice extent.
type Type of sea ice image
anom: Extent Anomalies
conc: Concentration
extn: Extent
trnd: Concentration Trends
.csv Comma delimited ASCII text file
.dbf Attribute format file used by the shapefiles (.shp)
.kml Google Earth™ file
.kmz Zipped Google Earth™ file
.png PNG image file
.prj Projection file used by the shapefiles (.shp)
.shp Shapefile
Comes with three associated ancillary files: .dbf, .prj, and .shx
.shx Shape index format file used by the shapefiles (.shp)
.txt ASCII text file
.zip Zipped data file (data compression format)

5.2 Directory Structure

Data files, monthly image files, and shapefiles are available on the FTP site in the ftp://sidads.colorado.edu/DATASETS/NOAA/G02135 directory. Within the top level directory, G02135, there are a number of subdirectories. They are described in detail in Table 7 and shown visually in Figure 19.

FTP Directory Structure
Figure 19.
Directory Structure
Table 7. Description of FTP Directory
Directory Description
Monthly directories (mmm) Each monthly directory contains all of the monthly data and image files for that month. The are identified by a 3-character month abbreviation (mmm).
north and south Contain the daily data and climatology data files. Underneath each of these directories are a daily directory and within that is a data directory that holds the daily data files.
shapefiles Contains all of the shapefiles for this data set subdivided into monthly directories identified by a 3-character month abbreviation (mmm). Within each monthly directory is a directory for the extent shapefiles (shp_extent) and the median shapefiles (shp_median)

 

5.3 File Size

Refer to Table 8 for a listing of file types and their sizes.

Table 8. File Sizes on the FTP Archive
File Type File Size
Monthly Images 50 KB to 310 KB
Monthly Extent Data Files 2 KB to 3 KB
Daily Extent and Area Data Files Near-real-time file: ~80 KB
Final file: ~1.7 MB
Shapefiles ~3 KB to 40 KB zipped

5.4 Data Access and Tools

Obtain the most recent daily and monthly image files from the Sea Ice Index Web site. Archived monthly image files and daily and monthly data files are available via FTP. The Browse Image Spreadsheet Tool is a quick and easy way to display selected monthly images in tabular form, in order to show past images side by side, Figures 20a and 20b are examples. There is also a tool for animating Sea Ice Index images. See Table 9 for details.

Table 9. Tools for Viewing Sea Ice Index Images
Tool Description
Browse Image Spreadsheet Tool (BIST): Extent, Concentration, and Concentration Anomalies Display archived images of extent, concentration, and concentration anomalies in tabular format, with up to 12 columns (one for each month), and up to as many rows as there are years in the data set. Figure 20a.
Browse Image Spreadsheet Tool (BIST): Extent and Concentration Trends Display archived images of extent anomaly graphs with trend lines and concentration trend images in tabular format, with 1 or 2 columns (for anomaly graphs, concentration trend images, or both) and up to 12 rows (one for each month). Figure 20b.
Sea Ice Index Animation Tool Animate extent, concentrations, concentration anomalies, or concentration trend images.
BIST Tool Screen Shot
Figure 20a. A screenshot of the Sea Ice Index Extent Anomaly BIST being used to compare March and September ice extent in 1979 and 2007. Click image for a larger version.
BIST Tool Screen Shot
Figure 20b. A screenshot of the Sea Ice Index Concentration Trend BIST being used to compare March and September ice extent in 1979 and 2007. Click image for a larger version.

6. Additional Information on Source Data, Processing, Algorithms, and Accuracy

The most important consideration when making inferences about trends in sea ice over decades is to have a consistently processed, reasonably accurate data record. More accurate records are available for shorter time periods, but these cannot be used to infer long-term trends with as much confidence. Other satellite data or charts from national ice centers are better choices when one wants the best (most accurate and precise) assessment of ice conditions over a shorter time period or in a particular region. The International Ice Chart Working Group has information on national ice centers on their Participating Agencies Web page. Yet, for tracking the response of Arctic and Antarctic sea ice to changing climate, the passive microwave data sets that the Sea Ice Index relies on are a good choice.

Here we briefly discuss information on accuracy and precision and a few of the factors that can result in inconsistencies or inhomogeneities in the record.

6.1 Accuracy and Precision

Sea Ice Index versus MASIE
Figure 21. Screenshots of the Sea Ice Index daily extent (top) and MASIE daily extent (bottom) from 08 September 2011. The red ellipses show one area, between Banks Island and Victoria Island, where SII and MASIE differ significantly. MASIE is the more accurate product because of the visible band data used as input. Click image for larger version.

The accuracy of Arctic sea ice concentration at a grid cell in the source data is usually cited as within +/- 5 percent of the actual sea ice concentration in winter, and +/- 15 percent during the summer when melt ponds are present on the sea ice (GSFC Confidence Level), but some comparisons with operational charts report much larger differences (Agnew 2003, Partington et al 2003). Accuracy tends to be best within the consolidated ice pack where the sea ice is relatively thick (greater than 20 cm) and ice concentration is high. Accuracy decreases as the proportion of thin ice increases (Cavalieri 1995). See the GSFC product documentation for more information.

The accuracy of the median sea ice extent edge position for Sea Ice Index products has not been rigorously assessed. It would be difficult to do so, because ice edge is not a well-defined parameter. For our purposes, it is where source data grid cells transition from greater than 15 percent to less than 15 percent concentration. Operational services usually speak of a marginal ice zone of varying width over which concentration transitions from more than 90 percent to 0 percent. Spot checks of the sea ice edge position using a 15 percent concentration cutoff against National Ice Center (NIC) ice charts show that when there is a broad, diffuse ice edge, the NRTSI and GSFC products sometimes do not detect sea ice where the concentration can be as high as 60 percent (Fetterer 2003 poster). When the sea ice edge is more compact, the 15 percent concentration cutoff reflects its location fairly well (Fetterer 2002).

Ice concentration from low-resolution passive microwave data is not highly accurate; and for this reason, it is best not to use Sea Ice Index ice concentration images alone, out of temporal context, especially those from a single day. Ice extent images are more reliable, because the difference in emissivity between open water and sea ice, even at low concentrations, is great (Comiso and Kwok 1996). Still, the instrument's low resolution (see Table 5 for the field of views) means that the ice edge, whether it is a compact or diffuse marginal ice zone, will not be represented well. For example, the daily SII extent product for 08 September 2011 is shown in Figure 21 with the 4 km Multisensor Analyzed Sea Ice Extent (MASIE) product from the same day. MASIE resolves the ice edge with greater precision and accuracy, but it is not a long and consistently processed record. The Sea Ice Index daily product does a reasonable job, but it is evident why we place higher confidence in monthly than in daily products. Many errors due to missing data and transient weather effects are averaged out when we average daily data over a month.

6.2 Land-to-ocean spillover

Ice can be falsely detected along coasts due to contamination of ocean pixels by the passive microwave emission of land. While the nominal grid cell size of the gridded products is 25 km x 25 km, the -3dB footprint of the 19.35 GHz SSM/I passive microwave channel is 69 km x 43 km (Hollinger et al 1990). To remove spurious Northern Hemisphere coastal ice, the NRTSI product processing uses the land spillover correction used for the GSFC product and is described in NASA Technical Memorandum 104647 (Cavalieri et al, 1997) as well as in the Sea Ice Concentrations from Nimbus-7 SSMR and DMSP SSM/I Passive Microwave Data guide document. The rational behind this land spillover approach is that ice will have retreated from most coasts in late summer, so that coastal ice observed at this time by passive microwave instruments is probably a false detection. To reduce the chance of removing ice where it really does exist, the method searches for and requires the presence of open water in the vicinity of the grid cell to be corrected. The method uses the monthly data from 1992 as a basis for correcting SSM/I data and monthly data from 1984 for correcting the SMMR data. It is not foolproof, as Figure 21 illustrates. The ice between Banks Island and Victoria Island (red circled area in the images) is not shown in the MASIE product and is probably the result of land spillover.

6.3 Removing Residual Weather Effects

Weather effects can cause the passive microwave signature of seawater to appear like that of ice (Cavalieri 1995). Atmospheric water vapor is often the reason behind false ice detection. Most of these false ice signatures are removed with a standard brightness temperature filter, but some are too close to those of real ice. Sea surface temperature fields that show where water is usually too warm for ice, or maximum ice extent fields that show where ice has never been before (in the satellite record), are used to mask out residual weather effects.

The steps to remove residual weather effects are taken in the processing for the input NRTSI product and GSFC product before these data are used in the Sea Ice Index processing.

6.4 Removing Residual Weather Effects from the GSFC Product

In processing done at GSFC, a mask or filter based on monthly climatological sea surface temperature (SST) is used to remove residual weather effects from daily concentration fields before those fields are averaged to make monthly gridded concentration. For more information, see the Land Spillover and Residual Weather-Related Effects in the Processing Steps section of the GSFC product documentation. Some subjective, manual false ice removal is done as well. These steps are taken before these data arrive at NSIDC.

6.5 Removing Residual Weather Effects from the NRTSI Product

In processing done at NSIDC, filtering for the Antarctic and the Arctic is done differently.

For the northern hemisphere, a mask or filter based on monthly climatological maximum sea ice extent is used. The base period for these monthly maximum extent masks is 1978 to 2002 (there is some variation based on month). A maximum ice extent mask is used in the Arctic because, in addition to removing spurious false ice from weather effects, it sometimes removes ice that the land-to-ocean spillover filter sometimes fails to remove. The sea surface temperature mask does not have this added benefit.

In July 2013, NSIDC adjusted the ocean climatology mask for June to better filter out erroneous data along the coast and due to weather effects and remove the need for manual Quality Checks (QC). Prior to July 2013, W. Meier would manually QC the data; looking to see if a change in mask would suddenly remove or add ice as the transition from one month's mask to the next happens. This is a possibility for those months in which ice is changing rapidly, because at those times even though the mask is the 1978 to 2001 maximum for a month, the beginning or end of the month might fall outside that maximum on occasion. An example is the Bering Sea where ice has become more extensive, so the old masks risked cutting off real ice, thus the need for manual QC. However, NSIDC has now created a new mask that accommodates the higher ice extent in the Bering, thus eliminating the need for manual QC of the data. Note: A slight change in the extents and areas for the month of June will occur due to this change in masks, but the change is within the error of the data.

For the southern hemisphere, a mask or filter based on monthly climatological SST is used to remove residual weather effects from daily concentration fields. This is the same mask that GSFC uses for their Antarctic processing. For more information, see the Land Spillover and Residual Weather-Related Effects in the Processing Steps section of the GSFC product documentation. No subjective, manual false ice removal is done.

Table 10 shows key differences in the processing of daily gridded sea ice concentration data between the GSFC product and the NRTSI product. This processing takes place before these data are used by the Sea Ice Index processing code.

Table 10. Key differences in processing of daily gridded sea ice concentration data.
Processing elements GSFC Product NRTSI Product
Brightness temperature source GSFC for SMMR, NSIDC/RSS for SSM/I and SSMIS NOAA CLASS for F17 forward
Weather filter* same same
Sea ice algorithm and brightness temperature tie points same same
Land-to-ocean spillover correction* same same
Residual weather effects (false ice detection)* Removed based on monthly climatological SSTs, with associated subjective filtering Removed based on monthly climatological maximum ice extent (Arctic) or SST (Antarctic)
Missing data Filled by spatial or temporal interpolation Flagged as missing

* These processing methods are imperfect and do not remove all false sea ice detections.

6.6 Consistency of the data record

Inconsistencies or inhomogeneity in the GSFC product data record arise from problems encountered when deriving sea ice concentrations from brightness temperatures measured by sensors with slightly different orbital characteristics, frequencies, and calibrations. The techniques employed to solve these problems, or at least reduce their impacts, are covered in the GSFC product documentation.

The much shorter NRTSI product data record is the result of processing steps designed to mimic GSFC product processing to the greatest degree possible. Table 10 shows where processing is not identical.

The Sea Ice Index data record extends the GSFC product record with the NRTSI product record. Prior to 2004, K. Knowles evaluated consistency between the NRTSI and GSFC products for the year 2002 using brightness temperature data from NSIDC/RSS and near-real-time brightness temperature data from NASA Marshall Space Flight Center (MSFC) as input and then differencing monthly gridded concentration derived from NRTSI daily data with monthly gridded concentration derived from GSFC daily data. We found that the brightness temperature source made little difference in overall extent and area. The differences in the NRTSI- and GSFC-derived monthly area and extent values for the year 2002 were at most 1.6 percent (this was the area difference for June 2002) with most differences much lower or negligible.

This early work showed that joining the GSFC and NRSTI products to create Sea Ice Index products was reasonable: ice extent and area data are processed in a consistent way. Since then, the NRTSI brightness temperature source has changed to the NOAA CLASS, and more recent comparisons by M. Savoie have shown that changes in data source, from the NRTSI product to the GSFC product, can result in slight changes in the Sea Ice Index monthly extent and area values. The changes are generally less than 20,000 km2 which is equivalent to about 30 to 40 grid cells in area, or much less than 1.0 percent.

7. References and Related Data Resources

7.1 Sea Ice Index References

Fetterer, F., and K. Knowles. 2004. Sea ice index monitors polar ice extent. Eos: Transactions of the American Geophysical Society 85, 163.

Meier, W., J. Stroeve, F. Fetterer, K. Knowles. 2005. Reductions in arctic sea ice cover no longer limited to summer. Eos: Transactions of the American Geophysical Society 86, 326.

Fetterer, F. 2003. Recent Arctic Ice Extent Minima Observed with the Sea Ice Index. ARCUS Study of Environmental Acrtic Change (SEARCH) Open Science Meeting, October 27-30, 2003, Seattle, Washington.

Fetterer, F. 2002. Sea Ice Index: Interpretation Resources for Sea Ice Trends and Anomalies. NSIDC Informal Technical Report. http://nsidc.org/data/docs/noaa/g02135_seaice_index/interpretation.html.

To help users interpret the images and figures within the Sea Ice Index correctly, the Sea Ice Index: Interpretation Resources for Sea Ice Trends and Anomalies document (Fetterer 2002) discusses the variability of sea ice, the applicability of statistical methods for trend detection, and the validity of passive microwave images of sea ice.

7.2 Documentation References

Agnew, T. A., and S. Howell. 2002. Comparison of digitized Canadian ice charts and passive microwave sea-ice concentrations. Geoscience and Remote Sensing Symposium, 2002. IGARSS '02. 2002 IEEE International 1: 231- 233. doi: 10.1109/IGARSS.2002.1024996

Cavalieri, D. J., C. L. Parkinson, P. Gloersen, and H. J. Zwally. 1997. Arctic and Antarctic Sea Ice Concentrations from Multichannel Passive-Microwave Satellite Data Sets: October 1978-September 1995 - User's Guide - NASA TM 104647. Goddard Space Flight Center, Greenbelt, MD 20771, pp17.

Cavalieri, D.J., K.M. St. Germain and C.T. Swift. 1995. Reduction of weather effects in the calculation of sea ice concentration with the DMSP SSM/I. Journal of Glaciology 41: 455-464.

Cavalieri, D.J., P. Gloersen and W. J. Campbell. 1984. Determination of sea ice parameters with the NIMBUS-7 SMMR. Journal of Geophysical Research 89 (D4): 5355-5369.

Comiso, J. C., and R. Kwok. 1996. Surface and radiative characteristics of the summer arctic sea ice cover from multi-sensor satellite observations. Journal of Geophysical Research 101 (C12): 28, 397-28, 416.

Gloersen, P. and F. T. Barath. 1977. A Scanning Multichannel Microwave Radiometer for Nimbus-G and SeaSat-A. IEEE Journal of Oceanic Engineering 2:172-178.

Hollinger, J.P., J.L. Peirce, and G.A. Poe. 1990. SSM/I Instrument Evaluation. IEEE Transactions on Geoscience and Remote Sensing 28 (5): 781-790. doi: 10.1109/36.58964.

Northrop Grumman. 2002. Algorithm and Data User Manual (ADUM) for the Special Sensor Microwave Imager/Sounder (SSMIS). Report 12621. Contract No: F04710-00-C-0001. Northrop Grumman Corporation: Space Systems Division. Azusa, California.

Parkinson, C.L., D.J. Cavalieri, P. Gloersen, H.J. Zwally, and J.C. Comiso. 1999. Arctic sea ice extents, areas, and trends, 1978-1996. Journal of Geophysical Research 104 (C9): 20,837-20,856.

Partington, K., T. Flynn, D. Lamb, C. Bertoia, and K. Dedrick. 2003. Late twentieth century Northern Hemisphere sea-ice record from U.S. National Ice Center ice charts. J. Geophys. Res. 108(C11): 3343. doi:10.1029/2002JC001623.

7.3 Related NSIDC Data Resources

7.4 Related Data Sets at Other Institutions

7.5 Educational Resources

  • Sea Ice Index on Virtual Globes: View September minimum extents and March maximum extents in Google Earth™. To access these files, see the September Sea Ice Extent and Sea Ice: Minimum and Maximum Extents sections on the View NSIDC Data on Virtual Globes: Google Earth Web page. To download a free version of Google Earth, visit the Google Earth Web site.
  • Exploring the Cryosphere Using Data from the National Snow and Ice Data Center :Created by the Science Education Resource Center (SERC) through the National Science Digital Library's (NSDL) Using Data in the Classroom program, this K-12 classroom activity uses the Sea Ice Index to teach skills in cryospheric studies and climate change.

8. Contacts, Product History, and Acknowledgments

8.1 Investigators

Florence Fetterer, Walt Meier, and Matt Savoie
National Snow and Ice Data Center
University of Colorado
Boulder, CO 80309

8.2 Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

8.3 Product History

The Sea Ice Index originated with ideas discussed between Florence Fetterer, NSIDC's NOAA liaison, and Ken Knowles, senior software developer (at NSIDC 1990-2006). The Index was envisioned as a Web site that would meet a need for readily accessible, easy-to-use information on sea ice trends and anomalies, with products that would assist in monitoring and diagnosing the ice extent minima that were gaining increasing attention in the research community in the late 1990s. Ice conditions, trends, and anomalies presented graphically would give at-a-glance answers to general questions such as "Is the ice extent in the Chukchi about where it usually is this time of year?" and "Was there less ice in the Beaufort Sea last spring than is typical?". The Index was and still is intended for both researchers and the scientifically inclined general public.

The Index was developed in 1999 with financial support from the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) and in cooperation with the NOAA National Geophysical Data Center (NGDC). This first version of the Sea Ice Index, published online in 2002, did not include SMMR but started with SSM/I in 1987. For this reason, it was a short time series; and ice extent trends derived from it were often statistically insignificant. In 2000, funding from the NOAA Oceanic and Atmospheric Research (OAR) Arctic program allowed us to do the work necessary to make the SMMR record of ice concentration homogenous with that of SSM/I. This was a contribution to the Study of Environmental Arctic Change (SEARCH) research project. The new Sea Ice Index, published online in 2004, is a record that begins in 1978.

Originally the Index was not intended to be a data set on its own but to be an information product that simply repackaged existing NSIDC passive microwave sea ice data and presented them graphically as an ongoing series. However, additional processing was necessary to build a consistent, homogenous ice concentration time series from SMMR and SSM/I data and to append the NRSTI product to the GSFC product. This led to the decision to make the Index a more traditional NSIDC data set, with standard NSIDC documentation, in 2008.

Major steps in its evolution follow.

2002

The Sea Ice Index prototype site was published online. It only used SSM/I data, for which the record began in 1987. Anomalies were shown using a base period of 1988-2000.

2003

We automated the task of updating the site every month and began archiving images and tables of ice extent. NSIDC developer Julia Collins modified the Web Image Spreadsheet Tool (WIST, now the BIST), to work with Index images. The WIST, developed by NGDC's Geospatial group, makes displaying images in tabular form easy and intuitive.

2004

The SMMR record was added, extending the record back to 1978. To accomplish this, we needed to adjust our near-real-time passive microwave ice concentration data stream to be consistent with the1978-2002 record (a standard product from GSFC). This required adjusting algorithm tie points. Ken Knowles led this work, as well as that of building the Sea Ice Index processing system into a new operational environment at NSIDC. In September of 2004, approximately 2,500 users visited the site online.

2006

The color bar for the concentration anomaly and trend images was changed from rainbow to shades of red (positive anomalies and trends) and blue (negative anomalies and trends), in keeping with best practices for graphically presenting maps of these types of data. NSIDC project lead Lisa Ballagh made extent maps for September and March (months of minimum and maximum extent) available in kml format, for use with Google Earth and other virtual globes.

The NSIDC Science Communications group published Arctic Sea Ice News 2006. This "ice blog" was created to help manage increasing media attention. It became a regular feature and is now called Arctic Sea Ice News and Analysis (ASINA). The ASINA science team needed to show how Arctic-wide ice extent was changing as the summer melt season progressed, so they used the daily NRTSI gridded ice concentration fields to obtain ice extent values for the ASINA summer melt season graph, updated daily.

At that time, the daily NRTSI fields were a by-product of Sea Ice Index processing. They were used to create the most recent monthly extent and concentration fields as well as the extent and area numbers but were not retained by the Sea Ice Index. The reason for this is that the Sea Ice Index focused on tracking changes in sea ice that are climatologically significant. To do this, a long, consistently processed record is needed. Satellite passive microwave provides that record, but only when concentration fields are averaged over a month. Day to day variability in the ice concentration record can be the result of short-term weather, of imperfect algorithms, and of sensor problems.

During 2006, approximately 67,000 users visited the site; and in September of 2006, specifically, approximately 8,600 users visited the site.

2007

Arctic Sea Ice News quickly grew in popularity, sending many new users to the Sea Ice Index site. We improved the way the Sea Ice Index processing code worked within the larger passive microwave data processing environment operated by the NSIDC DAAC and interacted with the Arctic Sea Ice News team so that Sea Ice Index products would support that site. NSIDC senior software developer Matt Savoie and ASINA lead scientist Walt Meier led this work and were added to the Sea Ice Index product citation.

During 2007, approximately 108,900 users visited the site; and in September of 2007, specifically, approximately 17,900 users visited the site.

2008

During 2008, approximately 319,800 users visited the site; and in September of 2008, specifically, approximately 47,400 users visited the site.

2009

The site was redesigned. Images could now be displayed on NASA's Blue Marble view of the Earth, and the chart of daily ice extent values was included as a Sea Ice Index product. Before this, it had appeared on the ASINA site only. Walt Meier and Matt Savoie were added to the data product citation in recognition of their role effecting this change.

During 2009, approximately 320,000 users visited the site.

2010

During 2010, approximately 323,400 users visited the site; and in September of 2010, specifically, approximately 27,600 users visited the site.

2011

During 2011, approximately 266,800 users visited the site; and in September of 2011, specifically, approximately 22,300 users visited the site.

2012

The daily data files were made available through the Sea Ice Index site in order to meet the needs of users who want the data that are used for the daily chart of sea ice extent. Sea Ice Index documentation was rewritten to give a fuller, cleared explanation of processing.

During 2012, approximately 301,800 users visited the site; and in September of 2012, specifically, approximately 35,400 users visited the site.

2013

The following improvements occurred:

  • The reference period for anomalies and climatologies changed from a 22-year period (1979-2000) to a 30-year period (1981-2010). A page of supplemental information was published about this change: Baseline Period Change.
  • The June mask was updated: Removing Residual Weather Effects from the NRTSI Product
  • The SII Web site was redesigned to make it easier to navigate.
  • An FAQ page, similar to the one for the MASIE product, was added to the SII Web site.
  • Image files are now labeled with data source: NRTSI product or GSFC product.
  • The color scheme of trend and anomaly images was changed slightly to make the images easier to interpret.

During 2013, approximately 311,000 users visited the site; and in September of 2013, specifically, approximately 34,000 users visited the site.

2014

March 2014

The citation for this data product was changed from

Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated 2009. Sea Ice Index. [indicate subset used]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5QJ7F7W.

to the following:

Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated daily. Sea Ice Index. [indicate subset used]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5QJ7F7W.

The change was made to address the fact that the Sea Ice Index is a near-real-time, ongoing data set that has new data added to it everyday.

September 2014

The daily portion of the Sea Ice Index was updated for 14 September 1984 and the monthly portion was updated for September 1984 due to an error found in the input data source. This caused the daily extent number for that day to change from 7.40367 million km2 to 6.46729 million km2 for the Arctic and 18.17876 million km2 to 18.29700 million km2 for the Antarctic. It caused the September monthly extent value to change from 7.17 million km2 to 7.11 million km2 and from 18.66 million km2 to 18.65 million km2 for the Arctic and Antarctic, respectively. The correction did not cause a change in the 1981-2010 average, reported to the nearest 10,000 square kilometers. For more information on the error in the input data, see the Version History and Data Updates section of the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data documentation (NSIDC-0051).

In September of 2014, approximately 32,400 users visited the site.

Future Plans for the Sea Ice Index

As of July 2013, the Sea Ice Index team is planning updates and improvements with the larger NSIDC DAAC passive microwave team. Our team includes Ann Windnagel, NSIDC technical writer and Web architect, and Kara Gergely, NSIDC User Services representative. The passive microwave team is lead by Donna Scott, with Walt Meier as science advisor. Funding from the NOAA Climate Data Record program and the NASA DAAC support its work.

Improvements will include the following:

NSIDC does not have a set schedule for these planned improvements. Work on the Sea Ice Index depends on available funding.

8.4 Acknowledgements

Distribution of the data set from NSIDC is supported by the NOAA@NSIDC Team with funding from NOAA and with assistance from the NSIDC NASA DAAC. This site is maintained with assistance from the NSIDC NASA DAAC.

9. Document Information

9.1 Acronyms and Abbreviations

The acronyms used in this document are listed in Table 11.

Table 11. Acronyms and Abbreviations
Acronym Description
ARCUS Arctic Research Consortium of the United States
ASINA Arctic Sea Ice News and Analysis
BIST Browse Image Spreadsheet Tool
CIS Canadian Ice Service
DAAC Distributed Active Archive Center
DMSP Defense Meteorological Satellites Program
DOY Day of Year
FOV Filed of View
FTP File Transfer Protocol
GIS Geographic Information System
GSFC Goddard Space Flight Center
KML Keyhole Markup Language
KMZ Zipped KML Files
MSFC Marshall Space Flight Center
NASA National Aeronautics and Space Administration
NESDIS National Environmental Satellite, Data, and Information Service
NGDC National Geophysical Data Center
NIC National Ice Center
NOAA National Oceanic and Atmospheric Administration
NSDL National Science Digital Library
NSIDC National Snow and Ice Data Center
NRT Near-Real Time
NRTSI Near-Real-Time SSM/I Polar Gridded Sea Ice Concentrations
OAR Oceanic and Atmospheric Research
PNG Portable Network Graphics
RSS Remote Sensing Systems
SEARCH Study of Environmental Arctic Change
SERC Science Education Resource Center
SII Sea Ice Index
SMMR Scanning Multichannel Microwave Radiometer
SSM/I Special Sensor Microwave/Imager
SSMIS Special Sensor Microwave Imager/Sounder
SST Sea Surface Temperature
URL Uniform Resource Locator

9.2 Document Creation Date

October 2008

9.3 Document Revision History

  • September 2014: A. Windnagel added an Errors and Updates section.
  • October 2013: A. Windnagel updated figures 1-4 to reflect that GSFC daily data is used to create an intermediate final SII monthly product. Previously, the documentation erroneously said that the monthly portion of the GSFC product (NSIDC-0051) was used.
  • September 2013: A. Windnagel updated the Arctic portion of Table 2 in the baseline-change.html document because the month labels were off by six months.
  • June 2013: A. Windnagel updated the document for the new 30-year (1981-2010) base period. Updated documenation for the new June ocean climatology mask used to filter weather effects.
  • February 2013: A. Windnagel added text about the addition of the data type label.
  • July 2012: A. Windnagel and F. Fetterer updated documentation to cover daily processing and added a product history section.
  • May 2012: A. Windnagel did a major revision of this document for clarity. Items updated are image descriptions and processing as well as putting the text into the new guide doc template.
  • October 2011: A. Windnagel added a table containing the time period each instrument is used. This was done in response to the F-17 Goddard final data that has now been processed.
  • March 2011: A. Windnagel added information on how the shapefiles are created.
  • April 2010: A. Windnagel doing a full review/revision of this document.
  • June 2009: A. Windnagel added note about transition from F13 to F17.
  • October 2008: D. Miller took documentation from the Sea Ice Index Web site and consolidated it into a formal Sea Ice Index guide document.

9.4 Document URL

http://nsidc.org/data/docs/noaa/g02135_seaice_index/index.html