Table of Contents

1. Summary
2. Overview of Sea Ice Index Processing
3. Detailed Data Description
   • Monthly Image and Data Files
   • Monthly Shapefiles
   • Daily Image and Data Files
   • Google Earth Files
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 NSIDC Data Resources
8. Contacts, Product History, and Acknowledgments
9. Document Information

Register

NSIDC encourages you to register as a user of this data product. As a registered user, you will be notified of updates and corrections. Register.

Citing These Data

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

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 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_1979-2000.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 reference period of 1979 through 2000.

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

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.

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 1979 to December 2000 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 22 mean concentration fields for that month between 1979 and 2000, 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.

Data Sources

All Sea Ice Index images and data are derived from daily or monthly 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. The NRTSI product is available at a daily temporal resolution, and the GSFC product is available at a daily and monthly resolution.

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:

• Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081)
• Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051)

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.

Note: At this time (July 2012), the GSFC product documentation describes it as having both preliminary and final components. Only the final GSFC data are used for Sea Ice Index processing.

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 - 30 September 1995
DMSP-F13 SSM/I	01 October 1995 - 31 December 2007
DMSP-F17 SSMIS	01 January 2008 - present

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

• Scanning Multi-channel Microwave Radiometer (SMMR)
• Special Sensor Microwave Imager (SSM/I)
• Special Sensor Microwave Imager/Sounder (SSMIS)

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

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.

Because the NRTSI product is only provided at a daily resolution, the Sea Ice Index processing must create its own version of a monthly gridded sea ice concentration from the NRTSI product when the GSFC product is not available. When the GSFC monthly product is available, it is used. The differences in processing can be seen in Figure 2 (NRTSI data) and in Figure 3 (GSFC data). Figure 2 contains an extra intermediate monthly concentration product (blue box) that is not present in the GSFC data flow chart.

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:

• Monthly Image and Data Files
• Monthly Shapefiles
• Daily Image and Data Files
• Monthly Google Earth Files

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.

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.

• Monthly Sea Ice Concentration Images
• Monthly Sea Ice Extent Images
• Monthly Sea Ice Concentration Anomaly Images
• Monthly Sea Ice Concentration Trend Images
• Monthly Sea Ice Extent Anomalies Graph
• Monthly Sea Ice Extent and Area Data Files

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:

• Extent Shapefiles
• Median Shapefiles

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.

• Daily Sea Ice Concentration Images
• Daily Sea Ice Extent Images
• Daily Sea Ice Extent Time Series Graphs
• Daily Sea Ice Extent Data Files
• Daily Climatology Data Files

Google Earth Files

4. Coverage and Resolution

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 July 2012) is:

• GSFC Product data: 26 October 1978 through 31 December 2010. There are no data from 3 December 1987 to 13 January 1988 due to satellite problems.
• NRTSI Product data: 01 January 2011 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. For the SSM/I and later portions of the record, a given polar region was, depending on latitude, sensed every day.

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

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)

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.

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)

 

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

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.

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.

Accuracy and Precision

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.

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.

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.

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.

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 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.

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 2003 (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.

W. Meier does some manual QC; 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.

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.

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 NSIDC Data Resources

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.

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.

Related NSIDC Data Resources

• Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product)
• Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product)
• MASIE (Multisensor Analyzed Sea Ice Extent) daily Arctic-wide ice extent at 4km resolution
• DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures (NSIDC-0001)
• Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures (NSIDC-0080)
• Arctic Sea Ice News and Analysis (ASINA)
• Sea Ice Index on Virtual Globes
• All About Sea Ice
• Frequently Asked Questions on Arctic Sea Ice
• Sea Ice Data at NSIDC
• Greenland Icesheet Today

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, 1979-2011 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

Investigators

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

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

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 2500 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.

In September of 2006, approximately 4100 users visited the site online.

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.

In September of 2007, approximately 6400 users visited the site online.

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.

In September of 2009, approximately 9000 users visited the site online.

2010

In September of 2010 approximately 15000 users visited the site online.

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.

Plans for the Sea Ice Index

As of July 2012, 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:

• The SII Web site will be redesigned to make it easier to navigate.
• An FAQ page, similar to the one for the MASIE product, will be added to the SII Web site.
• The color scheme of images will change slightly to make them easier to interpret.
• The reference period for anomalies and climatologies will change from the current 22-year period (1979-2000) to a 30-year period (1981-2010).
• Image files will be labeled with data source (NRTSI product or GSFC product).

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

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

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

Document Creation Date

October 2008

Document Revision History

• 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.

Document URL

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