Data Description

Parameters

The main parameter for this data set is sea ice age, measured in years.

File Information

Format

Data are provided in georeferenced netCDF (.nc) format. 

PNG (.png) browse images are also provided.

File Contents

Each netCDF file contains 52 weeks of sea ice ages, coded as integers in a 722 x 722 gridded subset of the 12.5 km Northern Hemisphere EASE-Grid. Table 1 lists the coded integer values and their meanings. 

Sample Browse Image

One browse image displaying sea ice age is provided for every week of data. Figure 1 shows a sample browse image.

Directory Structure

Data are available for download via HTTPS; the link is accessible through the "Download Data" tab. Within the file directory, there are two folders, data and browse. The data folder contains the sea ice age netCDF files, while the browse folder contains the PNG browse images, which are further subdivided into folders by year.

NetCDF File Naming Convention

The data files are named according to the following convention and as described to Table 2:
iceage_hh_rrrr_<start-date>_<end-date>_v##.nc

Example:
iceage_nh_12.5km_19840101_19841231_v4.1.nc

Browse Image File Naming Convention

The browse images are named according to the following convention and as described to Table 3:
iceage_hh_rrrr_<start-date>_<end-date>_v##.png

Example:
iceage_nh_25km_20000101_20000107_v4.1.png

File Size

NetCDF files are approximately 3.5 - 4.0 MB.

Browse images are approximately 400 KB.

Spatial Information

Coverage

This data set covers the Arctic Ocean within the boundaries defined below:
Southernmost Latitude: 48.4° N
Northernmost Latitude: 90.0° N
Westernmost Longitude: 180.0° W
Easternmost Longitude: 180.0° E

Resolution

12.5 km

Geolocation

Data are projected using a 12.5 km Northern Hemisphere EASE-Grid. The grid is shifted one-half grid cell relative to the standard version of EASE-Grid, which have the center of the grid right over the pole. More details can be found in tables 4 and 5 below. More details on EASE-Grid can be found on the EASE Grids website.

Temporal Information

Coverage

January 1984 to December 2018

Resolution

Weekly

Data Acquisition and Processing

Background

The method used to estimate sea ice age involves Lagrangian tracking of sea ice from week-to-week using gridded ice motion vectors (Maslanik et al. 2011; Tschudi et al. 2019). Starting in late 1978, ice age can be estimated by treating each grid cell that contains ice as a discrete, independent Lagrangian parcel and tracking the parcels at weekly time steps as they are advected by the weekly ice motions. The process can be viewed as a set of stacked planes overlying the grid used, with each plane corresponding to an age category. Parcels move around on their respective planes, independent of parcels of other age categories, which in turn lie in their own planes. To produce maps of ice age, the set of parcels for each weekly time increment is rasterized by assigning parcels to the 12.5 km x 12.5 km grid cell within which each parcel's position lies. In cases where parcels of different ages fall within a single grid cell, the age of the grid cell is assigned to the oldest parcel (Maslanik et al. 2011; Tschudi et al. 2019). Physically, this approach assumes that younger ice deforms more easily than older ice, and as such older ice will cover a greater fraction of the area within the grid cell. For example, if two parcels, one that represents first-year ice and one that represents third-year ice, both fall within the domain of a single grid cell, then the age of that cell will be assigned as third-year ice.

If the ice concentration of a grid cell remains at or above 15 percent throughout the melt season, then that parcel is assumed to have survived the summer minimum sea ice extent (typically reached in September), and the parcel's age is incremented by one year. The age of the ice is categorized as first-year ice (0-1 years old), second-year ice (1-2 years old), and so forth based on how many summer melt seasons the ice parcel survives (Tschudi et al. 2010). Note that grid cells with less than 15 percent sea ice concentration are treated as open water, even though the cells could still contain some ice.

Acquisition

The input ice motion vectors used to create this sea ice age data set are the weekly Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, Version 4. For details on how these ice motion vectors are created, see the Data Acquisition and Processing section of the sea ice motion vectors documentation.

Processing

• Input ice motion data - weekly fields from the Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, Version 4 data set - are bilinearly interpolated to a 12.5 km x 12.5 km EASE-Grid.
• Ice parcel position is computed weekly.
• Each year's ice is tracked from year to year as a Lagrangian tracer parcel that starts at the center of each grid cell and moves according to the weekly mean ice velocity.
• Ice age is discretized in yearly increments, where a year defined as the melt season which runs from one season's minimum Arctic ice extent (usually in September) to the next year's minimum.
• If a parcel remains at 15 percent or more for a melt season, then it is aged one year. If a parcel travels to a grid cell that has less than 15 percent ice concentration, the tracer parcel is assumed to have melted away.
• The age of a grid cell is the age of the oldest tracer parcel that exists in the grid cell. 

Quality, Errors, and Limitations

Quality Assessment

A 15% sea ice concentration threshold was chosen to provide the most conservative possible estimates of change in areas where multiyear ice is present. For example, at the end of summer melt, a grid cell within the marginal ice zone might have a total passive microwave-derived concentration of 15 percent. Even though, upon freeze-up, 85 percent of the grid cell would consist of first-year ice, the age of that grid cell is assigned to the oldest ice that survived within that grid cell. Hence, the maps indicate the coverage of areas that contain at least some (15 percent or more) multiyear ice but do not provide information on proportions of ice of different ages within individual grid cells. The 15 percent threshold is also the standard used by other sea ice index projects at NSDIC (e.g. ASINA). 

Error Sources and Limitations

When age classes are aggregated into first-year and multiyear ice categories, the information is comparable to the passive and active microwave satellite-derived time series of first-year and multiyear ice analyzed by previous studies. Overall, this remote sensing-based age product is similar in nature and information content to the buoy-derived age fields produced by Rigor and Wallace (2004), but with greater spatial detail. The age estimates are restricted to open ocean areas only, where ice motion can be resolved in the microwave data. Note that this excludes the passages in the Canadian Archipelago. The cited values for ice coverage are therefore less than the actual amount of ice present in the Arctic.

Errors in the method of estimating sea ice age depend on the following ice motion errors:

• Resolution of the satellite sensor
• Geolocation and binning errors of each image pixel
• Atmospheric effects and temporal variability of the surface, especially during the summer months

The sea ice age shown in this dataset is the oldest age within each grid cell and does not necessarily indicate that all ice in that cell is of that age. Ice may also be present in grid cells that are designated as open water if the concentration is less than 15 percent.

Version History

Related Data Sets

Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors

MEaSUREs Arctic Sea Ice Characterization 25 km EASE-Grid 2.0

Contacts and Acknowledgments

Mark Tschudi
University of Colorado Boulder
Colorado Center for Astrodynamics Research
Boulder, CO 80309

Walter N. Meier
University of Colorado Boulder
449 UCB
Boulder, CO 80309-0449

J. Scott Stewart
University of Colorado Boulder
Boulder, CO 80309-0449

Chuck Fowler 
University of Colorado Boulder
Boulder, CO 80309

Jim Maslanik 
University of Colorado Boulder
Boulder, CO 80309

References

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. doi: 10.1029/JD089iD04p05355.

Fowler, C., W. J. Emery, and J. Maslanik. 2004. Satellite-derived evolution of Arctic sea ice Age: October 1978 to March 2003. Geoscience and Remote Sensing Letters, IEEE, 1(2), 71-74. doi: 10.1109/LGRS.2004.824741.

Maslanik, J. A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi and W. Emery. 2007. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss. Geophys. Res. Lett., 34(L24501). doi: 10.1029/2007GL032043.

Maslanik, J., J. Stroeve, C. Fowler, and W. Emery. 2011. Distribution and trends in Arctic sea ice age through spring 2011. Geophys. Res. Lett., 38(L13502). doi: 10.1029/2011GL047735.

Rigor, I.G., and J.M. Wallace, 2004. Variations in the age of Arctic sea-ice and summer sea-ice extent. Geophysical Research Letters 31: L09401. doi: 10.1029/2004GL019492.

Tschudi, M. A., Fowler, C, Maslanik, J. A., Stroeve, J. 2010. Tracking the movement and changing surface characteristics of Arctic sea ice. IEEE J. Selected Topics in Earth Obs. and Rem. Sens., 3(4). doi: 10.1109/JSTARS.2010.2048305.

Tschudi, M. A., Meier, W. N., and Stewart, J. S. 2019. An enhancement of sea ice motion and age products. The Cryosphere Discussion. doi: 10.5194/tc-2019-40.