Monthly Ocean Masks and Maximum Extent Masks

NSIDC has created monthly ocean masks for users who want to know the maximum sea ice extent during a particular month, or who wish to mask out false sea ice concentrations caused by weather effects and/or land contamination. The ocean masks are very similar to the Monthly Climatology of Sea Ice Concentrations from NSIDC. However, the ocean mask is a binary indicator of ice/no ice for ocean along with coast, land, and lake masks, whereas the monthly means give the climatological percentage of monthly mean sea ice concentration (0 to 100 percent). NSIDC has also created maximum extent masks that show a maximum of sea ice extent during the entire time period.

The ocean masks were derived for each month from the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data using the NASA Team sea ice algorithm. The first processing step involved averaging the sea ice concentrations from this data set for each series of months throughout the time series, November 1978 through the most current processing, to create 12 monthly ice concentration climatologies for the entire series for each hemisphere. To derive the monthly ocean masks, individual pixels containing any fraction of ice in these monthly-averaged sea ice concentrations were flagged as ice-covered; all other non-land pixels were flagged as open water.

Data Access

04 September 2013: Please note that the ocean mask files and maximum extent mask files have been temporarily removed as they are not the masks used in the production of the Sea Ice Trends and Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS data sets. NSIDC is currently standardizing masks across all sea ice products and will distribute the replacement masks as soon as they are available.

Data Format

For both the ocean masks and the maximum extent masks, the pixel values are the same for all data files and are described in Table 1.

Table 1. Pixel Values
Pixel Value Description
0 Water
1 Ice
2 Coast
3 Land
4 Lakes

Ocean Masks

Ocean mask data are in flat, one-byte binary format with a grid size of 304 x 448 pixels (bytes) for Arctic data and 316 x 332 pixels (bytes) for Antarctic data. Ocean mask files exist for each month of the time series. PNG images of the ocean masks are also provided in the browse directory on the FTP site.

Maximum Extent Masks

Maximum ice extent data files are in flat one-byte binary format with a grid size of 304 x 448 pixels for the Arctic and 316 x 332 pixels for Antarctica. They are derived from the ocean masks. If ice was present in any of the twelve monthly ocean masks, the corresponding pixels were classified as ice in the final ice extent image, thus making the maximum extent masks a maximum of sea ice extent during the time period of the data. PNG images of maximum extent are also provided in the browse directory on the FTP site.

File Naming Convention

Ocean Masks

The ocean masks are named according to the following convention, and as described in Table 2:

oceanmask.mmm.yyyy1-yyyy2.h

Where:

Table 2. Ocean Mask File Naming Convention Description
Variable Description
oceanmask Identifies this as an ocean mask file
mmm 3-character month
yyyy1 4-digit year of the first year for which data was available
yyyy2 4-digit year of the last year for which data was available
h Hemisphere (n: Northern, s: Southern)

Maximum Extent Masks

The maximum extent masks are named according to the following convention, and as described in Table 3:

max_extent_mask.yyyy1-yyyy2.h

Where:

Table 3. Maximum Extent Mask File Naming Convention Description
Variable Description
max_extent_mask Identifies this as an maximum extent mask file
yyyy1 4-digit year of the first year for which data was available
yyyy2 4-digit year of the last year for which data was available
h Hemisphere (n: Northern, s: Southern)