Numeric data of sea ice extent in the Arctic Ocean (CSV file)
Numeric data of sea-ice extent (unit: square km) in the Arctic Ocean from June 2002 to the present and decadal averages of 1980's, 1990's, and 2000's are included.
Number "-9999" represents that we couldn't conduct the observation during the period for the reason that the satellite went into constrained operation mode or stand-by mode to avoid harmful effects by meteor showers and solar flares.
The sea-ice extent is calculated as the areal sum of sea ice covering the ocean where sea-ice concentration (SIC) exceeds a threshold (15% for AMSR-E). SICs are derived from various satellite-borne passive microwave radiometer (PMR) sensors using the algorithm developed and provided by Dr. Comiso of NASA GSFC through a cooperative relationship between NASA and JAXA. The following sensor's data were used;
Jan. 1980 - Jul. 1987
Jul. 1987 - Jun 2002
Jun. 2002 - Oct. 2011
Oct. 2011 - the present
The black dot seen at the North Pole is an area lacking data where PMR sensors cannot observe the Earth's surface due to the limit of its observational coverage (i.e., orbit inclination of 98deg. and swath width of 1600km for AMSR-E). Please note that this area is also counted as sea-ice cover in our estimation of sea-ice extent.
In principle, SIC data could have errors of 10% at most, particularly for the area of thin sea ice seen around the edge of sea-ice cover and melted sea ice seen in summer. Also, SIC along coastal lines could also have errors due to sub-pixel contamination of land cover in an instantaneous field of view of PMR data.
Averaging period and the update timing of daily data
In general, sea-ice extent is defined as a temporal average of several days (e.g., five days) in order to eliminate calculation errors due to a lack of data (e.g., for traditional microwave sensors such as SMMR and SSM/I). However, we adopt the average of latest two days (day:N & day:N-1) to achieve rapid data release. Only for the processing of WindSat data (Oct. 4, 2011 to the present) the data of the day before yesterday (day:N-2) is also sometimes used to fill data gaps.
Definition of sea-ice cover (extent and area)
The area of sea-ice cover is often defined in two ways, i.e., sea-ice "extent" and sea-ice "area." These multiple definitions of sea-ice cover may sometimes confuse data users. The former is defined as the areal sum of sea ice covering the ocean (sea ice + open ocean), whereas the latter "area" definition counts only sea ice covering a fraction of the ocean (sea ice only). Thus, the sea-ice extent is always larger than the sea-ice area. Because of the possible errors in SIC mentioned above, satellite-derived sea-ice concentration can be underestimated, particularly in summer. In such a case, the sea-ice area is more susceptible to errors than the sea-ice extent. Thus, we adopt the definition of sea-ice extent to monitor the variation of the Arctic sea ice on this site.
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