Data of Sea Ice Extent
Numeric data of sea ice extent in the Arctic Ocean (CSV file)
- Data from June 2002 to the present are included.
- The format is "month, day, year, extent (unit: square km)".
- 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.
- When you use our data and graph, please follow our site policy.
Notes on the sea-ice extent data
Data period and format
- Numeric data of sea-ice extent in the Arctic Ocean from June 2002 to
the present are contained in a CSV file. Please note that only the sea-ice
pixels in the browse image are counted for estimating the values of
sea-ice extent, and thus sea ice outside the image is not taken into
account in this data.
- The format is "month, day, year, sea-ice extent (unit: square km)".
- "-9999" indicates that we couldn't conduct the observation during the
period because the satellite entered a constrained operation mode or
stand-by mode to avoid harmful effects of meteor showers or solar flares.
Method for calculating sea-ice extent
- The sea-ice extent is calculated as the areal sum of sea ice covering
the ocean where sea-ice concentration (SIC) exceeds 15%. SIC data of
JAXA’s AMSR-E standard products are used for this purpose (http://sharaku.eorc.jaxa.jp/AMSR/products/pdf/alg_des.pdf).
The algorithm for calculating SIC was developed and provided by Dr. Comiso
of NASA GSFC through a cooperative relationship between NASA and JAXA.
- The black dot seen at the North Pole is an area lacking data where
AMSR-E 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). Please note that this area is also counted as sea-ice cover in
our estimation of sea-ice extent. We may change the policy (i.e., filling
the gap with full coverage of sea ice) in the near future due to the
recent drastic reduction of Arctic sea ice. We will announce this if it is
implemented.
- The current version of data processing produces an erroneous blip of
sea-ice extent on June 1 and October 15, which is seen in the graph of
sea-ice extent as a small peak on these dates. The apparent blip arises
due to switching of some parameters in the processing on those dates. The
parameter switching is needed because the surface of the Arctic sea ice
becomes wet in summer due to the melting of ice, drastically changing the
satellite-observed signatures of sea ice. We will soon improve the
processing to make the graph much smoother.
- 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 AMSR-E 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 two days to achieve rapid data release.
The wider spatial coverage of AMSR-E enables reducing the data-production
period.
- Usually the latest value of daily sea-ice extent is fixed and updated
at around 1 p.m. (4 a.m.) JST (UT). Before the value is fixed, we also
assign a preliminary value of daily sea-ice extent several times (usually
three to four times) as an early report, which is determined without the
full two-day observation coverage. (The fixed values of sea-ice extent are
determined with the full coverage of observation data.)
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.
Updated daily
