Notes on numeric data of sea-ice extent
- 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.
- Sea-ice data is updated at around 3:00(UTC) every day.
- When you use our data and graph, please follow our site policy.
- Further long-term daily sea-ice extent from 1978 to the present year are available at the JASMES-Climate web site)
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 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||：||SMMR
|•||Jul. 1987 ～ Jun. 2002||：||SSM/I
|•||Jun. 2002 ～ Oct. 2011||：||AMSR-E
|•||Oct. 2011 ～ Jul. 2012||：||WindSat
|•||Jul. 2012 ～ the present||：||AMSR2
- 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.
Method for calculating sea ice thickness and melt ice concentration ratio products
The product of sea ice thickness and the melt pond concentration which is shown in the VISHOP is calculated from AMSR-E and AMSR2 data by using a research algorithm1) developed by K. Tateyama (Kitami Institute of Technology) and others in the Arctic research projects utilizing the IARC-JAXA Information System (IJIS) and satellite imagery (1st - 4th generations). This product is an essentially experimental and research product. This product has the effectiveness in the relative dry freezing seasons such as autumn, winter and spring (September – May), but cannot provide the accurate sea ice thickness in melting wet season (June - August) because the sea ice surface is covered by melt ponds.
This product is opened to the public for the usages of research and validation of algorithms. The Arctic Data archive System (ADS) is not liable for any loss or damage resulting from the use of this data.
Krishﬁeld, R. A., A. Proshutinsky, K.Tateyama, W. J. Williams, E. C. Carmack, F. A. McLaughlin, and M.-L.
Timmermans (2014), Deterioration of perennial sea ice in the Beaufort Gyre from 2003to 2012 and its impact on he oceanic freshwater cycle, J. Geophys. Res.Oceans, 119, doi:10.1002/2013JC008999.
||The AMSR2 brightness temperatures and products data are provided by Japan Aerospace Exploration Agency (JAXA).
||The AMSR-E brightness temperatures and products data were provided by Japan Aerospace Exploration Agency (JAXA).
||The WindSat Sensor Data Record (SDR) brightness temperatures are being provided by the Naval Research Laboratory (NRL) and NOAA's National Environmental Satellite, Data, and Information Service (NESDIS).
||The SSM/I Antenna Temperature (TA) data were produced by Remote Sensing Systems (RSS).
||The Nimbus-7 SMMR Pathfinder Brightness Temperature Data were provided by the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, under contract with the National Aeronautics and Space Administration (NASA).