September daily extents are now fully reported and the 2021 September monthly results can be compared with those of the previous 14 years.  MASIE showed 2021 at 5.2M km2  and SII was close behind, reaching 4.9M for the month.  Analysis below shows that the 2021 Minimum was 1/2 Wadham ( 1M km2) higher than the 14 year average, and 1¼ Wadhams more than 2007

In August, 4.4M km2 was the median estimate for the September monthly average extent from the SIPN (Sea Ice Prediction Network) who use the reports from SII (Sea Ice Index), the NASA team satellite product from passive microwave sensors. The SII actual ice extent was 1/2 Wadham higher.

The graph below shows September comparisons through day 273 (Sept. 30).

Note that both MASIE and SII started the month higher than average, hit bottom earlier and then increased the surplus. SII tracked much lower up to ~400k km2 less than MASIE before ending just 143k km2 down.  The other years, 2007, 2019 and 2020 were much lower than average. The animation below shows the ice extents for day 273 each of the last 15 years.

The table shows ice extents in the regions for 2021, 14 year averages and 2007 for day 273. Averages refer to 2007 through 2020 inclusive.

Region	2021273	Day 273 Average	2021-Ave.	2007273	2021-2007
(0) Northern_Hemisphere	5371075	4879409	491666	4086883	1284192
(1) Beaufort_Sea	776885	542936	233948	498743	278142
(2) Chukchi_Sea	485260	189321	295939	51	485209
(3) East_Siberian_Sea	416429	299075	117355	311	416118
(4) Laptev_Sea	58277	150394	-92117	235245	-176967
(5) Kara_Sea	71952	23909	48043	15367	56585
(6) Barents_Sea	18	15305	-15287	4851	-4833
(7) Greenland_Sea	124689	241619	-116930	353210	-228522
(8) Baffin_Bay_Gulf_of_St._Lawrence	59581	52823	6758	42247	17334
(9) Canadian_Archipelago	441822	391718	50104	307135	134687
(10) Hudson_Bay	2260	3832	-1572	1936	323
(11) Central_Arctic	2932779	2967366	-34586	2626511	306268

Deficits in Laptev and Greenland Seas are more than offset by surpluses in Beaufort, Chukchi, and East Siberian Seas.  Extents in Kara and Canadian Archipelago are also well above average.   Overall, the NH ice extent is surplus by 492k km2 or 10% over 14 year average.

Summary

Earlier observations showed that Arctic ice extents were low in the 1940s, grew thereafter up to a peak in 1977, before declining.  That decline was gentle until 1996 which started a decade of multi-year ice loss through the Fram Strait.  There was also a major earthquake under the north pole in that period.  In any case, the effects and the decline ceased in 2007, 30 years after the previous peak.  Now we have a plateau in ice extents, which could be the precursor of a growing phase of the quasi-60 year Arctic ice oscillation.

For context, note that the average maximum has been 15M, so on average the extent shrinks to 30% of the March high before growing back the following winter.  In 2021 about 35% of the March maximum was retained, so the melt season losses were considerably less than in the past.

Background 

A commenter previously asked, where do they get their data? The answer is primarily from NIC’s Interactive Multisensor Snow and Ice Mapping System (IMS). From the documentation, the multiple sources feeding IMS are:

Platform(s) AQUA, DMSP, DMSP 5D-3/F17, GOES-10, GOES-11, GOES-13, GOES-9, METEOSAT, MSG, MTSAT-1R, MTSAT-2, NOAA-14, NOAA-15, NOAA-16, NOAA-17, NOAA-18, NOAA-N, RADARSAT-2, SUOMI-NPP, TERRA

Sensor(s): AMSU-A, ATMS, AVHRR, GOES I-M IMAGER, MODIS, MTSAT 1R Imager, MTSAT 2 Imager, MVIRI, SAR, SEVIRI, SSM/I, SSMIS, VIIRS

Summary: IMS Daily Northern Hemisphere Snow and Ice Analysis

The National Oceanic and Atmospheric Administration / National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) has an extensive history of monitoring snow and ice coverage.Accurate monitoring of global snow/ice cover is a key component in the study of climate and global change as well as daily weather forecasting.

The Polar and Geostationary Operational Environmental Satellite programs (POES/GOES) operated by NESDIS provide invaluable visible and infrared spectral data in support of these efforts. Clear-sky imagery from both the POES and the GOES sensors show snow/ice boundaries very well; however, the visible and infrared techniques may suffer from persistent cloud cover near the snowline, making observations difficult (Ramsay, 1995). The microwave products (DMSP and AMSR-E) are unobstructed by clouds and thus can be used as another observational platform in most regions. Synthetic Aperture Radar (SAR) imagery also provides all-weather, near daily capacities to discriminate sea and lake ice. With several other derived snow/ice products of varying accuracy, such as those from NCEP and the NWS NOHRSC, it is highly desirable for analysts to be able to interactively compare and contrast the products so that a more accurate composite map can be produced.

The Satellite Analysis Branch (SAB) of NESDIS first began generating Northern Hemisphere Weekly Snow and Ice Cover analysis charts derived from the visible satellite imagery in November, 1966. The spatial and temporal resolutions of the analysis (190 km and 7 days, respectively) remained unchanged for the product’s 33-year lifespan.

As a result of increasing customer needs and expectations, it was decided that an efficient, interactive workstation application should be constructed which would enable SAB to produce snow/ice analyses at a higher resolution and on a daily basis (~25 km / 1024 x 1024 grid and once per day) using a consolidated array of new as well as existing satellite and surface imagery products. The Daily Northern Hemisphere Snow and Ice Cover chart has been produced since February, 1997 by SAB meteorologists on the IMS.

Another large resolution improvement began in early 2004, when improved technology allowed the SAB to begin creation of a daily ~4 km (6144×6144) grid. At this time, both the ~4 km and ~24 km products are available from NSIDC with a slight delay. Near real-time gridded data is available in ASCII format by request.

In March 2008, the product was migrated from SAB to the National Ice Center (NIC) of NESDIS. The production system and methodology was preserved during the migration. Improved access to DMSP, SAR, and modeled data sources is expected as a short-term from the migration, with longer term plans of twice daily production, GRIB2 output format, a Southern Hemisphere analysis, and an expanded suite of integrated snow and ice variable on horizon.

http://www.natice.noaa.gov/ims/ims_1.html

Footnote

Some people unhappy with the higher amounts of ice extent shown by MASIE continue to claim that Sea Ice Index is the only dataset that can be used. This is false in fact and in logic. Why should anyone accept that the highest quality picture of ice day to day has no shelf life, that one year’s charts can not be compared with another year?

MASIE is rigorous, reliable, serves as calibration for satellite products, and continues the long and honorable tradition of naval ice charting using modern technologies. More on this at my post Support MASIE Arctic Ice Dataset