[1] Recent progress in sea ice concentration remote sensing by satellite microwave radiometers has been stimulated by two developments: First, the new sensor Advanced Microwave Scanning Radiometer-EOS (AMSR-E) offers spatial resolutions of approximately 6 × 4 km at 89 GHz, nearly 3 times the resolution of the standard sensor SSM/I at 85 GHz (15 × 13 km). Second, a new algorithm enables estimation of sea ice concentration from the channels near 90 GHz, despite the enhanced atmospheric influence in these channels. This allows full exploitation of their horizontal resolution, which is up to 4 times finer than that of the channels near 19 and 37 GHz, the frequencies used by the most widespread algorithms for sea ice retrieval, the NASA-Team and Bootstrap algorithms. The ASI algorithm used combines a model for retrieving the sea ice concentration from SSM/I 85-GHz data proposed by Svendsen et al. (1987) with an ocean mask derived from the 18-, 23-, and 37-GHz AMSR-E data using weather filters. During two ship campaigns, the correlation of ASI, NASA-Team 2, and Bootstrap algorithms ice concentrations with bridge observations were 0.80, 0.79, and 0.81, respectively. Systematic differences over the complete AMSR-E period (2002–2006) between ASI and NASA-Team 2 are below −2 ± 8.8%, and between ASI and Bootstrap are 1.7 ± 10.8%. Among the geophysical implications of the ASI algorithm are: (1) Its higher spatial resolution allows better estimation of crucial variables in numerical atmospheric and ocean models, for example, the heat flux between ocean and atmosphere, especially near coastlines and in polynyas. (2) It provides an additional time series of ice area and extent for climate studies.

Sea ice remote sensing using AMSR‐E 89‐GHz channels

Contributing Authors: J. Box (USA), D. Bromwich (USA), R. Brown (Canada), J.G. Cogley (Canada), J. Comiso (USA), M. Dyurgerov (Sweden, USA), B. Fitzharris (New Zealand), O. Frauenfeld (USA, Austria), H. Fricker (USA), G. H. Gudmundsson (UK, Iceland), C. Haas (Germany), J.O. Hagen (Norway), C. Harris (UK), L. Hinzman (USA), R. Hock (Sweden), M. Hoelzle (Switzerland), P. Huybrechts (Belgium), K. Isaksen (Norway), P. Jansson (Sweden), A. Jenkins (UK), Ian Joughin (USA), C. Kottmeier (Germany), R. Kwok (USA), S. Laxon (UK), S. Liu (China), D. MacAyeal (USA), H. Melling (Canada), A. Ohmura (Switzerland), A. Payne (UK), T. Prowse (Canada), B.H. Raup (USA), C. Raymond (USA), E. Rignot (USA), I. Rigor (USA), D. Robinson (USA), D. Rothrock (USA), S.C. Scherrer (Switzerland), S. Smith (Canada), O. Solomina (Russian Federation), D. Vaughan (UK), J. Walsh (USA), A. Worby (Australia), T. Yamada (Japan), L. Zhao (China)

Observations: Changes in Snow, Ice and Frozen Ground

Changes apparent in the arctic climate system in recent years require evaluation in a century-scale perspective in order to assess the Arctic’s response to increasing anthropogenic greenhouse-gas forcing. Here, a new set of centuryand multidecadal-scale observational data of surface air temperature (SAT) and sea ice is used in combination with ECHAM4 and HadCM3 coupled atmosphere‐ice‐ocean global model simulations in order to better determine and understand arctic climate variability. We show that two pronounced twentieth-century warming events, both amplified in the Arctic, were linked to sea-ice variability. SAT observations and model simulations indicate that the nature of the arctic warming in the last two decades is distinct from the early twentieth-century warm period. It is suggested strongly that the earlier warming was natural internal climate-system variability, whereas the recent SAT changes are a response to anthropogenic forcing. The area of arctic sea ice is furthermore observed to have decreased ∼8 × 10 5 km 2 (7.4%) in the past quarter century, with record-low summer ice coverage in September 2002. A set of model predictions is used to quantify changes in the ice cover through the twenty-first century, with greater reductions expected in summer than winter. In summer, a predominantly sea-ice-free Arctic is predicted for the end of this century.

/pdf/arctic-climate-change-observed-and-modelled-temperature-and-b7f0aeecwk.pdf

Arctic climate change: observed and modelled temperature and sea-ice variability

[1] Satellite-derived estimates of sea-ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present. These data show that in addition to the well-documented loss of perennial ice cover as a whole, the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly. The oldest ice types have essentially disappeared, and 58% of the multiyear ice now consists of relatively young 2- and 3-year-old ice compared to 35% in the mid-1980s. Ice coverage in summer 2007 reached a record minimum, with ice extent declining by 42% compared to conditions in the 1980s. The much-reduced extent of the oldest and thickest ice, in combination with other factors such as ice transport that assist the ice-albedo feedback by exposing more open water, help explain this large and abrupt ice loss.

http://sites.fas.harvard.edu/~scia30/2008/SOURCEBOOK/2007_Maslanik_GRL.pdf

A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss

The perennial ice area was drastically reduced to 38% of its climatological average in 2007 but recovered slightly in 2008, 2009, and 2010 with the areas being 10%, 24%, and 11% higher than in 2007, respectively However, trends in extent and area remained strongly negative at -122% and -135% decade (sup -1), respectively The thick component of the perennial ice, called multiyear ice, as detected by satellite data during the winters of 1979-2011 was studied, and results reveal that the multiyear ice extent and area are declining at an even more rapid rate of -151% and -172% decade(sup -1), respectively, with a record low value in 2008 followed by higher values in 2009, 2010, and 2011 Such a high rate in the decline of the thick component of the Arctic ice cover means a reduction in the average ice thickness and an even more vulnerable perennial ice cover The decline of the multiyear ice area from 2007 to 2008 was not as strong as that of the perennial ice area from 2006 to 2007, suggesting a strong role of second-year ice melt in the latter The sea ice cover is shown to be strongly correlated with surface temperature, which is increasing at about 3 times the global average in the Arctic but appears weakly correlated with the Arctic Oscillation (AO), which controls the atmospheric circulation in the region An 8-9-yr cycle is apparent in the multiyear ice record, which could explain, in part, the slight recovery in the last 3 yr

/pdf/large-decadal-decline-of-the-arctic-multiyear-ice-cover-2g0ltmxw8v.pdf

Large Decadal Decline of the Arctic Multiyear Ice Cover

Results are presented of a series of coordinate special sensor microwave imager (SSM/I) underflights that were carried out during March 1988 with NASA and Navy aircraft over portions of the Bering, Beaufort, and Chukchi seas. NASA DC-8 AMMR data from Bering Sea ice edge crossings were used to verify that the ice edge location, defined as the position of the initial ice bands encountered by the aircraft, corresponds to an SSM/I ice concentration of 15 percent. Direct comparison of SSM/I and aircraft ice concentrations for regions having at least 80 percent aircraft coverage reveals that the SSM/I total ice concentration is lower on average by 2.4 +/-2.4 percent. For multiyear ice, NASA and Navy flights across the Beaufort and Chukchi seas show that the SSM/I algorithm correctly maps the large-scale distribution of multiyear ice: the zone of first-year ice off the Alaskan coast, the large areas of mixed first-year and multiyear ice, and the region of predominantly multiyear ice north of the Canadian archipelago.

Aircraft active and passive microwave validation of sea ice concentration from the Defense Meteorological Satellite Program special sensor microwave imager

Passive Microwave Signatures of Sea Ice

Examination of springtime ice drafts obtained from submarine profiles in a narrow band of the western Arctic Ocean from offshore Alaska to 89°N indicates that the mean ice draft decreased 15 m between the mid-1980s and early 1990s No similar trend was evident in ice drafts near the North Pole The 1980s drafts were composed largely of ice exceeding 35 m, while the early 1990s drafts contained more ice in thinner categories The differences in drafts between the two periods appear to be related largely to ice dynamics effects associated with the presence and strength of the Beaufort Gyre, which weakened considerably in the early 1990s

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL012967

Evidence for rapid thinning of sea ice in the western Arctic Ocean at the end of the 1980s

Classification of sea ice types with single‐band (33.6 GHz) airborne passive microwave imagery

An aircraft data set of coincident Ka band (33.6 GHz, vertical polarization) passive microwave images and aerial photographs acquired in the Chukchi-Beaufort Sea region in March 1983 was analyzed to evaluate radiometric signatures of deformational features that occur in sea ice. A total of 115 fractures and 197 pressure ridges were examined with respect to physical appearance (relative age, snow cover, ice type, width, orientation) as observed in photographs, and radiometric character (brightness temperature, radiometric contrast with respect to adjacent ice, radiometric profile across the feature) as measured from digital passive microwave images. Of the deformational features that were observed in aerial photographs, 82% had radiometric signatures of sufficient contrast to be observed in passive microwave images. Fractures and ridges have equal chance of detection, but fractures cannot be distinguished from pressure ridges on the basis of brightness temperature, radiometric contrast, or characteristics of radiometric profiles measured across these features. However, visual analysis of passive microwave images, which takes into account contextual information, does allow fractures to be distinguished from pressure ridges in many instances. Radiometric signatures of both fractures and ridges are more likely to be radiometrically warmer (as opposed to cooler) than adjacent ice, which suggests that saline ice is a significant constituent of most deformational features. New ridges are more likely to be radiometrically warm than old ridges, probably because brine drains from the ridge as it ages (which reduces emissivity) and snow accumulates in drifts along the ridge trend (which enhances scattering). However, brightness temperatures of snow-covered ridges extend across a range that is approximately 15 K cooler, and 10 K warmer than the range observed for snow-free ridges. Old features show higher radiometric contrast with respect to adjacent ice than new features, which increases their probability of detection.

Duane T. Eppler

Papers

Aircraft active and passive microwave validation of sea ice concentration from the Defense Meteorological Satellite Program special sensor microwave imager

Passive Microwave Signatures of Sea Ice

Evidence for rapid thinning of sea ice in the western Arctic Ocean at the end of the 1980s

Classification of sea ice types with single‐band (33.6 GHz) airborne passive microwave imagery

Passive microwave signatures of fractures and ridges in sea ice at 33.6 GHz (vertical polarization) as observed in aircraft images