Antarctic sea ice elevation from satellite radar altimetry

In situ measurements of sea ice thickness from ship and upward-looking sonar are used to assess the potential for satellite radar altimetry to provide information on Antarctic sea ice thickness. A climatology of satellite ice elevation estimates is compared to an Antarctic sea ice thickness climatology made from the Antarctic Sea Ice Processes and Climate ( ASPeCt) data set. In addition monthly, regional, satellite ice elevation estimates are compared to ULS ice draft data. The results show reasonable spatial agreement between the satellite and in-situ data, and show regional signals of change in ice elevation in line with that which would be expected. The results show some promise for providing information on Antarctic ice thickness from radar altimetry missions such as CryoSat. However, further studies into snow and ice density and the radar penetration into the Antarctic snow cover are required

Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum

September 2007 marked a record minimum in sea ice extent. While there have been many studies published recently describing the minimum and its causes, little is known about how the ice thickness has changed in the run up to, and following, the summer of 2007. Using satellite radar altimetry data, covering the Arctic Ocean up to 81.5 degrees North, we show that the average winter sea ice thickness anomaly, after the melt season of 2007, was 0.26 m below the 2002/2003 to 2007/2008 average. More strikingly, the Western Arctic anomaly was 0.49 m below the six-year mean in the winter of 2007/2008. These results show no evidence of short-term preconditioning through ice thinning between 2002 and 2007 but show that, after the record minimum ice extent in 2007, the average ice thickness was reduced, particularly in the Western Arctic. Citation: Giles, K. A., S. W. Laxon, and A. L. Ridout (2008), Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum, Geophys. Res. Lett., 35, L22502, doi: 10.1029/2008GL035710

On large outflows of Arctic sea ice into the Barents Sea

Winter outflows of Arctic sea ice into the Barents Sea are estimated using a 10-year record of satellite ice motion and thickness. The mean winter volume export through the Svalbard/Franz Josef Land passage is 40 km(3), and ranges from - 280 km(3) to 340 km(3). A large outflow in 2003 is preconditioned by an unusually high concentration of thick perennial ice over the Nansen Basin at the end of the 2002 summer. With a deep atmospheric low situated over the eastern Barents Sea in winter, the result is an increased export of Arctic ice. The Oct-Mar ice area flux, at 110 x 10(3) km(2), is not only unusual in magnitude but also remarkable in that > 70% of the area is multiyear ice; the ice volume flux at similar to 340 km(3) is almost one-fifth of the ice flux through the Fram Strait. Another large outflow of Arctic sea ice through this passage, comparable to that in 2003, is found in 1996. This southward flux of sea ice represents one of two major sources of freshwater in the Barents Sea; the other is the eastward flux of water via the Norwegian Coastal Current. The possible consequences of variable freshwater input on the Barents Sea hydrography and its impact on transformation of Atlantic Water en route to the Arctic Ocean are examined with a 25-year coupled ice-ocean model

Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre

The Arctic Ocean’s freshwater budget comprises contributions from river runoff, precipitation, evaporation, sea-ice and exchanges with the North Pacific and Atlantic. More than 70,000km3 of freshwater are stored in the upper layer of the Arctic Ocean, leading to low salinities in upper-layer Arctic sea water, separated by a strong halocline from warm, saline water beneath. Spatially and temporally limited observations show that the Arctic Ocean’s freshwater content has increased over the past few decades, predominantly in the west. Models suggest that wind-driven convergence drives freshwater accumulation. Here we use continuous satellite measurements between 1995 and 2010 to show that the dome in sea surface height associated with the western Arctic Beaufort Gyre has been steepening, indicating spin-up of the gyre. We find that the trend in wind field curl—a measure of spatial gradients in the wind that lead to water convergence or divergence—exhibits a corresponding spatial pattern, suggesting that wind-driven convergence controls freshwater variability. We estimate an increase in freshwater storage of 8,000±2,000km3 in the western Arctic Ocean, in line with hydrographic observations, and conclude that a reversal in the wind field could lead to a spin-down of the Beaufort Gyre, and release of this freshwater to the Arctic Ocean

Tracer-derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007

We investigate the freshwater composition of the shelf and slope of the Arctic Ocean north of the New Siberian Islands using geochemical tracer data (delta O-18, Ba, and PO4*) collected following the extreme summer of 2007. We find that the anomalous wind patterns that partly explained the sea ice minimum at this time also led to significant quantities of Pacific-derived surface water in the westernmost part of the Makarov Basin. We also find larger quantities of meteoric water near Lomonosov Ridge than were found in 1995. Dissolved barium is depleted in the upper layers in one region of our study area, probably as a result of biological activity in open waters. Increasingly ice-free conditions compromise the quantitative use of barium as a tracer of river water in the Arctic Ocean. Citation: Abrahamsen, E. P., M. P. Meredith, K. K. Falkner, S. Torres-Valdes, M. J. Leng, M. B. Alkire, S. Bacon, S. W. Laxon, I. Polyakov, and V. Ivanov (2009), Tracer-derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007, Geophys. Res. Lett., 36, L07602, doi:10.1029/2009GL037341

A comparison of the performance of the ice and ocean tracking modes of the ERS‐1 radar altimeter over non‐ocean surfaces

The European Space Agency's ERS-1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non-ocean surfaces. As part of the ERS-1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non-ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions

Living on Cold Substrata: New Insights and Approaches in the Study of Microphytobenthos Ecophysiology and Ecology in Kongsfjorden

Organisms in shallow waters at high latitudes are under pressure due to climate change. These areas are typically inhabited by microphytobenthos (MPB) communities, composed mainly of diatoms. Only sparse information is available on the ecophysiology and acclimation processes within MPBs from Arctic regions. The physico-chemical environment and the ecology and ecophysiology of benthic diatoms in Kongsfjorden (Svalbard, Norway) are addressed in this review. MPB biofilms cover extensive areas of sediment. They show high rates of primary production, stabilise sediment surfaces against erosion under hydrodynamic forces,and affect the exchange of oxygen and nutrients across the sediment-water interface. Additionally, this phototrophic community represents a key component in the functioning of the Kongsfjorden trophic web, particularly as a major food source for benthic suspension- or deposit-feeders. MPB in Kongsfjorden is confronted with pronounced seasonal variations in solar radiation, low temperatures, and hyposaline (meltwater) conditions in summer, as well as long periods of ice and snow cover in winter. From the few data available, it seems that these organisms can easily cope with these environmental extremes. The underlying physiological mechanisms that allow growth and photosynthesis to continue under widely varying abiotic parameters, along with vertical migration and heterotrophy, and biochemical features such as a pronounced fatty-acid metabolism and silicate incorporation are discussed. Existing gaps in our knowledge of benthic diatoms in Kongsfjorden, such as the chemical ecology of biotic interactions, need to be filled. In addition, since many of the underlying molecular acclimation mechanisms are poorly understood, modern approaches based on transcriptomics, proteomics, and/or metabolomics, in conjunction with cell biological and biochemical techniques, are urgently needed. Climate change models for the Arctic predict other multifactorial stressors, such as an increase in precipitation and permafrost thawing, with consequences for the shallow-water regions. Both precipitation and permafrost thawing are likely to increase nutrient-enriched, turbid freshwater runoff and may locally counteract the expected increase in coastal radiation availability. So far, complex interactions among factors, as well as the full genetic diversity and physiological plasticity of Arctic benthic diatoms, have only rarely been considered. The limited existing information is described and discussed in this review

Classification of CryoSat-2 Radar Echoes

Sea-ice thickness at global scale is an important variable in the polar climate system. Only satellite altimeters such as onboard the CryoSat-2 mission allow us to obtain sea-ice thickness on hemispherical scale. Accurate CryoSat-2 altimeter range measurements provide surface elevations which have to be referenced to the local sea level to obtain sea-ice freeboard that can be converted into sea-ice thickness assuming hydrostatic equilibrium. The local sea-surface height is determined by careful detection of leads in the ice surface using the specific characteristics of the radar signal. Off-nadir reflections from leads can significantly affect the range retracking and hence bias the surface elevations of leads and sea ice. This can finally lead to a negative freeboard and hence also affects the thickness and volume retrieval. We present a method for the classification of CryoSat-2 radar echoes to correctly discriminate between valid and off-nadir biased echoes. We apply our classification to a CryoSat-2 track from December 15 where 50 leads over a distance of 2,300 km are identified. Overall 22 % of the surface elevations are associated with biased radar echoes