112 research outputs found

    Seasonal and inter-annual variations in Antarctic sea ice extent as mapped by radar altimetry

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    Previous work has shown that interannual variations in total sea ice extent may provide a sensitive indicator of global climate change. Data from passive microwave instruments have allowed mapping of global sea ice extents from 1973–76 and from 1978 up to September 1987, [Gloerson and Campbell, 1988]. In this paper data from another microwave instrument, the Geosat radar altimeter, have been used to map the Antarctic sea ice extent for the period November 1986 to January 1989. Comparison with total Antarctic sea ice extents derived from the Scanning Multichannel Microwave Radiometer (SMMR) show excellent agreement during the freeze up period but show significant differences during the late part of the melt period

    ERS‐1 altimeter fast delivery data quality flagging over land surfaces

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    Over land ice and land, satellite altimeters provide valuable topographic information, in spite of having been designed primarily to operate over the ocean. There is a need, however, for careful data quality assessment and screening as erroneous elevation measurements can be included within the telemetered data, especially when the range tracker encounters complex echoes, or rapidly varying topography. The ERS-1 Fast Delivery (FD) data product provides an excellent source of near real-time data, which has already been used for ice sheet mapping. This is a reduced data set consisting of onboard parameters generated once per second. In this paper we show that by applying thresholds to two parameters, significant amounts of poorly tracked data can be eliminated. The effectiveness of this filtering technique is demonstrated by a comparison of filtered and unfiltered altimeter data with a digital elevation model. This filtering technique is applied to the first 35 day repeat cycle of FD data obtained over land to produce the first map of global topography from ERS-1

    Recent loss of floating ice and the consequent sea level contribution

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    We combine new and published satellite observations and the results of a coupled ice-ocean model to provide the first estimate of changes in the quantity of ice floating in the global oceans and the consequent sea level contribution. Rapid losses of Arctic sea ice and small Antarctic ice shelves are partially offset by thickening of Antarctic sea ice and large Antarctic ice shelves. Altogether, 746 +/- 127 km(3) yr(-1) of floating ice was lost between 1994 and 2004, a value that exceeds considerably the reduction in grounded ice over the same period. Although the losses are equivalent to a small (49 +/- 8 ÎŒm yr(-1)) rise in mean sea level, there may be large regional variations in the degree of ocean freshening and mixing. Ice shelves at the Antarctic Peninsula and in the Amundsen Sea, for example, have lost 481 +/- 38 km(3) yr(-1)

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

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    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

    Ku-band radar penetration into snow cover Arctic sea ice using airborne data

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    Satellite radar altimetry provides data to monitor winter Arctic sea-ice thickness variability on interannual, basin-wide scales. When using this technique an assumption is made that the peak of the radar return originates from the snow/ice interface. This has been shown to be true in the laboratory for cold, dry snow as is the case on Arctic sea ice during winter. However, this assumption has not been tested in the field. We use data from an airborne normal-incidence Ku-band radar altimeter and in situ field measurements, collected during the CryoSat Validation Experiment (CryoVEx) Bay of Bothnia, 2006 and 2008 field campaigns, to determine the dominant scattering surface for Arctic snow-covered sea ice. In 2006, when the snow temperatures were close to freezing, the dominant scattering surface in 25% of the radar returns appeared closer to the snow/ice interface than the air/snow interface. However, in 2008, when temperatures were lower, the dominant scattering surface appeared closer to the snow/ice interface than the air/snow interface in 80% of the returns

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

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    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
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