C14A-01 INVITED
Interannual and Seasonal Variability in the Arctic Ocean as Observed with GRACE and In Situ Bottom Pressure Measurements
The comparison between GRACE-derived bottom pressure and Arctic Bottom Pressure Recorder (ABPR) measurements at the North Pole is quite good through 2007. Both GRACE and the ABPRs show a declining bottom pressure trend amounting to 10 cm from 2002 to 2006. This trend was associated with a drop in upper ocean salinity near the North Pole due to an anticyclonic shift in circulation. There is also reasonable agreement between the spatial distribution of bottom pressure trends from GRACE Release 1 data and a decrease in bottom pressure in the Makarov Basin associated with the hypothesized return of less saline, Pacific-derived, upper ocean water to that region. GRACE Release 4 data from 2006 to 2008 show a reversal of the 2002-06 Arctic Ocean trends, as does the hydrographic structure of the region. The North Pole ABPR and GRACE data also indicate an annual variation in area-averaged Arctic Ocean bottom pressure of about 2 cm water equivalent with a maximum in late summer to early fall, a couple of months after peak runoff. The sophisticated modeling study of Dobslaw and Thomas [2007] shows a similar variation in Arctic Ocean mass due to runoff, but with maximum bottom pressure in phase with runoff. A very simple, first order model of the Arctic Ocean response to runoff and atmospheric pressure illustrates a mechanism for retaining runoff mass in the basin at seasonal time scales and shows good agreement with the observed seasonal variation in area-averaged Arctic Ocean bottom pressure. Dobslaw, H., and M. Thomas (2007), Impact of river run-off on global ocean mass redistribution, Geophys. J. Int., 168, 527-532, doi:10.1111.j.1365-246X.2006.03247.x.
C14A-02 INVITED
Basin-wide thinning of Arctic sea ice following the 2007 record ice extent minimum
September 2007 marked a record minimum in Arctic sea ice extent, 24% lower than the previous record low in September 2005, and 37% below the climatological mean. Model studies have suggested that ice thickness and ice extent are intrinsically linked, and 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°North, we investigate changes in sea ice thickness in the run up to, and following, the 2007 minimum. These results show no evidence of 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.
C14A-03
Impact of Freeboard and Snow Depth Variability on Satellite Altimetric Sea Ice Thickness Retrievals
Combinations of sea ice freeboard and snow depth measurements from satellite data provide a means to derive global sea ice thickness values, however there are large differences in spatial coverage and resolution between the measurements which can lead to uncertainties when combining the data. Airborne measurements of sea ice freeboard and snow depth taken during the March 2006 EOS Aqua Advanced Microwave Scanning Radiometer sea ice validation campaign are analyzed to provide knowledge of the spatial variability of these quantities as well as optimal methods for combining the data. Errors in the retrieved ice thickness values due to spatial variability are shown to be small, justifying the use of large-scale snow depth measurements in combination with small-scale freeboard measurements to obtain the sea ice thickness. Two methods are presented to address the spatial resolution with which the sea ice thickness is retrieved. A low resolution method which brings the freeboard and snow depth measurements to the same scale through spatial averaging is presented to provide a large-scale average thickness value. A method to scale the snow depth measurement to the higher resolution of the altimeter is also presented and is shown to be capable of providing a more detailed measure of the sea ice thickness distribution.
C14A-04 INVITED
CryoSat-2 and the CryoSat Mission
CryoSat was chosen as the first of ESA's Earth Explorer Opportunity missions in late 1999, following a
competitive selection process. Its goal is the measurement of secular change in the cryosphere, particularly
in the elevation of the ice caps and the thickness of sea ice. The required accuracy corresponds to about
half of the variation expected due to natural variability, over reasonable scales for the surfaces concerned.
The selected technique is radar altimetry, although the instrument has been modified to provide the
enhanced capabilities needed to significantly extend the spatial coverage of previous altimetry missions,
particularly ERS and EnviSat. Thus the radar includes a synthetic aperture mode which enables the along-
track resolution to be improved to about 250 m. This will enable detection of leads in sea-ice which are
narrower than those detected hitherto, so that operation deeper into pack-ice can be achieved with a
consequent reduction in errors due to omission. Altimetry over the steep edges of ice caps is hampered by
the irregular topography which, since the radar ranging is performed to the closest reflector rather than the
point directly below, introduces uncertainty into the exactitude of repeat measurements. CryoSat's radar
includes a second antenna and receiver chain so that interferometry may be used to determine the arrival
angle of the echo and so improve localisation of the reflection.
The satellite payload, which includes a DORIS receiver for precise orbit determination and a set of star
trackers to measure the orientation of the interferometer, is quite complex and demanding. The satellite was
launched on 8 October 2005, just less than 6 years after the start of the programme. Unfortunately the
launch vehicle, a Rockot launcher derived from the Russian SS-19 ICBM, suffered an anomaly at the end of
its second-stage flight, with the result that the satellite was lost, the debris falling close to the North pole.
Determination to rebuild the satellite and carry out the mission was extremely widespread: within 6 months all
of the necessary funding issues, legal procedures, industrial commitments and resource demands had been
solved and the programme restarted. The new satellite, inevitably called CryoSat-2, includes a large number
of improvements compared to its predecessor, although many are internal changes to improve the reliability
and ease of operations. More significantly, the expected lifetime has been increased.
The satellite measurements will be supported by a comprehensive set of validation data, collected on the
surface and from airborne platforms. These validation data, designed to specifically address the
uncertainties in the interpretation of the radar echoes, have been collected during a series of carefully co-
ordinated measurement campaigns over several years. Additionally, techniques to enable the collocation of
surface and satellite measurements over the moving sea-ice have been developed and rehearsed, ready to
support the dedicated validation campaigns during the mission.
CryoSat-2 is near completion, less than 3 years after the start of the industrial contract. Launch was originally
planned for March 2009, again with a Rockot. But lack of availability of this vehicle (more specifically, the
versatile third stage added to the ICBM) has induced a change to the Dnepr launcher, also an ICBM: the SS-
18. As a result of this change the launch is now planned for November 2009. So finally, about 10 years after
it was first selected, the CryoSat mission will start collecting data.
http://www.esa.int/livingplanet/cryosat
C14A-05
Glaciogenic bedforms on the Chukchi Borderland, Morris Jesup Rise and Yermak Plateau: three prolongations of the Arctic Ocean continental margin
The US Coast Gard Cutter Healy and Swedish icebreaker Oden have collected multibeam bathymetry and subbottom profiles from the Chukchi Borderland, extending out from the continental shelf of northern Alaska, the Morris Jesup Rise north of Greenland and the Yermak Plateau protruding out from the northwestern Svalbard continental margin. The collected data show glaciogenic bedforms in the form of mega scale glacial lineations, flutings, iceberg scours, morainic ridges and conspicuous erosional channels cutting into a glacially striated seabed. These Arctic Ocean glaciogenic seafloor features show similarities to features mapped on the Antarctic continental margin, both regarding their morphology and dimensions. In this presentation, results from geophysical mapping in the Arctic Ocean with icebreaker Oden during the LOMROG 07 expedition and with USCGC Healy during the HOTRAX 05 and HLY0703 expeditions are presented and compared with published results from the Antarctic continental margin. On the Yermak Plateau, multibeam data from the LOMROG 07 expedition show subdued glacial striations, flutes, which are extending in a northwesterly direction towards the central Arctic Ocean. The mapped portion of the Yermak Plateau where the flutes exist is between 500-600 m deep. Chirp sonar profiles collected along with the multibeam bathymetry reveal that these flutes comprise the top of a glacially eroded surface which extends down to a water depth of approximately 830 m below present sea level, although the glacially eroded surface is draped by sediments from a depth of approximately 600 m. This suggests that the Yermak Plateau has been overridden by an ice sheet. Results from the nearby shallow continental margin suggest that the Svalbard ice sheet during the Last Glacial Maximum (LGM), however, never reached the deeper Yermak Plateau and, thus, the flutes must originate from older glaciations. On the Morris Jessup Rise deep iceberg scours are mapped down to a water depth of 1012 m. These iceberg scours cut across the Morris Jesup Rise and it is possible to see from the multibeam data that the deep drafting icebergs causing them came from a westerly direction. Studies of sediment cores acquired directly from the scours indicate that the scouring took place during Marine Isotope Stage (MIS) 6. On the Chukchi Cap the multibeam data from USCGC Healy show closely-spaced striations and bedform-like features at approximately 410 m depth at approximately 760 32' N, 1630 49' W. The striations are curvilinear and oriented roughly northwest-southeast. They are typically 1-3 m deep and spaced 100 – 200 m apart. The bedform-like features are about 10 m high with wavelengths of 1-2 km. These features, currently more than 660 km from the nearest coastline suggest the presence, at some time, of a grounded ice sheet on the Chukchi Cap. Conspicuous erosional channels found slightly deeper than the striations, possible suggest intense sub-glacial water flow. Taken together, the mapped glacial morphology combined with coring results providing chronostratigraphic information constrain the extents of former ice sheets and ice shelves on the continental margins respective deep Arctic Ocean during past glacial periods.
C14A-06
Marine Geophysical Investigations of Arctic and Antarctic margins: implications for paleoglaciology
Ice sheets have advanced and retreated across Arctic and Antarctic continental shelves many times over the Late Cenozoic glacial age. The most recent retreat of ice from the Last Glacial Maximum has left a well- preserved suite of submarine glacial landforms on the sea floor of these shelves. Swath-bathymetric and other acoustic methods allow these landforms to be imaged at very high spatial resolution. We show assemblages of submarine landforms produced at the base and margins of ice sheets from both Arctic and Antarctic shelves, and use these data to reconstruct the past flow directions and dynamics of these ice sheets. The positions of former ice streams can be located, and the nature and rate of ice-sheet retreat can be inferred. We also show submarine landforms and shallow acoustic profiles that allow inferences to be made about the processes occuring at former ice-sheet beds. Glacier surging also produces a characteristic suite of superimposed landforms that are well preserved in Arctic fjords, and we use of these features to identify past surges in the geological record.
C14A-07
New 3-D view of a middle-shelf grounding-zone wedge in Eastern Basin Ross Sea, Antarctica
A new large-area multibeam survey of a previously identified grounding zone wedge on the central Ross Sea middle continental shelf was acquired during NBP0802 and NBP0803 in February 2008. Within a regional framework, this wedge corresponds to the third grounding event since the WAIS began the post-LGM retreat. The survey reveals the 3-D detailed view of a lineated topset with iceberg gouges, a smooth multi-lobed foreset and distinct downdip pinchout of the grounding zone wedge. Beyond the down-dip pinchout, older subglacial lineations, oblique to the younger lineations, are evident. The multibeam survey along with sub- bottom profiler records permitted us to precisely position piston cores for each of these morphologic sectors. The combined data may serve as a proxy for evaluating some aspects of the WAIS's modern grounding- zone system. For example, sediment cores at the wedge's thin landward and basinward limits obtained homogenous gray mud below a thin olive-green pelagic drape. The absence of a similar pelagic drape embedded in the homogenous gray muds suggests that grounded ice did not retreat past this location before the WAIS occupied the middle-shelf grounding position. In other words, the pause in WAIS retreat was not associated with any significant re-advance.
C14A-08
Tidally Influenced Iceberg Furrows on the Ross Sea Continental Shelf Indicate Rapid Flow and Likely Collapse of Ice Sheet
Side-scan sonar records from the central and western Ross Sea show deep (up to approximately 650 meters water depth) iceberg furrows that record and episode of massive calving at the margin of the retreating ice sheet. These furrows overprint mega-scale glacial lineations and are buried beneath diatomaceous glaciomarine sediments. Hence they mark the final stage of ice sheet grounding on the continental shelf. These features are unique in that they display internal ridges that are interpreted as having a tidal origin. Two sets are distinguished based on their orientations. One set shows a strong northwestward-directed flow on the inner shelf. A second set displays more random orientations with more variable spacing of internal ridges. The most plausible explanation for these features is that they were formed at the calving margin of the ice sheet as it decoupled from the sea floor under tidal influence. The spacing of the ridges implies high rates of flow and disintegration. Ongoing efforts to determine the timing of this event focuses on the continental slope where ice-rated debris layers have been identified but efforts to date these layers have thus far been unsuccessful.